Let me try to answer your questions:
1.What is the minimum current which we can be measure through 100uOhms shunt
In principle you can measure with a resolution of 1mA. The reachable accuracy is 0.5% gain error plus 0.5uV offset error. So at 0A is should be in the range of ~+/- 5mA (0.1uV = 1mA with 100uOhm shunt). One further aspect is that there is quite an amount of random noise added to each individual measurement result, therefore you should evaluate the measurements using averaging over multiple results (for accuracy evaluations on ISENSE I recommend something like 1000 values). E.g. with ADC settings DEC=512, IIR=1/32 and I=0A => gain256 the noise is ~1.07uVrms or ~10.7mArms. Assuming a typical spread individual results can vary by 3*rms => ~+/-30mA.
Note: To reach the full accuracy a calibration on system level is required, which eliminates errors introduced by e.g. soldering.
2.What is the effect of placing the PCB near to the shunt.
3.What happens if the shunt leads does not match PCB test points.
The proximity/distance is not immediately introducing an measurement error. (I used a shunt connected with 2 twisted 25cm long wires without issues "on my desk"). The errors are introduced by coupling noise and by unintentional currents flowing. Remember that 100uOhm is a pretty small resistance and 0.1uV (= 1mA) is a small signal. So I would state that a careful layout is most important and proximity is important in particular under EMI conditions.
4.Can we measure 1mA or less in no load condition with our hardware .
See above.
Please suggest us to do any modification on our hardware to measure minimum current in no load condition.
The 400mA and 80-90mA seems to big! There a multiple sources for errors, like schematics, layout, software.I suggest the following steps:
- check that one side of the shunt is referenced to AGND
- measurements at 3 different currents, e.g. 1A, 0A, -1A (averaged results N=1000)
- schematics and layout review
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