Tweaks and mods for HP 3457A 6.5-digit DMM

Intro

After my first article about 6½-digit HP 3456A there was a desire to attempt improvement of later DMM from Hewlett-Packard, Model 3457A. Let’s take a close look on the device in question.

HP 3457A is a versatile system DMM with 7 functions (DCV, TrueRMS ACV up to 450 VAC, DCI, 2W and 4W resistance, Frequency, Period), providing readout with up to 6½-digit resolution (extendable to 7½ over GPIB) with measurement speed up to 1350 SPS. Basic DCV accuracy is specified better than 5 ppm. HP 3457A can also use optional expansion card with multiplexer scanner, to add up to 10 channels for input signals. Being benchtop/system multimter front panel operation have a lot of flexibility and programmable functions. Meter also comes with standard HP-IB/GPIB interface port for remote control/operation. Many of inteface and overall design ideas in this meter are used as a base for later HP 3458A monsters.

This multimeter was introduced to market in 1989, at MSRP $2800 USD.

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Details and modifications were tested on actual single HP 3457A unit, and may or may not work in your particular instrument. Beware!

If you willing to contribute or add your experience regarding HP/Agilent/Keysight instruments repairs or provide extra information, you can do so following these simple instructions

Manuals and service information

Related component datasheets

Exterior

This meter was also first in HP system-level flagship line to feature integrated measurements for both direct and alternating current and alternating voltage frequency and period. Previous models, such as HP 3455A and HP 3456A did not have these features.


Image 1: HP 3457A overview


Image 2: HP 3457A bottom side

This DMM also features new “screwdriver-free” calibration to allow electronic adjustments for related functions, using external known reference standard and using onboard ADC to have automatic calculation of affected calibration constants by embedded processor. This means that disassembly and cover removals not required for adjustment and calibration (with few exceptions, covered later). Calibration coefficients are stored in SRAM, which is powered even if instruments powered off, with help of on-board lithium battery. Yes, it also does mean that calibration will be lost if meter too old and battery discharged.


Image 3: HP 3457A internal calibration RAM battery check

Special circuitry protects calibration data from unauthorized or random overwrite. Also calibration commands protected by password, which can be configured by user following special procedure. If password is lost (such as when buying meter on eBay, or previous owner didn’t know the password) it can be reset by jumper toggle on the digital board.

More advanced firmware also greatly improve system use of HP 3457A in automated test jigs and ATE environments. There is possibility to save and recall multiple operation modes with all function settings, even after power cycle. Also each function and mode can be configured either from front panel or remotely over GPIB interface. Special set of macro keys allow programming for frequently used functions/settings. It has keyboard, just like white “Steinway & Sons”, with 44 keys, not counting power switch! For comparison with other HP meters (power switch, rear/front terminal selector, shield selector are not taken into account):

  • HP 3456A – 37 buttons
  • HP 3455A – 28 buttons
  • HP 3458A – 36 buttons

HP 3457A is truly a record holder for HP DMMs in this regard :).


Image 4: HP 3457A front face

Also this is first meter to have “magical” AutoCal button, which corrects:

  • 3 GΩ Resistance range (correction for gain and offset)
  • Flatness of AC input attenuator for ACV function
  • Offset for input amplifier for DCV function

Also as bonus HP promoted ability to get 7th digit from special register access. To my opinion, this was bit overdue addition, as previous model, 3456A even with official specifications without any tweaks can be more precise and less noisy than 3457A. Sounds too good to be true? Check the table below with uncertainty calculations. Data from specs for 24 hour and 90 day period was used . Low ranges like 0.1 and 1 V emulated by using lowest range on 3458A. Areas where 3457A have worse spec marked in red color.

U, V HP 3456A, 24h specs, ±V HP 3457A, 24h specs, ±V HP 3456A, 90 day specs, ±V HP 3457A, 90 day specs, ±V
0.1 0.0000026 0.0000029 0.0000058 0.0000064
0.1 0.0000049 0.0000029 0.0000064 0.0000064
0.3 0.0000067 0.0000061 0.0000112 0.0000114
0.5 0.0000085 0.0000068 0.0000160 0.0000145
1 0.000013 0.0000085 0.000028 0.000023
1 0.000028 0.0000085 0.000043 0.000023
2 0.000036 0.000012 0.000066 0.000040
3 0.000044 0.0000155 0.000089 0.000057
5 0.00006 0.0001225 0.000135 0.000365
10 0.0001 0.000155 0.000250 0.000540

Table 1: HP 3457A vs HP 3456A DCV specifications

Now, we clarified DCV specifications, where 7½-digit resolution could be worth considering. On alternating voltage, 3457A is much weaker than older 3456A. Note that 3456A conversion from ADC is actually 7 digits, but least digit is averaged out and not reported to the front-end, nor the GPIB port remote controller.

3457A allow using multi-channel mux, replacing rear terminal block. There are few options for mux cards, one is high-voltage/high-current slow HP 44491A or faster low-voltage card HP 44492A.

Important feature, absent from previous 3456A, is ability to select front or rear terminal input by remote GPIB command. Even top end 8½ HP 3458A does not have this ability to preserve input signal quality and protection. As result there is no mechanical switch on front panel to change terminal side manually.

Also this unit has least weight in class, due to use of plastic instead of metal as covers material. However it comes at expense of worse noise/RFI immunity. Later mainstream benchtop 34401A meter was also reverted back to metal chassis + shielding covers.


Image 5-6: Inner protection shields


Image 7: Inguard shield removed, revealed AC and front end main board

Overall design architecture is similar to 3456A and also has out-guard power supply/digital/interface and in-guard analog/measurement sections. More details presented below.


Image 8: Overall block diagram

Inguard isolated section has next circuits: – input terminals with protection, switching circuits and current shunts,


Image 9: Electronic input terminal switching


Image 10: COTO relay for front end


Image 11-13: Resistance shunts for current measurement function

- high-voltage divider 1:100 for DCV function. (also part of hybrid assembly U101), – input DC amplifier with 1:3.33, 1:33.3 and 1:333 gain,


Image 14: Input amplifier after my modifications

- AC/DC converter with it’s own attenuator and amplifier


Image 15-16: AC TrueRMS converter board

- Main voltage reference for ADC and current sources for resistance function,


Image 17-18: LM399-based reference module soldered directly to PCBA

- multi-slope integrating ADC itself with control logic,


Image 19: LT1122 opamp

- linear voltage regulators and optoisolated interface to outguard digital “brains”.


Image 20-21: Linear power regulator and optoisolation interface between the domains

Outguard earth-referenced domain contains of: – same optoisolated section for interface, – main processor and it’s memory, – GPIB interface, – front panel with display and keyboard, – main power supply.


Image 22: HP 3457A digital out-guard section

Theory of operation, how things work

Heart of the meter, ADC built using same principle as 3456A, but now majority of critical components (switches, comparator, current setting resistor networks, logics and clock tree) is embedded in proprietary hybrid ASIC U511:


Image 23: HP 3457A U511 hybrid

ADC’s input reference voltage is 10 V. Reference voltage source is same module from 3456A, using LM399 ovenized buried zener circuit. This module even have same HP part number. There is no service manual schematics for this module in 3457A documents, but such schematics available in 3456A.


Image 24: Original reference schematics

Hmm, hold on a moment? Input signal scale of the ADC as result is ±10V, but measurement scale is ratio of 3? How did that happened?

This is not a mistake, but a design approach of the 3457A. To scale input signal amplitude input amplifier is always engaged, but has different gain coefficient. On ranges 30 mV, 300 mV and 3 V amplifier gain (later A) is 3333, 33.3 or 3.33 as a result. Higher voltage ranges are even trickier. For 30 V and 300 V additional 100:1 divider is added in front of the amplifier, which is configured for A=33.3 or 3.33.

Most of key circuitry in contained in another proprietary ASIC – HP U101.


Image 25: U101 hybrid design block diagram

As result of such complex arrangement 3457A front-end does have more input noise compared to 3456A, and also does not have 1:1 direct path on any of the DCV ranges. Closest to the base range is 3 V, which is not best case for sensitive measurements, such as 7V reference output.

Going back to input amplifier, it’s based on combined schematics of two matched FET pairs, Q111 and Q112 together with dual opamp U111 (TL072).


Image 26: Modifications for 3457A input amplifier

Main goal of modifications was to reduce noise in DCV function, so let’s take close look on two main sources of such noise, input amplifier front-end and ADC itself.

Input amplifier have U112,Q114 which set and maintain stable current on the FETs. U112b used to invert voltage reference level +10V to negative -12V, -2V of which going to R115 (top terminal have -10V, bottom terminal -12V). This stable 2V level supplied to voltage-current converter around U112a,Q114 with current setting by resistor R114. 2V over 5.11 kΩ produce current around 400 µA , or about 200 µA per side. R113 help to maintain balance when input is shorted, or in other words – cancel out voltage offset.


Image 27: R113 trimpot adjutment for offset correction

There is special procedure for voltage offset correction, but we will get back to it later.

Resistors R111 and R112 act as load and define gain of the FET-amlifier, while current mirror circuit around U111a,Q113 balance equal DC current between the sides, increasing overall gain as result. Actual amplifier part is Q111, and Q112 help for constant voltage between source and drain of Q111. Diodes CR113, CR114 ensure operation of the current mirror with their voltage drop. CR111 and CR112 protect opamp U111b from overload conditions. This opamp provide most of the gain. Worth to note, that this operational amplifier must be stable with both high and low gains, and as we will see later, most troublesome range for stability is 3 V.

It’s clear, that due to custom hybrid ASIC design, we cannot do much in terms of noise, other than improving stability and quality of the supply voltages. So main interest go to other possible noise sources. Suspects are listed below:

  • Pair Q111 and operation of this stage (usually higher working current produce less noise).
  • U111
  • U112
  • Noise on +10V reference voltage. (Even with shorted input, when reference noise irrelevant to ADC! This is due to use of reference voltage for input amplifier operation)
  • Much less probable – noise from Q112, Q113, Q114, CR113 and CR114.

So in next chapter we will try some experiments for first four circuits.

But before that, take a look on the ADC itself.


Image 28: HP 3457A ADC

Here is composite integrator Q514,U512 with integrating capacitor C511 and pre-amp for comparator circuit U513,R513,R515. Everything else, like charge rate switches, counters and control logic integrated on U511 hybrid ASIC, or perform auxilary roles (reset comparator U515a, switches). These auxiliary circuits do not affect noise performance of the ADC.

Likely, largest contributors to noise performance are

  • C511
  • Q514
  • U512
  • U513

Now we have just two more things to do.

Modifications worklog

Some might think that HP 3457A is ancient technology instrument, and it should be very easy just to swap some opamps and components to modern high-performance devices, and everything will become much better. Just like replace CPU and GFX in computer from old Pentium 4 to modern Core i9, eh?

Well, in analog design world things are much more complicated, and swapping components without understanding what the circuit do is a road to a disaster and broken equipment.

Upgrades and modifications on this HP 3457A were attempted for long time, multiple times for period over a year.The longest process was getting required component samples. It’s not easy to get high-performance components which are not stocked in regular retailers, and also there is a lot of counterfeit on Aliexpress (more than half of sales, in my own experience)

Another time consuming challenge – complex approach type. It’s not possible to isolate just one circuit and test it’s performance, without rest of the instruments. That mean each modification test is full reassembly, few hours warmup and then multiple hours for datalogging (each low range separately!). Add time to assemble/disassemble, replace parts, etc.

I tried to combine modifications in groups, so can test multiple things at same time, when that was possible. Often modification result was negative (performance got worse), so reverse modification was required to determine what went wrong. So to save time for a reader, I’ve collected all modifications (that I remember) into a table, with summary result if it helped the instrument or not, and also decision on each modification. Hope this can be helpful for other HP 3457A owners.

Modification type Successful? Noise reduced? Modification result
Add backlight for LED display Yes No impact Applied
Add internal temperature sensor for in-guard section Yes No impact Applied
Additional ferrite common mode choke between transformer and in-guard section Yes Looks like yes Applied
Improve thermal isolation for LM399 reference Yes Yes Applied
Replace connectors to hard soldered connection for VREF PCBA Yes Looks like yes Applied
Increase capacitor value C901 to 2200 µF Yes No difference detected Reverted
Increase capacitor values C911 and C921 to 470 µF Yes No difference detected Applied
Additional ceramic capacitor 4.7 µF right on pins 3 and 5 of the U511 Yes Worse Reverted
Same modification as above, but 0.1 µF Yes Still worse noise Reverted
Replacement of U512 and U513 to LT1122 Yes Yes Applied
Attempt to remove Q514 Yes Worse Reverted
Attempt to increase preamplifier gain (U513) Yes Worse Reverted
Replaced U111 to ADA4077 Yes Yes Applied
Replaced U112 to AD706 Yes Yes Applied
Replaced Q111 to pair 2SK30A Yes Yes Applied
Increase current thru Q111 Yes Partially better, but worse on 3V Reverted
Replaced U501 to AD707 Yes Yes Applied

Table 2: Modifications log

Thermal sensor and mounting location:


Image 29-30: Additional temperature sensor and output BNC port for it

Let’s go thru rest of the modifications, targeted on measurement noise reduction.

Improve thermal isolation for LM399 reference covered in detail in my article about HP 3456A meter.
Adding common mode ferrite choke and removing connections between reference module and mainboard does not need additional explaining either.

Since all microchips in HP 3457A are in thru-hole type DIP style, all replaced components are carefully desoldered and positions on the board populated with high-quality collet-type sockets. This allows to experiment easy without risk of damaging the board taking everything apart for each component swap. Operational amplifier LT1122 have both variants for DIP8 and SMT packages, so they can be installed instead of original LT318 (U512 and U513) with ease.


Image 31-32: LT1122 circuitry near DCV reference

On second photo you can see removed resistors, missing Q512 and R513 1 kΩ instead of factory 5.11 kΩ. All this had to be reverted to original state in the end result, as noise only got worse.


Image 33: Replaced opamps with collet sockets

Situation with U111 is more difficult, as I got only single-channel version ADA4077-1 but original TL072 is dual opamp component. I did not want to wait, so adapter board bodge to the rescue to fit two chips in DIP8 socket.


Image 34: ADA4077 with DIY adapter board

PCB Layout:


Image 35: Adapter PCB layout

To use AD706 in U112 position together with AD707 in position U501 I’ve used factory adapters SO8/DIP8. When I tried to increase current thru FETs it revealed low quality of R114, at best about bad quality of USSR-made MLT-series general purpose resistors.


Image 36: HP 3457A original damaged R114 resistor

It is possible, than bad contacts and extra noise was caused by this resistor, as it’s directly related to noise performance of the amplifier circuitry. I’ve used foil resistors S5-61 10.4 kΩ as replacement, with very close but opposite by polarity resistance temperature coefficients:


Image 37-38: Replacement foil resistors S5-61

After Q111 replacement (with faster curve than original) and U111 (more gain) input amplifier circuitry started to oscillate with frequency around 150 kHz. To get it all stable again I’ve used mica capacitors:


Image 39: Capacitors used for stability compensation

I used middle sweet spot device for replacement Q111. Key importance at this stage to probe output with oscilloscope, to ensure stable operation without any oscillation, with actual DC signal applied on the input, from zero all the way to the full scale 3 V. If there is lack of compensation, then amplifier will work normally when input voltage is zero or less then 0.1 V, but will oscillate in sync with auto-zero operation with larger signal amplitudes.

It was the most troublesome, to pick and sort Q111 FET replacement. I’ve bought few 2SK30A, and each FET was characterized for I/V performance. I’ve used functional generator (U3), voltmeter (U4) with GPIB, two multimeters in current measurement mode (U1 and U2) with USB interface, PC and custom test jig. Usually testing for such done using SMUs or curve tracers, but I’ve got my setup working with what I had available:


Image 40: DIY voltage-current curve plot detector

Detail description – VT1 + DA2 is current limiting for 12 mA, VT2 + DA4 is voltage regulator to provide 10 V, DA3 is middle point generator at +5 V level from power ground return. This was required to enable trimpot RP1 setting for both negative and positive gate voltage. When test jig controlled remotely by PC position of RP1 is in middle.

R6 and R14 with their protection diodes prevent gates from excessive current. Diodes in drain circuits create virtual gaps to allow current tapping by DMMs. Fixture to accept DUT FET was implemented using collet socket for 5 positions. For original dual-FET from HP 3457A I made custom adapter due to difference in pinout.

As result I can get two curves, to enable testing pair of 2SK30A’s at once. Final test data rainbow with all tested FETs:


Image 41: Test data generated from many 2SK30A FETs

Original HP 3457A FET shows curve close to ideal:


Image 42: HP 3457A original Q111 matching

Our matched FET pair isn’t bad either:


Image 43: Replacement Q111 matching

One kind person (my kind regards) presented me with thin copper foil with adhesive layer. So I taped both FETs using the copper foil tape, to ensure good thermal contact between both FETs. Make sure total height of the component in board is not taller then 12 mm, as there is AC module PCBA above the circuitry here.


Image 44: HP 3457A input amplifier after all modifications


Image 45-46: Input amplifier overview


Image 47-48: Input amplifier ready for testing

Before and after condition measurement data results

For results testing I’ve used 7½-digit measurements with shorted and thermally PE-insulated inputs. Sampling rate was equal to 0.5 Hz, getting reading every 2 seconds. Best-case “optimistic” data in 10 minute time length was selected from captured data. So we get 600 samples, where each point processed from sum of 6½-digit reading and 7th digit adder. RMS deviation was calculated, in ppm scale from the average. To measure ADC’s noise (line ADC only) JM101 jumper was desoldered and test points TP5 and TP2 were shorted together.


Image 49: Input amplifier ready for testing

Meter was connected to PC over GPIB interface to capture data over time periods.

HP 3457A scale Original condition, ppm RMS After modifications, ppm RMS
0.03V 5.0525 2.9879
0.3V 0.4851 0.2969
3V 0.1137 0.0783
30V 0.3625 0.3206
300V 0.0896 0.0824
ADC only 0.0735 0.0648
ADC only + cap (4.7 µF) 1.6
ADC only + cap (0.1 µF) 0.9

Table 3: RMS noise data measurements

Note, that on low voltage ranges (0.03, 0.3, 3 V) meter can pickup all the environment noise and interference, due to very high input impedance. Also ambient temperature, humidity and pressure variations affect the stability and noise performance of the meter as well.

As you can see, modifications can help to get quieter readings, especially on most sensitive ranges (about 40% reduction). This is good, even though not as impressive as HP 3456A tweaking.

Before modifications After modifications
HP 3457A, ADC noise
HP 3457A, 30 mVDC range
HP 3457A, 300 mVDC range
HP 3457A, 3 VDC range
HP 3457A, 30 VDC range
HP 3457A, 300 VDC range

Table 4: Measurement noise data plots

Few practical notes about modifications

After Q111 replacement the complete input amplifier balance had to be adjusted and re-calibrated. This is little tricky. In service manual this procedure was explained for only case when self-test diagnistics related to this section unable to pass. Perhaps because of this note in service manual typical calibration in the service shops does not include this calibration either.

I had no problems with self-test, as I had calibrated the balance during initial repairs and troubleshooting in meter, before any modifications. Experimentally, I figured out that there is indeed balance offset. This also revealed that level of self-test fail threshold is ±1.5 µV, while calibration is required to level ±0.5 µV.

Calibration procedure for balancing does not adjust anything, it just shorts input amplifier input and measure if the output voltage is in spec, higher or lower than spec. This information is displayed on the screen and operator is expected to adjust R113 for best result.

So I got idea to prevent running this procedure for intermediate testing. To do so I’ve followed next workflow:
First I check if the adjustment necessary at all (why to waste time on extra drift). Press blue button, then D_AUTOZERO (display shows DELAY), then arrow down (display showss DIAGNOSTIC). Now press 4 and ENT. If PASSED is displayed, then we can exit this mode by long press of right arrow key (RESET) and not bother with anything else.

If display shows FAILED LO or FAILED HI – some work need to be done.

Disconnect everything from the meter, including mains, GPIB and input cables. Turn the meter upside down, undo 6 screws (if there are original rubber gaskets, there is no need to remove screws from cover). Now turn the meter again, front panel facing to us, remove top plastic cover and under it we can find top shield, holder of the multiplexer board.


Image 50: HP 3457A with shields installed

Those who have 3457A with multiplexer module, you’ll have to remove it. Service manual demands of removal rear terminal panel, but I have custom one in my unit, and it’s not required.

Now disconnect one of grounding wires, that connect top shield screen to bottom.


Image 51-52: Grounding wire terminal

Carefully lift rear side of the shield, disconnect cable from rear terminal and turn screen up. Now we have access to test holes, which marked as Vos Adj A2-R113. Carefully insert isolated flat screwdriver to fit the pot port.


Image 53: Vos adjustment trimmer access port in shield cover

Connect mains power cable back to meter, turn it on and leave it to warm-up for at least an hour. Manual does not tell to warm-up delay, but I think it’s good idea to let things stabilize in near-use case temperatures to avoid unwanted errors. After warm-up complete, adjust the trimmer till you get PASSED result in the test as described above.

Now we can disconnect everything, assemble parts back and turn on for warming up and final tests, to make sure that meter reports still PASSED after a day or two.

Second, similar adjustment procedure should be performed in diagnostics mode 2, to correct AC frequency flatness for input divider. I had it PASSED without any issues, so I didn’t need to adjust anything. First rule of repair – do not change or fix things, that are still working fine.

If you opening the meter, it’s worth to check good connections between all binding post terminals to board headers, make sure all connections are clean and tight. Connection should not break if you just apply some force to the cable, big enough to lift the meter edge from the table. Weak contacts should be tighten carefully, but not too much. Reliability of these connections is the key for two lowest resistance ranges (30 Ω and 300 Ω).

It was common problem in other instrument (HP 3458A), reading incorrect resistances when connections were loose. If you get NEGATIVE resistance after tightening the terminals and contacts, that would mean last calibration was done with loose contacts. And calibration of that meter is very expensive, to let such mistakes pass by.

And some extra photos for bonus:


Image 54: HP 3457A input frontend and signal conditioning schematics


Image 55: HP 3457A A/D converter section schematics

RAMTRON memory modifications:


Image 56-57: Ramtron FRAM memory in HP 3457A


Image 58: Shields locked in place

Backlight modification for display

Probably all of the HP 3457A owners are “very happy” when trying to see what the heck is small LCD showing, especially in dim environment conditions. Unlike bright red LEDs on HP 3456A or fuzzy warm VFD on HP 3458A, this meter does not have ANY back-light, only passive LCD glass. Urge to attach something generating light to aid readout visibility is very hard to resist.

So I removed LED display and here’s the sad story about it:


Image 59: LCD display removed from assembly

Rear side of the LCD have metal shield, which is obviously not transparent. That makes possibility of rear back-light impossible. I did not want to risk full disassembly, due to risk of killing the only working display. So I’ve decided to do my best without taking everything apart, and keeping external parts to minimum.

  1. External parts easy to get damaged
  2. Can cause unexpected short circuits, or ESD issues (running meter in winter with people wearing charged sweaters).
  3. Would require modification of the chassis.

So I found dead chinese cell phone in junk box, which donated six flat white SMD-LEDs. These LEDs were moved to narrow PCB:


Image 60: Tiny PCB to hold LEDs in place

Backlight boards with LED diodes installed on mount frame to position light for the edge of display glass.


Image 61: LED mount for display backlight

I’ve used digital power from GPIB controller board regulator.


Image 62: LED supply power connections

Current-limiting resistors are mounted on additional board for the board with four LEDs. Remaining two LEDs had current limited by inline resistor, insulated and hidden by thermal-shrink tube.


Image 63: HP 3457A rear view on modified screen

Result at night.


Image 64: LCD display with DIY backlight at night

In daylight.


Image 65: LCD display with DIY backlight, with daylight conditions

To be fair, contrast of this LCD got bit worse, but readings are still clearly visible. Much more important fact is, that display is now visible and usable at night time.

Calibration and performance verification

This section is added by xDevs.com support team, covering calibration procedure and testing of 24-hour stability performance on DIFFERENT, non-modified HP 3457A meter. This is non-traceable calibration for education purpose only.

To run some performance numbers on HP 3457A, let’s connect one to our Fluke 5700A/03 working standard multi-function calibrator and use Python application to crunch some numbers.


Image 66: HP 3457A 6½-digit DMM with Fluke 5700A Multi-Function Calibrator in background.

Reference source value = (Low limit + Hi limit) / 2;

DCV Function performance test

Here is automated performance verification test data before the calibration adjustment.

Before calibration adjustment, AS RECEIVED condition. Last calibration date or history of the unit is unknown.

DCV Test HP 3457A Data Source uncertainty Low Limit Hi limit Measured 24hr spec Result
0.03 VDC (0.03 Range) 0.0299997 3.81 ppm 0.0299973 0.0300026 -9.6 ppm 83.7 ppm PASS 11.01 %
-0.03 VDC (0.03 Range) -0.0300017 3.81 ppm -0.0300026 -0.0299973 57.3 ppm 83.7 ppm PASS 65.55 %
1.0 VDC (3 Range) 1.000021 1.47 ppm 0.9999933 1.0000066 21.0 ppm 5.2 ppm FAIL 314.84 %
2.0 VDC (3 Range) 2.000042 1.47 ppm 1.9999866 2.0000133 21.0 ppm 5.2 ppm FAIL 314.84 %
3.0 VDC (3 Range) 3.000063 1.47 ppm 2.9999799 3.0000200 21.0 ppm 5.2 ppm FAIL 314.84 %
-1.0 VDC (3 Range) -1.000019 1.47 ppm -1.0000066 -0.9999933 19.0 ppm 5.2 ppm FAIL 284.86 %
-2.0 VDC (3 Range) -2.000041 1.47 ppm -2.0000133 -1.9999866 20.5 ppm 5.2 ppm FAIL 307.35 %
-3.0 VDC (3 Range) -3.000069 1.47 ppm -3.0000200 -2.9999799 23.0 ppm 5.2 ppm FAIL 344.83 %
10 VDC (30 Range) 10.000179 2.36 ppm 9.9998814 10.000118 17.9 ppm 9.5 ppm FAIL 150.93 %
20 VDC (30 Range) 20.000358 2.36 ppm 19.999762 20.000237 17.9 ppm 9.5 ppm FAIL 150.93 %
30 VDC (30 Range) 30.0005 2.36 ppm 29.999644 30.000355 16.6 ppm 9.5 ppm FAIL 140.53 %
-10 VDC (30 Range) -10.000159 2.36 ppm -10.000118 -9.999881 15.9 ppm 9.5 ppm FAIL 134.06 %
-20 VDC (30 Range) -20.000358 2.36 ppm -20.000237 -19.999762 17.9 ppm 9.5 ppm FAIL 150.93 %
-30 VDC (30 Range) -30.000649 2.36 ppm -30.000355 -29.999644 21.6 ppm 9.5 ppm FAIL 182.41 %
100 VDC (300 Range) 100.00119 2.85 ppm 99.99638 100.00361 11.9 ppm 33.3 ppm PASS 32.92 %
200 VDC (300 Range) 200.00299 2.85 ppm 199.99277 200.00723 14.9 ppm 33.3 ppm PASS 41.36 %
300 VDC (300 Range) 300.0069 2.85 ppm 299.98915 300.01084 23.0 ppm 33.3 ppm PASS 63.62 %
-100 VDC (300 Range) -100.001 2.85 ppm -100.00361 -99.99638 10.0 ppm 33.3 ppm PASS 27.66 %
-200 VDC (300 Range) -200.0029 2.85 ppm -200.00723 -199.99277 14.5 ppm 33.3 ppm PASS 40.11 %
-300 VDC (300 Range) -300.0072 2.85 ppm -300.00724 -299.99275 24.0 ppm 33.3 ppm PASS 99.38 %

Table 5: Different non-modified HP 3457A performance test, as received, DCV function

As we can see, meter cannot meet it’s 24 hour specification for 3V and 30V ranges, but actually not that much off. 300 mVDC range was not tested due to bug in control code.

After calibration adjustment, AS RETURNED condition. Calibrated vs same Fluke 5700A on January 21, 2018.

Table below present measured data after calibration adjustment and performance verification. This is 48 hours of running time, to ensure that short-term stability of the meter is taken into account.

DCV Test HP 3457A Data Source uncertainty Low Limit Hi limit Measured 24hr spec Result
0.03 VDC (0.03 Range) 0.0300005 3.81 ppm 0.0299973 0.0300026 17.8 ppm 83.70 ppm PASS 20.39 %
-0.03 VDC (0.03 Range) -0.0300012 3.81 ppm -0.0300026 -0.0299973 41.9 ppm 83.70 ppm PASS 47.91 %
0.3 VDC (0.3 Range) 0.3000015 2.45 ppm 0.2999953 0.3000046 5.2 ppm 13.00 ppm PASS 34.09 %
0.2 VDC (0.3 Range) 0.2000013 2.45 ppm 0.1999969 0.2000030 6.8 ppm 13.00 ppm PASS 44.34 %
0.1 VDC (0.3 Range) 0.1000010 2.45 ppm 0.0999984 0.1000015 9.9 ppm 13.00 ppm PASS 64.51 %
-0.1 VDC (0.3 Range) -0.0999998 2.45 ppm -0.1000015 -0.0999984 -1.3 ppm 13.00 ppm PASS 8.44 %
-0.2 VDC (0.3 Range) -0.2000003 2.45 ppm -0.2000030 -0.1999969 1.7 ppm 13.00 ppm PASS 11.54 %
-0.3 VDC (0.3 Range) -0.3000014 2.45 ppm -0.3000046 -0.2999953 4.7 ppm 13.00 ppm PASS 30.85 %
1.0 VDC (3 Range) 1.000003 1.47 ppm 0.9999933 1.0000066 3.4 ppm 5.20 ppm PASS 50.97 %
2.0 VDC (3 Range) 2.000004 1.47 ppm 1.9999866 2.0000133 2.2 ppm 5.20 ppm PASS 34.23 %
3.0 VDC (3 Range) 3.000007 1.47 ppm 2.9999799 3.0000200 2.4 ppm 5.20 ppm PASS 36.65 %
-1.0 VDC (3 Range) -0.999997 1.47 ppm -1.0000066 -0.9999933 -2.4 ppm 5.20 ppm PASS 37.18 %
-2.0 VDC (3 Range) -1.999998 1.47 ppm -2.0000133 -1.9999866 -0.7 ppm 5.20 ppm PASS 10.74 %
-3.0 VDC (3 Range) -3.000004 1.47 ppm -3.0000200 -2.9999799 1.6 ppm 5.20 ppm PASS 24.49 %
10 VDC (30 Range) 10.000053 2.36 ppm 9.9998814 10.0001186 5.3 ppm 9.50 ppm PASS 45.25 %
20 VDC (30 Range) 20.000069 2.36 ppm 19.9997628 20.0002372 3.4 ppm 9.50 ppm PASS 29.23 %
30 VDC (30 Range) 30.000093 2.36 ppm 29.9996442 30.0003558 3.1 ppm 9.50 ppm PASS 26.33 %
-10 VDC (30 Range) -10.000005 2.36 ppm -10.0001186 -9.9998814 0.5 ppm 9.50 ppm PASS 4.53 %
-20 VDC (30 Range) -20.000053 2.36 ppm -20.0002372 -19.9997628 2.6 ppm 9.50 ppm PASS 22.48 %
-30 VDC (30 Range) -30.000182 2.36 ppm -30.0003558 -29.9996442 6.0 ppm 9.50 ppm PASS 51.15 %
100 VDC (300 Range) 99.999103 2.85 ppm 99.996385 100.003615 -8.9 ppm 33.30 ppm PASS 24.80 %
200 VDC (300 Range) 199.9989 2.85 ppm 199.99277 200.00723 -5.5 ppm 33.30 ppm PASS 15.21 %
300 VDC (300 Range) 300.00069 2.85 ppm 299.989155 300.010845 2.3 ppm 33.30 ppm PASS 6.39 %
-100 VDC (300 Range) -99.99869 2.85 ppm -100.003615 -99.996385 -13.0 ppm 33.30 ppm PASS 36.15 %
-200 VDC (300 Range) -199.99843 2.85 ppm -200.00723 -199.99277 -7.8 ppm 33.30 ppm PASS 21.72 %
-300 VDC (300 Range) -300.00057 2.85 ppm -300.007245 -299.992755 1.9 ppm 33.30 ppm PASS 7.96 %

Table 6: Different non-modified HP 3457A performance test, as returned after calibration adjustment, DCV function

Calibration revealed no problems, worst deviation is +64% for +⅓ scale of the 0.3V range. Base range, which is 3.0 VDC on this meter is now better than 3 ppm relative to calibrator.

Resistance function test (4-wire with offset compensation enabled)

Before calibration adjustment, AS RECEIVED condition. Last calibration date or history of the unit is unknown.

Resistance data as received, before any adjustment:

DCV Test HP 3457A Data Source uncertainty Low Limit Hi limit Measured 24hr spec Result
1 Ω 0.999676 40.2 ppm 0.99969222 0.99990257 -120.7 ppm 65 ppm FAIL 114.79 %
1.9 Ω 1.899333 8.3 ppm 1.89938396 1.89966243 -99.9 ppm 65 ppm FAIL 136.36 %
10 Ω 9.999936 8.3 ppm 9.99917100 10.0006369 3.2 ppm 65 ppm PASS 4.46 %
19 Ω 18.99904 4.3 ppm 18.9977783 19.0004116 -2.8 ppm 65 ppm PASS 4.18 %
100 Ω 100.00203 4.3 ppm 99.9977999 100.005460 4.0 ppm 34 ppm PASS 10.53 %
190 Ω 189.99533 3.3 ppm 189.987913 190.002086 1.7 ppm 34 ppm PASS 4.70 %
1.0 kΩ 1000.003 3.3 ppm 999.952200 1000.0288 12.5 ppm 35 ppm PASS 32.64 %
1.9 kΩ 1900.0177 3.3 ppm 1899.92333 1900.06887 11.3 ppm 35 ppm PASS 29.64 %
10 kΩ 10000.1 3.3 ppm 9999.68499 10000.451 3.2 ppm 35 ppm PASS 8.36 %
19 kΩ 18999.74 3.3 ppm 18998.9573 19000.4126 2.8 ppm 35 ppm PASS 7.56 %
100 kΩ 100002.17 3.3 ppm 99996.8799 100005.540 9.5 ppm 40 ppm PASS 22.09 %
190 kΩ 189994.63 5.3 ppm 189984.113 190001.326 10.0 ppm 40 ppm PASS 22.23 %
1.0 MΩ 999992.33 5.3 ppm 999949.6 1000050.2 -7.5 ppm 45 ppm PASS 15.04 %
1.9 MΩ 1899942.7 14.3 ppm 1899842.63 1900067.96 -6.6 ppm 45 ppm PASS 11.21 %
10 MΩ 9999620 14.3 ppm 9996723.16 10002008.8 25.4 ppm 250 ppm PASS 9.61 %
19 MΩ 18999773 60.3 ppm 18993129.6 19004920.4 39.3 ppm 250 ppm PASS 12.69 %
100 MΩ 100423000 60.3 ppm 99842216 100174303 4147.0 ppm 1600 ppm FAIL 249.78 %

Table 7: Different non-modified HP 3457A performance test, as received, 4-wire resistance function

After calibration adjustment, AS RETURNED condition. Calibrated vs same Fluke 5700A on January 21, 2018.

After adjustment and calibration to ⅔ of the full-scale (example is 19 kΩ instead of 30 kΩ, as Fluke 5700A cannot provide arbitrary values).

DCV Test HP 3457A Data Source uncertainty Low Limit Hi limit Measured 24hr spec Result
1 Ω 0.9997266 40.2 ppm 0.99969222 0.99990257 -70.7 ppm 65 ppm PASS 67.25 %
1.9 Ω 1.89947 8.3 ppm 1.89938396 1.89966243 -28.0 ppm 65 ppm PASS 38.21 %
10 Ω 9.9999833 8.3 ppm 9.99917100 10.0006369 7.9 ppm 65 ppm PASS 10.82 %
19 Ω 18.999227 4.3 ppm 18.9977783 19.0004116 6.9 ppm 65 ppm PASS 10.00 %
100 Ω 100.00203 4.3 ppm 99.9977999 100.005460 4.0 ppm 34 ppm PASS 10.53 %
190 Ω 189.99547 3.3 ppm 189.987913 190.002086 2.4 ppm 34 ppm PASS 6.58 %
1.0 kΩ 999.99433 3.3 ppm 999.952200 1000.0288 3.8 ppm 35 ppm PASS 10.01 %
1.9 kΩ 1900.001 3.3 ppm 1899.92333 1900.06887 2.5 ppm 35 ppm PASS 6.73 %
10 kΩ 10000.1 3.3 ppm 9999.68499 10000.451 3.2 ppm 35 ppm PASS 8.36 %
19 kΩ 18999.74 3.3 ppm 18998.9573 19000.4126 2.8 ppm 35 ppm PASS 7.56 %
100 kΩ 100001.6 3.3 ppm 99996.8799 100005.540 3.9 ppm 40 ppm PASS 9.01 %
190 kΩ 189993.47 5.3 ppm 189984.113 190001.326 3.9 ppm 40 ppm PASS 8.68 %
1.0 MΩ 1000002.3 5.3 ppm 999949.6 1000050.2 2.4 ppm 45 ppm PASS 4.84 %
1.9 MΩ 1899962 14.3 ppm 1899842.63 1900067.96 3.5 ppm 45 ppm PASS 5.95 %
10 MΩ 9999443.3 14.3 ppm 9996723.16 10002008.8 7.7 ppm 250 ppm PASS 2.93 %
19 MΩ 18999373 60.3 ppm 18993129.6 19004920.4 18.3 ppm 250 ppm PASS 5.91 %
100 MΩ 100006630 60.3 ppm 99842216.2 100174303 -16.2 ppm 1600 ppm PASS 0.98 %

Table 8: Different non-modified HP 3457A performance test, as returned after calibration adjustment, 4-wire resistance

Test on highest range with 1 GΩ standard was not performed.

Rest of the functions like AC voltage, DC and AC currents are also verified, and both calibration reports are available in PDF below.

Note out of 24-hour specification for negative 3.00000 mADC signal. Few attempts to calibrate it out (by rerunning DCV and DCI zero calibration and +3.00000 mA FS calibration) did not succed, as this measurement remained about -100 ppm from the reference. Wondering, if that is design issue of HP 3457A, or simply particular unit trait.

Credits for this testing and calibration goes to xDevs’s frequent contributor and metrology nut Todd. Thank you! :)

Conclusion

We learned few interesting things about 6½-digit HP 3457A meter, which had some unusual design choices. Today these meters are available on secondary market for good prices, and can be interesting tool for novice analog design engineers who’d like to peek deeper into analog precision design. Cost of maintenance on these meters is reachable even for hobby users to use a lab working tool.

xDevs.com would like to apologize to author of this article, TEKTRON for very long time on this publication. We sincerely promise to do better next time.

Discussion is very welcome thru comment section or at our own IRC chat server: irc.xdevs.com (standard port 6667, channel: #xDevs.com). Web-interface for access mirrored on this page.

Author: TEKTRON
Published: Dec. 31, 2016, 10:26 p.m.
Modified: Feb. 5, 2018, 1:21 a.m.

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