How to calculate the value of a current sense resistor for use with an ADC?

My goal is to measure the output current of a 4-output voltage regulator (LTM4644) whose maximum current per output is 4A. I use the LTC2975 Power System Manager.

The datasheet of this component provides the following information about the built-in ADC:

I therefore assumed that I have to use the ADC's full-scale voltage to calculate the value of the current sense resistor:

Rsns = 170mV / 4A = 42.5mOhms

Then I found the DC2428A circuit diagram published by Linear Technology and whose application is similar to mine. The circuit diagram makes mention of a 10mOhms sense resistor, which implies:

Vin_adc = 0.01 * 4 = 40mV.

Why did LT engineers choose this specific sense resistor's value? Does it means that the measure will be less precise with a 10mOhms sense resistor than a 40mOhms one?

I have a very little understanding of note 4 and I suppose that's why I don't know why calculations are not based on the full-scale voltage value...

Thank you in advance for your help!

• This doesn't answer your question but as another route from your CSR to an ADC input voltage for measurement, take a look at the ZXCT1022E5TA. May 6, 2017 at 19:04
• Is it a 10 bit ADC? In which case it would get it down to 8 bits for faster or less resource intensive calculations etc May 6, 2017 at 20:22
• Actually, the LTC2975 has an internal 16-bit ADC, but that does not really answer the question. In any case, thank you for this remark! May 6, 2017 at 20:52