Problem with designing a resistance measurement device using current sense amplifier

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:

Problem with designing a resistance measurement device using current sense amplifier

And here is the results:

Problem with designing a resistance measurement device using current sense amplifier

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.

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Category: voltage Time: 2016-07-28 Views: 1

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