Keithley 2182 nV voltmeter repair worklog

Contents

Intro

Following good local tradition, we are back to Keithley instruments this time. Some might already read Keithley 182 nanovoltmeter review. In this article modern replacement for that voltmeter will be studied and repaired. Welcome Model 2182 (and very similar newer 2182A). It was developed back in year 2000 to compete with 7½-digit HP’s 34420A µOhm/nV-meter. Tektronix (Keithley merged with Tek from 2010) still produces and sell 2182A starting at USD $3410. This instrument is not a common tool on most labs, due to it’s specialty and use specifics.

Function Ranges Accuracy
DC Voltage CH1 ±10mV, ±100mV, ±1V, ±10V, ±100V Best 1 Year Specification: ±25 ppm + 2
DC Voltage CH2 ±100mV, ±1V, ±10V Best 1 Year Specification: ±25 ppm + 2
DC Noise spec 6 nV & 25s NPLC5, FILT, 10mV, 2.5µV & 1s NPLC1, FILT, 10V
Temperature -200°C to +1820°C, Type J,K,N,T,E,R,S,B Accuracy: ±0.2 °C, 0.001°C resolution
ADC linearity ±0.8ppm of reading + 0.5ppm of range
Input impedance >10 GΩ for 10mV-10V ranges

Table 1: Model 2182A key specifications

As far as we know, there are no additional options available to upgrade Keithley 2182/2182A.

Disclaimer

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As usual, all photos are clickable for high-resolution version.

Manual references

Unfortunately, there is no schematics available in public service manuals for Model 2182.

Initial inspection and basic disassembly


Image 1: Keithley Model 2182 nV-meter front

2182 is same footprint and size as other Keithley’s meters, such as 2001, 2002, 2400 series SMU, etc. Input connector is specialized type, LEMO V series, same as one used in HPAK 34420A. It’s compatible with Keysight’s 34102A Low Thermal Input Cable.

For calibration purposes original Keithley 2188 shorting plug or compatible Keysight 34103A shorting plug can be used.

If you need to make custom cables, you can get original Keithley “2182-KIT connector” or compatible Keysight 34104A input connector for USD $121. Based on EEVBlog discussion LEMO’s part number for connector is FVN.1S.304.CLYC37Z, referncing LEMO catalog. It’s not cheap, Digikey retailer sells these connectors for USD $109.2

Teardown

After few screws removal, cover is easily removed, just like most of other half-rack Keithley devices.

Let’s take a look inside:

Overall construction is similar to 6½-digit DMM Model 2000, but with very different mainboard front end. One big mainboard with all components located on middle of chassis, and mains transformer mounted on front side of the instruments. Bit surprised to see just regular wiring from input LEMO connector to PCB. Expected triax or similar low-noise cable instead.

Animated photograph of both 2000 and 2182 boards illustrates very small difference in digital domain of both meters. No surprises, engineers usually reuse existing design blocks. This approach works fine for digital due to firmware flexibility, but analog sections are completely different. One notable difference is presence of Analog Devices AD1851, which is 16-bit ±3V PCM Audio DAC. This DAC is used for ±1.2V analog output, which takes measured signal from digital domain and provides analog output with programmable gain from 10-9 to 106. This feature is similar to analog output of Keithley 182 and can be useful for datalogging with external devices, if ±0.1% accuracy is sufficient.

Onboard power supply is separated into independent digital and analog sections, coming from separate transformer sections. Digital supply, powering main processor, front panel and communications interface is routed thru J1015 connector. As visible on photos, set of capacitors around PSU inputs already went bad (no wonder, 16 years already for this instrument) and urge for replacement and PCB cleanup.

Analog power provided from J1016 connector, rectified by CR310 bridge and regulated by U302 (+15VDC, 78M15), U303 (-15VDC, 79M15) and U301 (+5VDC, 7805) into main analog supplies. Few more regulators such as U304 (LM295T), U305 (LM317M) and references (U306,U310, TL431CD) provide local auxiliary voltages near by.

Initial checks are always starting from power supply, and service manual lists nice step-by-step procedure:

Step Item/component Required condition Remarks
1 Line fuse Check continuity Remove to check
2 Line voltage 120V/240V as required Check power module position
3 Line power Plugged into live receptacle, power on Check for correct power-up sequence
4 U304, pin 2 +5V ±5% +5VD, referenced to TP301
5 U305, pin 2 +37V ±5% +37V, referenced to TP301
6 CR313 anode -6V ±20% -6VD, referenced to TP301
7 Q304, pin 2 -5V, ±5% -5VD, referenced to TP301
8 U302, pin 3 +15V ±5% +15V, referenced to TP303
9 U303, pin 3 -15V ±5% -15V, referenced to TP303
10 U301, pin 3 +5V ±5% +5V, referenced to TP303
11 CR305 cathode +29V ±20% +29V, referenced to TP303
12 CR309 anode -29V ±20% -29V, referenced to TP303
13 CR310 V+, pin 1 +18V ±20% +18V, referenced to TP303
14 CR310 V-, pin 2 -18V ±20% -18V, referenced to TP303
15 CR306 anode -7V ±20% -7V, reference to TP303
16 CR308 cathode +7V ±5% +7V, referenced to TP303

Table 2: Model 2182 power supply check points

Digital construction is shown on nice block diagram from service manual:

Socketed ROM ICs U102 and U103 contain firmware code, which we downloaded and provided in firmware section below. U103 stores the D0-D7 bits of each data word, and U103 stores the D8-D15 bits. Calibration constants and settings are stored in I2C EEPROM U112 (24LC16B). Everything else is fairly usual, RS232 with level converter and GPIB interface based on National Instruments NAT9914BPL chipset. There is a speaker on board, so instrument can emit sounds when error or alarms occur.

Analog circuitry and front-end

Service manual again helpful with structural diagram of all analog sections and blocks.

Good old pre-selected 0.5ppm/°C Linear LM399 ovenized buried zener voltage reference is used (U753) similar to many other 6½-digit instruments on the market. R751 custom TF-245 resistor network located just near. This resistor network was known point of failure in other Keithley’s instruments, such as Model 2000 and Model 2400 SMU, so it will be among first to check during repair.

This 2182, manufactured in year 2000, already using custom logic array for integrating ADC control (U510), marked 2000-802A02 ORBIT. This chip is used on many Keithley instruments and is equivalent of ALTERA MAX EPM7160 CPLD logic (2000-802A01). Interface to digital board is isolated thru optocouplers U114 (HCPL-2531), U115,U116 (HCPL-2601).

There are only passive components located on bottom side of the board. PCB is four layer, with extensive use of guard rings in sensitive areas. Routing is a bit spaghetti-like, so it’s not that easy to trace signal path. I’ve learn this hard way during Model 2001 repairs.

Signals from input LEMO connector are routed to PCB front-end via four wires and protected with GDT tubes and varistors, to reduce chance of damage from over-voltage conditions. Per service manual, transistors Q666 and Q667 on front-end provide protection for CH1 HI, while Q661 and Q662 provide protection for CH2 HI signal.

Pair of metal cans Q619 and Q652 are paired matched NPN BJTs, LM394. All low-noise analog magic and critical components of front-end is covered by big plastic covers, mounted on both sides of PCB. This is aided to avoid any stray airflow around circuits, as even small thermal gradients can generate thermal EMF, which can easily cause signal corruption on these nanovolt and microvolt levels! Great attention to detail, like shown here by Keithley is a must to guarantee circuit’s performance. These blocks also have separation on PCB by cutouts to further reduce leakage path and mechanical stresses.

Front-end low-noise preamplifier is located under bigger box covers, so let’s take a look on it first.

This low-noise preamp is made up of Q624, U614, and associated components. Q624 and U614 form a composite op amp with switchable gain depending on the selected measurement function. Q624 forms the differential amplifier input, while U614 provides the required high open-loop gain. Circuit gain is controlled through feedback elements and signal paths by switches Q611-Q614, Q622, Q623, and U645. Selected Q624 have Keithley’s own part number 2010-600A, which hints for it’s use in 7½-digit 2010 DMM as well.

U601 is a unity-gain amplifier that provides signal buffering for the divider common signal path. A 100:1 resistive divider is switched in to attenuate the signal for the 100V range.

There is 10 KΩ precision network from BI, R700, as well as metal film SMT resistors R670, R673, R687, R688, R681, R699, R684, and pair of 100 Ω Vishay foil resistors R641 and R717 with tolerance 0.05%.

Second smaller box protects ADC’s gain amplifier from airflow.

There are some passives and big polycarbonate film 2.2µF capacitor.

Our FR4 PCB have Keithley’s model number 2182-102-01E, meaning Revision E.

Few more photos of bottom side:

Repair workflow

Step Item/component Required condition Remarks
1 Function DCV1 Use DCV1 for measurements
2 Input connections CH1 HI and LO Connect voltage source to CH1
3 Measurement range 100V Select 100V range
4 Input voltage 100V Apply 100V to CH1 input
5 U601 pin 6 +1V Buffer output
6 TP652 +10V Multiplexer output to A/D
7 Measurement range 10V Select 10V range
8 Input voltage +10V Apply 10V to CH1 input
9 TP651 +10V Preamp output
10 TP652 +10V Multiplexer output to A/D
11 Measurement range 1V Select 1V range
12 Input voltage +1V Apply 1V to CH1 input
13 TP651 +1V Preamp output
14 TP652 +10V Multiplexer output to A/D
15 Measurement range 100mV Select 100mV range
16 Input voltage +100mV Apply 100mV to CH1 input
17 TP651 +100mV Preamp output
18 TP652 +10V Multiplexer output to A/D
19 Measurement range 10mV Select 10mV range
20 Input voltage +10mV Apply 10mV to CH1 input
21 TP651 +1V Preamp output
22 TP652 +10V Multiplexer output to A/D

Table 3: Analog circuit checks

Diagnostics

Firmware

Firmware ROM dumps from two ROMs, read by general purpose ROM programmer.

Firmware version Combined binary ODD ROM EVEN ROM Motorola SREC Release notes
Model 2182 A07, V.54.1, Mar 8 12:18:37 EST 2000 2182-803-A07 CHK:00E69901 2182-804-A07 CHK:009A88AB
Model 2182A C02, V.103.1, Aug 10 13:58:16 EDT 2007 8Mb 2182-C02 2182 C02 PDF

Many other Keithley Instruments firmware dumps are available on xDevs.com documentation site

Other possible issues

Tweaks and tricks

Calibration

Performance verification

DCV Noise evaluation at shorted inputs

Restoration summary

This article would be impossible without work of Todd, who actually own units discussed and provided all raw content and testing to work with. It’s a joy for any passionate engineer to see these instruments working, despite all those years.

If you do have any questions, jump in comments! All the feedback is appreciated.

Author: Todd Micaleff
Published: Jan. 28, 2016, 9:41 a.m.
Modified: July 6, 2016, 10:28 a.m.

References

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