# Mass Production Ultra-Low Current Calibration Methods

I'm attempting to design calibration for mass production on a newly designed product. The product is designed to use sub 1mA, so any digital calibration is unlikely.

An example of this would be in the following scenario (note: this is not the actual design):

simulate this circuit – Schematic created using CircuitLab

What methods are available to calibrate each of the input resistors ranges on this circuit that are cost and power sensitive?

My own research/input: The only thing I can see as a feasible option is trimmer potentiometers and perhaps using automation to do the calibration. But this seems like a time-consuming process and the fact that the potentiometers can move over time and have significant temperature drift. I have seen some people put adhesive on the trimmer but then in future, the device can be calibrated which is required.

I don't know if there are any methods for digital potentiometer setting which can be powered off and remember its position?

• "The product is designed to use sub 1mA, so any digital calibration is unlikely. " what does that even mean? Where does the digital part come from, it isn't in any other part of the question. – PlasmaHH Oct 24 '18 at 14:29
• You can use things such as Digital-Potentiometers which are commonly used in mass production. Such that a test fixture will measure offsets ect and then set the Digi-Pots to calibrate the device. They are volatile such that they forget their state; so a Microprocessor remembers their position. This product has no digital circuity on-board so this is impossible. To low current on the product means an MCU cannot be added purely for calibration. – Fat Diode Oct 24 '18 at 14:33
• Not sure where you get that idea from, there are µC that deep sleep with only µA needs and digital pots are often in the order of nA – PlasmaHH Oct 24 '18 at 14:35
• There are non-volatile digital pots. – Steve G Oct 24 '18 at 14:38
• In the application you show (setting gain on an op-amp), you would use 1% or 0.5% or lase trimmed packs for the resistors if the gain ratio was important. Fitting trimpots would anathema to a high production item, and fitting digital trimpots in series with each resistor asinine. You could introduce a digital Epot such as the MAX5400 (maximintegrated.com/en/app-notes/index.mvp/id/864) but this implies additional cost and at minimum a way to probe and program the device on the production line. A voltage controlled gain op-amp configuration would be much more sensible. – Jack Creasey Oct 24 '18 at 15:16