Electronic Construction Project

Ultra Precise Voltage Reference

Surplus HP3458A 7-volt precision voltage reference board finds a home
as a bench reference.

Max Carter

The precison voltage reference board, used in the HP3458A 8½-place digital multimeter, incorporates the venerable LTZ1000 ovenized voltage reference chip. Some features of the LTZ1000 (from the data sheet) include:

  • 1.2µVpp Noise
  • 2µV√kHr Long Term Stability
  • Very Low Hysteresis
  • 0.05ppm/°C Drift
  • Temperature Stabilized
The output voltage for the reference board used in this project was specified by its supplier to 6 decimal places. The tag that accompanied it stated 7.096287 volts.

HP3458A Voltage Reference Board

HP3458A ultra-precise voltage reference

Reference Board Schematic

HP3458A voltage reference schematic
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Power Supply and Interconnect Board

The pin headers are gold-plated.
Note that the second pin from the right on the upper header has been removed.

voltage reference power supply

Power Supply and Interconnect Board Schematic

Note that all grounds are tied to a single point (star ground).

voltage reference power supply/interface board schematic

Negative 15 volt note: Sharp-eyed readers will notice that the negative 15 volts on pin 4 of J401 appears to go to a 2.67k resistor to ground (R419 on the reference board schematic above) and nowhere else. So what is the purpose of the -15 volts and this resistor? The answer is I don't know. I asked the supplier of the board, and he didn't know for sure either. He thought it might be to "neutralize offset change caused by ground current IR drop". It sounds reasonable. My reasoning for including it in the bench reference supply is simply that HP includes it in the HP3458A multimeter circuit. If HP thinks it's necessary, then so do I.

Reference Board Mated to Power Supply/Interconnect

voltage reference board mated to power supply/interconnect board

Completed Reference

completed voltage reference

The Voltage Reference Doing its Thing

Note the reading on the multimeter.

voltage reference doing its thing

Recommended Reading

For some insight into the issues involved in designing an ultra-precise voltage reference, please consult the data sheet for the LTZ1000 ultra precision voltage reference.

Schematics produced with DCCAD.