Just how good are the voltage measurements you make using USB based instruments such as the ADALM1000 or Analog Discovery? How would you find out? You could always check the readings against a bench DMM but a good DMM can cost hundreds of dollars. Instrument manufacturers also sell bench voltage calibration standards but again these can cost hundreds of dollars or more.
Michel Gallant, Department of Engineering, St. Francis Xavier University has written a quick guide on a simple voltage reference that can be used to calibrate and check the accuracy of voltmeters and oscilloscopes. It is built around the AD584 precision band-gap voltage reference ( one of the IC's included in the ADALP2000 analog parts kit ). This design uses a switch to select between 10 V and 2.5 V outputs and will require a 12 V or greater power source on the 10 V range. I might suggest instead switching between 5 V and 2.5 V modes which will allow operation from a fresh 9 V battery and is a better match to the 0 to 5 V input range of the ALM1000. I would also suggest soldering the circuit on to a small piece of circuit board rather than using a solder-less breadboard as shown.
AD584 DC voltage calibrator
In the 2.5 V configuration the AD584 can be powered directly from the +5 V supply of the ALM1000. In the version shown the two input channels of the ALM1000 can be switched between ground and the 2.5 V reference to calibrate both offset and gain.
Single unit cost for the AD584JNZ is $8.01 at Mouser. According to the AD584 datasheet for the J grade part the output voltage worst case should be within +/- 7.5 mV in the 2.5 V mode. The AD584 from my Analog Parts Kit was 2.4995 V or -0.5 mV off at room temperature.
Another 2.5 V precision band-gap reference from Analog Devices available in a DIP package is the REF43GPZ with an initial accuracy of 0.10% or +/- 2.5 mV. Single unit cost is $9.72 at Mouser. Measurements of the 7 units I had on hand showed a spread in the output accuracy from -1.7 mV to +1.1 mV with an average of -0.357 mV.
If you are not interested in building your own here are a couple of ready made ones to consider.
For a pre-made voltage calibration reference, Syscomp offers the 9V battery powered REF-101, for $29.00. It provides 2.5V and 250mV volt outputs with 0.5 % tolerance or +/- 12.5 mV at 2.5V.
Another similar voltage reference is offered by Malone Electronics at VoltageStandard.com. Model VREF5-01, a 0.01% accurate, 9V battery powered DC voltage reference. This board can be ordered with the following output voltages: 2.048V, 2.500V, 3.000V, 4.096V, 4.500V, and 5.000V and costs $25.50.
Any of these possibilities, while not supper accurate or traceable to NIST, are certainly good enough to calibrate an ALM1000 or check an Analog Discovery. Procedures for calibrating an ALM1000 for use with the ALICE software are included in the various user's guides.
As always I welcome comments and suggestions from the user community out there.