After retiring last year I purchased an HP34401A off eBay for repair, then repair an Agilent 34401A. Earlier had repaired a couple Tek 2465 scopes, so beginnings of a new to me lab at home
While revisiting and old unique voltage divider circuit (used a flip-flop) and measuring, the result the two DVM differed by a small amount, then realized I needed something to calibrate these 34401A and the two old Flukes (87 and 77) we had laying around. Instead of sending everything out for calibration, decided to get a new Keysight 34465A DVM and use this to help calibrate/check the other instruments.
Anyway, long story short a few custom PCBs were designed and fabricated. Features are listed below.
A) Resistor "On-DVM" PCB.
1) 4 resistor positions selectable by jumpers for 4 wire measurements.
2) Each resistor can be a Leaded type, 0603 type, 0805 type or 1206 type, or various combinations (for adjusting value).
3) 3 Thermistors types to read PCB temperature, leaded, 0603 or 0805. Separate pins which can be read by DVM or another meter.
4) Directly connects to DVM by Banana plugs, no leads required.
5) Rotated CCW 90 degrees becomes and good zero Volt & Ohm (2 or 4 wire) reference.
B) "On-DVM" Calibrator PCB,
1) Functions selectable by jumpers.
2) Single resistor position, can be Leaded type, 0603, 0805 or 1206, or various combinations.
3) DC Voltage reference, can be various voltages (2.5, 5, 10 volts with different pre-regulator) and has somewhat standard "footprint" of SOIC 8 pin. Pin 2 Vcc in, Pin 4 And, Pin 6 Vout, Pin 5 Trim. Layout supports various trim techniques for various references, and includes trim pot.
4) AC Voltage reference, creates accurate 50% duty cycle square waveform of Vref, thus rms value is Vref/2. Supports various crystals & frequencies , HC49 or leaded type. Jumper enables AC waveform and another jumper selects frequency from 14 stage counter (CD4060). Has discrete output buffer for lower impedance.
5) 3 Thermistors types to read PCB temperature, leaded, 0603 or 0805. Separate pins which can be read by DVM or another meter.
6) Standard Power 12V connector and also 2 pin battery connector, both reverse protected.
I built the 1st of the Resistor PCBs with 5ppm resistors, 100 ohm 0.05% 1206, 1K Vishay 0.1%, 10K 0.1% and 100K 0.1%. Used a hermetic 10K 1% leaded thermistor. 2nd PCB used all 1206 0.1% 15ppm resistors of 1K, 10K, 100K & 1M and a 0603 10K 1% thermistor.
The Calibrator used a 1206 10K 0.1% 10ppm resistor, a 10K 0603 1% thermistor, a 4.096MHz 20ppm Crystal and a Analog Devices ADR4550 5 volt reference.
All these used gold plated Banana plugs for direct DVM mounting, very convenient
With a nominal room temperature of ~77F they measured about ~82F or ~28C, so the inputs to the DVMs seems to be about ~5F warmer than ambient.. The Keysight 34465 has a nice feature of directly converting 5K and 10K thermistors to temperature, also the trend chart is really nice for observing readings of voltage, resistance and temperature.
All readings are very stable on the 34401As or 34465A after a time to settle in to DVM input temperature. Of course this isn't NIST certified, nor even ultra-precision, but adequate for my needs
Here's some images of the resistor PCBs.
Best,