HP/Agilent/Keysight 3458A 8.5-digit DMM modifications and tweaks

Draft

Intro.

Initial specifications

Let’s first take a look on HP/Agilent/Keysight 3458A meter specification with high-stability DC reference – Option 002 and measured data for temperature coefficient.

Function Range
DC Voltage 100mV 1 V 10 V 100V 1000 V
Accuracy (1y), Rdg ppm + Range ppm 5+3 (0.0005%) 4+0.3 (0.0004%) 4+0.05 (0.0004%) 6+0.3 6+0.1
Temperature coefficient, ppm 1.2+1 1.2+0.1 0.5+0.01 2+0.4 2+0.04
Measured tempco, ppm 1.2+1 1.2+0.1 0.5+0.01 2+0.4 2+0.04
RMS noise at NPLC100, Range ppm 0.2 0.02 0.01 0.02 0.01
AC Voltage 10mV 100mV 1V 10 V 100V 1000 V
Best accuracy (1y), Rdg ppm + Range ppm 200+110 (0.02%) 70+20 (0.007%) 200+20 (0.02%) 400+20 (0.04%)
Temperature coefficient, ppm 30+200 25+1
Measured tempco, ppm 30+200 25+1 25+1 25+1 25+1 25+1
Bandwidth frequency (max accuracy) 1 Hz to 10MHz (40Hz to 1kHz)
Resistance 4W 10 Ω 100 Ω 1 KΩ 10 KΩ 100 KΩ 1 MΩ 10 MΩ 100 MΩ 1 GΩ
Accuracy (1y), Rdg ppm + Range ppm 15+5 15 (0.0012%) 10+0.5 (0.0010%) 15+2 50+10 500+10 (0.05%) 5000+10 (0.5%)
Source current 10 mA 1 mA 100 µA 50 µA 5 µA 500 nA
Temperature coefficient, ppm 3+1 3+1 3+0.1 3+1 20+20 100+20 1000+20
Measured tempco, ppm 3+1 3+1 3+1 3+1 3+1 3+1 20+20 100+20 1000+20
RMS noise at NPLC100, Range ppm 0.3 0.3 0.03 0.045 0.06 3.6 36
DC Current 100 nA 1 µA 10 µA 100 µA 1 mA 10 mA 100 mA 1 A
Accuracy (1y), Rdg ppm + Range ppm 30 + 400 20+40 20+10 20+8 20+5 20+5 35+5 110+10
Temperature coefficient, ppm 10+200 2+20 10+4 10+3 10+2 10+2 25+2 25+3
Measured tempco, ppm 100+200 2+20 10+4 10+3 10+2 10+2 25+2 25+3
Shunt resistance (burden voltage) 545.2kΩ(55mV) 45.2kΩ(45mV) 5.2kΩ(55mV) 720 Ω(75mV) 100 Ω(100mV) 10 Ω(100mV) 1 Ω(250mV) 0.1 Ω (<1.5V)
RMS noise at NPLC100, Range ppm 2.5 0.25 0.025 0.025 0.025 0.025 0.025 0.025
AC Current 100 µA 1 mA 10 mA 100 mA 1 A
Accuracy (1y), Rdg ppm + Range ppm 600+300 600+200 600+200 600+200 800+200
Temperature coefficient, ppm 20

Table 1: 3458A key specifications

This is helpful to determine narrow spots which can attempted for improvement.

Better stability/lower noise voltage reference

Noise of the reference voltage as an important source – one would need to improve there too, to have lower noise not only for a zero reading. Also many signal sources have at least comparable noise – so the only thing to do there would be to reduce the noise bandwidth by having 2 amplifiers + ADCs and do a kind of commutating auto zero – so not missing any of the signal. So this is an obvious possible factor of 2 in speed – but at a price.

Modification for current shunt resistors and current source

Start with easy part – DCI function. Why easy? Because it’s simple component swap.

I have broken A1 board, from which I removed original R213, R212, R211, R207, R209, R206 resistors and tested their TCRs to establish our baseline expectations.
Test results:

I don’t have much better replacement for 40K VHP101, so decided to leave that one alone for now, as we as 90 ohm (1mA range) and 4.53K shunts (10uA range), for but replaced rest with better resistors:

After installing A1 board back, no SELF-TEST errors detected, ACAL ALL passed clean too.
Running GPIB app now collecting same 100mA running thru both meters from HP 3245A (LTZ-powered).

3458B : -4.65 ppm/K measured, 25ppm/K spec.
3458X : -1.88 ppm/K measured, 25ppm/K spec

3458B : -10.6 ppm/K measured, 25ppm/K spec.
3458X : +61.6 ppm/K measured, 25ppm/K spec

3458B : +3.36 ppm/K measured, 10ppm/K spec.
3458X : +10.92 ppm/K measured, 10ppm/K spec

Retest 10mADC range only confirms bad result

3458B : +3.47 ppm/K measured, 10ppm/K spec.
3458X : +12.35 ppm/K measured, 10ppm/K spec

1mA range

3458B : +0.55 ppm/K measured, 10ppm/K spec.
3458X : +5.48 ppm/K measured, 10ppm/K spec

100µA range

3458B : -1.42 ppm/K measured, 10ppm/K spec.
3458X : -0.45 ppm/K measured, 10ppm/K spec

No problem with 100µA range at all.

100nA range

3458B : <100 ppm/K measured, 210ppm/K spec.
3458X : <100 ppm/K measured, 210ppm/K spec

Error/tempco of 100nA range mostly delivered from amplification stage, so this measurement have no meaning. It matches unmodified 3458, so replacement of 500K resistor was for nothing and not required.

DCV comparison, 100mV range

3458B : +1.72 ppm/K measured, 2.2ppm/K spec.
3458X : +0.63 ppm/K measured, 2.2ppm/K spec

DCV comparison, 1000mV range

3458B : -0.2 ppm/K measured, 1.3ppm/K spec.
3458X : -0.128 ppm/K measured, 1.3ppm/K spec

As we can see, DCV 100mV and 1V ranges are good and cause zero problems, and not cause of DCI excessive tempco. We can close A3 tempco effect theory for now.

Original A1’s resistors TCR tests:

Test results:

Summary table:

Resistor Original A1 New resistor Range Original TCI Test result X1
R213 0.1R +7.50 ppm/K +0.55 ppm/K 1 A -4.65 ppm/K -1.88 ppm/K
R212 1R +1.70 ppm/K -0.86…-1.47 ppm/K 100 mA -10.6 ppm/K +61.6 ppm/K
R211 9R -1.38 ppm/K +1.5…+0.94 ppm/K 10 mA +3.36 ppm/K +12.35 ppm/K
R210 90R Unchanged unchanged 1 mA +0.55 ppm/K +5.48 ppm/K
R209 634R +4.65 ppm/K +1.2…+1.25 ppm/K 100 uA -1.42 ppm/K -0.45 ppm/K
R208 4.53K Unchanged unchanged 10 uA DNT Not tested
R207 40K Unchanged unchanged 1 uA DNT Not tested
R206 500KR +1.86 ppm/K +0.78 ppm/K 100 nA <100 ppm/K <100 ppm/K

Replace terminal block with triaxial ports

http://www.ebay.com/itm/HP-Agilent-16059A-BNC-Coaxial-Breakout-Cable-Adapter-Keysight-16058A-/282237951084?hash=item41b6b15c6c:g:SQUAAOSwo4pYFYZa
http://www.ebay.com/itm/Lot-of-11-BNC-Connectors-Termination-Pass-thru-TEI-AMP-North-Hills-NH14776-/141873831989?hash=item210856e035:g:yK8AAOSwKtlWkni9
http://www.ebay.com/itm/N-TO-N-PLUG-LEAD-PTFE-SILVER-PLATED-CABLE-36-NEW-1PC-/150611016375?hash=item23111dbab7:m:mh75AjrgeJZclwE8AWsmdDA
http://www.ebay.com/itm/HARBOUR-INDUSTRIES-M17-128-RG400-N-male-N-male-Coaxial-Cable-31-inch/262742272668?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D41402%26meid%3D4665808a489a4baf87caadc141d7bcf6%26pid%3D100005%26rk%3D1%26rkt%3D6%26sd%3D182252061200

Reversible terminal block

Route active OHM guarding to guard

Low-noise power regulation circuitry

-10V and other hidden calibrations

Except documented Zero, 10V and 10 KΩ calibrations meter does accept additional less known calibration values:

Function Accepted range Function
0.1 VDC -0.9 … 0.9 0.1V calibration
-10 VDC -12 … -1 -10V calibration
10 VDC 1 … 12 10V calibration (standard)
1000 Ω 1000 – 12000 ohm 10 KΩ (standard)

Isolated battery power supply

Author: Illya Tsemenko
Published: Dec. 28, 2016, 12:42 p.m.
Modified: July 22, 2017, 1:30 p.m.

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