ADC error for low-voltage measurements

Question

The application is high-side current sensing for a battery-operated device. The current drawn from the battery goes through a shunt resistor, the voltage across the resistor is amplified by an INA, which output goes into an ADC (ADS1262). An MCU retrieves the conversion results and forwards them to a computer for analysis.

I tested the circuit by generating a current [0 A - 1.5 A] through the shunt resistor (0.01 A steps from 0 A to 0.1 A, then 0.1 A steps). According to the resistor's value and the amplification, this gives a voltage range [0 V - 4.3 V] at the ADC's input, which has a 5 V reference. I calibrated the ADC before the test (offset and gain respectively at min and max input current). For each current step, I recorded 100 conversion results from the ADC synchronized with as many samples from a tabletop DMM (Agilent 3606) for the ground truth. Then I computed the average error for each step. As shown below, the absolute error between the generated current and the measured one decreases linearly with increasing input. At 1.5 A input, the error is smallest (about 50 μA).

Why is the error larger for smaller inputs? I expect some ADC nonlinearity, but in the ppm range, not several percent. I checked that the ADC's input is indeed linear, so this behavior doesn't come from the amplification stage. Is it a general property of ADCs or is it specific to my circuit?

(This was part of a prior question, where I gave more details on the project. I don't think the details are relevant here, though. This other question shows an error curve very similar to mine, but they used a MOSFET for the current sensing and its characteristic was nonlinear to begin with. So it doesn't help me much.)

Answer 1

You will be very lucky if you can use the bottom 50 mV of an ADC as a calibration point. I usually avoid the bottom and top 50 mV of the quoted range because of gain slope and zero offset problems.

It does indeed look like you are calibrating with 0 volts fed into the input but you will probably get the same "zero" digital output value with maybe +20 mV fed into the input.

I would calibrate at just below maximum and just above minimum to ensure lowest error for the major part of the range.

Answer 2

For zero load, can the INA generate 0 V out ? With offsets, it might need to generate a negative voltage which it can't unless you have a negative supply.