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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?
Hello, The SAR ADC input resistance is 2.2k and capacitor is 10pF. The equivalent circuit is same as Figure 17 in page no 17(it is applicable for psoc5lp) which is shown in below link: http://www.cypress.com/file/141176/download With SAR_hardware mux, it is not possible to connect the opamp after sar hardware mux. But you can use firmware AMUX bus and connect output of AMUX to opamp to route it to SAR ADC. I attached the project here as reference. Can you please check if this configuration is fine? Thanks Ramesh B