INA828 for sensing current down to tens of nA

Question

I need to measure the voltage drop across a test resistor in order to sense the current flowing through it. This test resistor has a value of 1 MOhm and it's connected between the positive voltage of a battery (or PSU) and the positive input bias pin of a DUT. The ground pin of the battery (GND1) is connected directly to the ground pin of the DUT (GND1). The current flowing in the resistor is around 50nA to 100nA, thus the voltage across the resistor is about 50mV to 100mV.

I cannot connect directly the test resistor to an ADC to measure the voltage drop across the resistor because the ADC has a low input resistance. I am therefore thinking of using the INA828 between this test resistor and an ADC (for instance the ADS1263).

I am hence thinking to realize something like the schematic below. The ground of the circuit containing DUT (GND1), battery and test resistor will not be common with the ground of the INA828 (GND2) and subsequent ADC. I have some questions:

1. Do you think the schematic is correct or am I making any mistake?
2. Is the INA828 a good choice for measuring these voltages or you have a better suggestion? My guess is that it should work well since the input bias current is 0.15nA thus negligible compared to the current flowing in the test resistor.
3. Do you agree with using a dual supply +/- 5V and to connect the REF pin to ground?
4. Will I need to use an ADC compatible with dual supply? My guess is that, since the current between battery and DUT will flow only in one direction, the output of the INA828 will be always positive. Or am I wrong?

Thanks in advance

Answer 1

From the IN828 datasheet we have this (in your case G=1 so the black diamond):

You need to stay within the 6-sided diamond figure.

If the highest possible output voltage is +/-100mV then you need to keep GND2 and (Vd-I*R/2) within about +/-3.5V of each other. If it is +/-3V then more like within +/- 2.5V of each other. It's up to you to maintain that relationship, and if you don't the amplifier will not work properly (it will saturate in one way or another).

Floating the entire front and circuit and using optical or other means to transmit the signal, as Andy suggests, is a possibility (maybe the only practical one, depending on your situation) but it comes with many possible complications. You still have to maintain the common-mode voltage as shown in the figure (so maybe you need a direct connection to Vd or a buffer amplifier to maintain the CM input voltage as with bio signals) and if you use something more pleasant than batteries such as a DC-DC converter it could disrupt the measurement due to AC noise and capacitive coupling. The means of transmitting the signal could also disrupt the measurement. A nA signal, while not inherently difficult to measure, could be challenging to galvanically isolate.

One off-the-shelf possibility might be a battery-powered DMM with serial interface, but there will still be some coupling through the interface which could disrupt the measurement. COM for the meter would best be connected to Vd, most likely, depending on your setup.

Answer 2

The ground of the circuit containing DUT (GND1), battery and test resistor will not be common with the ground of the INA828 (GND2) and subsequent ADC.

The whole scheme depends on knowing how the two disparate ground systems interact (if you are to rely on a measurement that involves a galvanic connection to the measurement amplifier). For instance, if the two grounds wobble with respect to each other at 10 volts p-p at some frequency then it's extremely likely that your measurement system will fail due to excessive common mode interference exceeding the InAmp supply rails.

Of course, your two disparate grounds could have 1,000 volts between them and this is a real problem. So, unless you can resolve this, all four of your questions are invalidated.

If this cannot be resolved then I suggest you use an isolated measurement method. This might mean battery powering a "front-end" circuit that locally monitors that current through the resistor and transmits that information optically (or via radio) to the main measurement circuit powered remotely. This is a real option.