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Original question:

ADS1015: About the signal source resistance

ADS1015: Signal source resistance & Impedance

ponchoM
Prodigy 80 points
Part Number: ADS1015

Hi, it's the first time designing an ADC circuits, I understand the basics, but I want to go deeply so I want to do it right with this first circuit.

Target: Sense voltage of a Solar Panel(Vmin= 16V - Vmax= 21V) and a LiPo Battery  (Vmin= 8V - Vmax= 12.1V)  with the ADS1015 at 3,3V 

ADS1015 Configuration: Single-Shot with PGA -> 1 @± 4.096V 

Circuit:

The voltage divider calcs are for maxVin=25V to get a Vout range between 0 to 3.3V

I think, for reach the target this is a solution, but no the best solution.

First I didn't take care about input impedance, please let me know what I'm doing wrong and how can do it better?

  • 0
    TI__Guru* 94455 points

    panchoM,



    In general, I think your measurement method should work. However, you could adjust the resistances for a better resolution.

    Right now you have the measurement set up so that your max measurement is basically 3.3V out of ±4.096V. That means the measurement would be covered by 1650 codes (that's 2^11*3.3/4.096). You could redo the divider so that R10 and R12 are both at 17.5k instead. That would make the divider output 2.011V. Using the ±2.048V FSR, you'd be using 2011 codes to represent the measurement. This way you'd have slightly better resolution.

    For the input impedance, there will be some error. However, the input impedance for the ADC is the same for the ±2.048V and the ±4.096V FSR ranges. Additionally, the equivalent resistance including the ADC input impedance is about 17.45k. This represents a small error to the measurement.



    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 80 points

    All clear!

    Thanks J Wu!