A 3V voltage reference

AD780 3V reference

I have had my Fluke 77 multimeter for something like 20 years now. I wondered how accurate it is after all that time (having never been sent for calibration).

A quick search, and I downloaded the manual, which includes the calibration procedure. It turns out that calibration is very simple, just one preset to adjust. but you need an accurate 3V voltage reference to do that.

So I took a look at Farnell components to see what voltage ref ICs they stock. I wanted something that could give 3V as accurately as possible. With a bit of research, I found the AD780, a single chip device that fits the bill. In the form with the BN suffix, the accuracy is given as 2.999 to 3.001V DC - good enough for my purposes. The data sheet is here.

The circuit is as simple as can be. A 9V battery makes a very clean power source, and two diodes protect the chip from accidental reversing of the supplies. The three capacitors are not strictly needed, but they reduce the output noise slightly.

Below, the finished project. It took maybe an hour to make. Because it is so simple, and only used occasionally, I did not bother with a box. I just mounted the battery and output terminals right on the veroboard. simple remove the battery when it is not in use.

3V reference, topside 3V reference, underside

This is the device in use. My Fluke said 2.996V before adjustment, an error of about 0.1%. Not bad after 20 years! (I also used it to test both channels of my (even older) Tek scope, which was also spot on.)

Calibrating the Fluke 77 with the 3V reference

The AD780BN costs about 13€ (other versions a little less, but this is most accurate), so this project probably costs around 15€, at the time of writing (June 2014). That is much less than any commercially available calibration, and good enough for most purposes. A nice easy little project.