Does the output of the sensor gets affected with C9 value. Please comment.
That method of current excitation is quite stable.
It depends mostly on the accuracy/stability of the shunt reference ZD1.
The only thing that could improve it slightly is to use a P-MOSFET in place of the PNP bipolar transistor to eliminate the small error due to the transistor base current.
That would require a logic-level type MOSFET [Vgs(th)≦2V max] to operate from a 3.3V supply.
5500 ohm multiply 0.5 mA = 2.75 V, not feasible with the shown circuit, no current regulation achieved. Even minimal value of 2600 ohm is marginal.
Already answered in post#6.ZD1 is LM385Z which is a adjustable shunt reference. Is this IC is sufficient or do you suggest?
Where do you see a difficulty?How can I connect the sensor with ADC_REF? Please guide me.
I assume you mean "ADC".Also watch out for the DAC reference being lower than the voltage you are measuring. The reference is only 377.5mV but the output of the sensor could be several times bigger.
You already know that you don't need absolute accuracy. I agree with this.whether it is 0.50 mA or 0.60mA, it is not a problem.
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