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
HP3458A Voltage Reference Board
Reference Board 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.
Power Supply and Interconnect Board Schematic
Note that all grounds are tied to a single point (star ground).
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
Completed Reference
The Voltage Reference Doing its Thing
Note the reading on the multimeter.
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.
