A Precision Voltage Reference

Don Wilhelm W3FPR

Voltage Reference

I found myself with the confusion of the ham with too many meters. The quip about the man who had 2 watches and didn't know what time it was certainly was hitting close to home. In the past I had access to calibrated lab equipment and could believe what the meters said, but I left that world behind, and found myself wondering just how accurate my voltmeters really were.

As I was looking at the fine print in a DigiKey catalog, I found a group of voltage reference devices and ordered one. The LM4040 device comes in several voltage flavors and have a tolerance of 0.1% -- not good enough for a calibration lab, but certainly good enough for the majority of ham use. With this device, I could calibrate my meters to an accuracy of 1% (based on the rule of thumb that the standard must be 10 times better than the instrument being calibrated.)

The LM4040-2.5 which I chose requires a minimum operating current of 60 microamps and a maximum operating current of 15 milliamps. I sized the series resistor (620 ohms) for a current of approximately 10 ma. so I could draw a small current without upsetting the operating parameters. I added a string of 100 ohm resistors to give me voltage steps at 0.5 volts increments. I hand selected these resistors from a pile and managed to find 5 resistors that measured right on 100 ohms with an ohmmeter calibrated at 0.1%. A reasonable alternative would be to use 1% resistors which would still yield a unit having an accuracy of 0.1% for the 2.5 volt output and a bit over 1% at the other voltage levels -- still good enough for most ham work.

As you can see in the photo above, I used manhatten style construction, and I chose not to create a case for this project. It is not something that is used all the time, so I felt no need for a case -- besides, bare electronics is beautiful in my opinion. Layout is not critical since we are dealing with DC -- as long as there are no unintentional voltage drops from bad connections or cheap connectors, this circuit can be built most any way one would chose.

The Schematic is shown below.

Voltage Reference Schematic