3 Replies Latest reply on Jan 19, 2017 3:23 AM by user_1269049

    Measure 40 high impedance analog signals with ADC_SAR


      Hi all,


      At a test stand we have 40 analog signals coming from voltage dividers.


      The problem is that the resistances are very high. The variable resistor, connected to 24VDC, is about 10MOhm, the resistor over that the voltage is measured is 1MOhm.


      I have verified by using PSoC Creator that I can configure a CY8C5868AXI-LP035 so that its ADC_SAR has 40 single ended inputs.


      The signals change very slowly (temperatures). The measurement may be slow, 100ms or less per channel are ok.


      However, I am not sure about the input resistance of the ADC_SAR. There is a very low current flowing through the voltage divider, about 2µA. If the input resistance of ADC_SAR is not extremely high, the input resistance will introduce an error and reduce the measured voltage significantly.


      1) What is the input resistance? Can it considered to be constant so that the error introduced by this resistance can be calculated?


      We could use OpAmps as impedance converter / voltage follower (I am German and am not sure if this is correct in English; in German it is "Impedanzwandler" and "Spannungsfolger"), but then we would need 40 of them (I know there are ICs with 8 OpAmps).


      2) The PSoC5 has 4 OpAmps on board.  Can they be used as impedance converters / voltage followers between the multiplexer part of the ADC_SAR and the ADC part?


      Thanks for input,