Precision Voltage Reference
Over the years most of us have purchased a few multimeters, and perhaps some of the older ones are still hanging around like faithful friends. You might prefer one ahead of the others, perhaps because it is easier to use and you think that it is probably more accurate, but you wonder just how accurate it is…
Determining the accuracy of a DMM is not easy. Often manufacturers will totally ignore accuracy in their specifications, or if it is a high quality model, they might say something like ±0.02% ±3 digits ±4mV/°C — which does not help either.
This gadget will output a precise 2.500V with an accuracy of ±1 mV. It is great for checking any meter, it does not cost much, and it must be one of the simplest projects around.
The Circuit
The reference voltage is generated by a MAX6325 chip from Maxim. This uses buried-zener technology for low noise and an internal amplifier that enables you to load the output without affecting accuracy. Factory trimming ensures an accurate output.
This series of chips come in a variety of voltage outputs but the MAX6325 is the best as its 2.5V output neatly suits most DMMs which are 3½ digit and have a full scale of 2.999V. The prototype was tested on a brand new (and expensive) 4½ digit DMM which produced a reading of exactly 2.5000V. A later test with an even more expensive meter (a 5½ digit Rigol DM3058) produced exactly the same result. It is possible for both DMMs and the reference to have the same error in the same direction, but that is extremely unlikely.
Measurements made three years later showed that its output had drifted by just 0.3mV. So, over the long term my MAX6325 seems to be accurate to about ±0.01% which is an excellent result.
The data sheet is available from here.
You can attach capacitors and a trimming resistor to improve noise performance and accuracy but these are not really needed, the basic chip does the job well enough.
This is such a simple circuit that it really does not warrant a circuit diagram.
Parts List
1 x Maxim MAX6325CPA voltage reference IC
1 x 1N4004 diode (or any general purpose diode)
1 x 8 pin IC socket Box, output terminals and other hardware.
The MAX6325 is available from Futurlec and direct from Maxim. As at September 2012 Futurlec charge more for the chip (about US$21) but they do not force you to pay for an expensive courier, so they will work out cheaper in the long run.
Construction
I built the circuit by soldering the wires to an 8 pin IC socket and housing it in a small plastic box. It is powered from a bench power supply, a more “sophisticated” design might use a 9V battery and an off/on switch, but then you would need some method of testing the battery.
