This question is regarding a experimental project that I previously discussed HERE. So I have build a test PCB with MAX9611 and I have a put a sense resistor which its value is exactly *2.5 Ohms* (I checked with a trusty bench meter). So the idea is if I successfully can calculate the same *2.5 Ohms* with this IC, then I want to convert it to a precision resistance meter.

But things have not gone smooth so far. I made some measurements which yielded the following results. For the measurement I used a good quality power supply in constant current mode and I forced in the circuit from *20mA* to *50mA* in *10mA* steps:

Here is my equivalent measurement circuit:

And here is the results:

As you can see I have calculated the resistor value roughly *3.7 Ohm*. I do not know where does this difference comes from? Here is what I guess:

1- It is simply the extra resistance of 2 pieces of wire (roughly *5 cm*) that I used to connect power supply with headers on PCB.

2- It is because I have not correctly configured setup the MAX9611. This I need to explain more:

**Voltage Measurement**I use the Common-Mode voltage measurement of MAX9611 which according to the datasheet is the average of the voltage at RS+ and RS- pins. The range is 0 to 57.3 and the ADC is 12 bit. So I multiply the raw ADC value by $${\frac{57.3}{4096}} = 0.0139892578125$$ to get the real voltage. The voltage that I measure (as you can see in the excel sheet) is always a few mV less than what the power supply display shows me.**Current Measurement**For current I use the CSA with 1x gain, which according to the datasheet with this gain setting, the full scale voltage will be*440mV*. In this range the LSB step size is*107.50uV*so to get the current in*mA*I just multiply the raw ADC value (12 bit) by*1.075*but still I am not really sure if I am doing the right conversion to read current from CSA.

So the question is where does that difference in calculating *Rsense* comes from.

Please let me know if you need more information.