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Input divider for ADC128S022

Vladimir Naumenkov
Expert 2970 points
Other Parts Discussed in Thread: LMV831, LMV771

Dear Sir

I need to protect ADC input. ADC supply tied to 230 V 50 Hz supply. Short circuit may be in the Rload, then protection need for one period of 230 V supply.

Please, see attach. R3 and R5 is voltage divider, which decreases signal at the ADC input, then R5 must be much greater than R3. Can I install R4 and R5 30 k or more? As I understand, to install big value for R4 and R5 undesirable? Analog input leakage current is 1 microA max, then current via R4, R5 must be at least 10 microA. If 5V/(30+30)k=83 microA, then I can set R4 = R5 = 51k?

TIA

Sinserely

Vladimir Naumenkov

www.agat.by

ADC input protection 2.docx

  • 0
    TI__Genius 16890 points
    Hello Vladimir,

    To get rid of the problem of the input DC Leakage current causing a voltage through your resistor, you can put a buffer made from an op amp just before your 100 ohm resistor. The buffer will have a low I bias, so that there is little voltage across the resistor from it and it will also have a low output impedance which is good for the ADC. You will want to keep the 100 ohm resistor between the buffer and the ADC so that the cap does not load the output of the op amp. The LMV771 or LMV831 may work good for you.

    Mike
  • 0 in reply to Michael Stout
    Expert 2970 points

    Dear Michael, thank you

    If I make circuit without op ampl buffer, then voltage acrsss R5 resistor (caused by the current from the ADC input) will be about 30k x 1 microA = 30 mV.  I need to measure only AC component.  In principle, I would consider this a permanent component (30 mV)  in the time, when an alternating current by the sense resistor R1 is not flow. Alternating current flows through the sense R1 resistor is not always - there are times when it does not flow.  All the more so, the divider R4, R5 too will not provide precisely 2,5 volts in an average point (since I'm going to take 5% resitors).  Measurement accuracy of 10-15% of AC component  is enough for me.  If I do so, I can give up the op amplifier ? Or there are other issues that still require an op ampl?

    TIA

    Sincerely

    Vladimir Naumenkov

    www.agat.by

  • 0 in reply to Vladimir Naumenkov
    TI__Genius 16890 points
    Hi Vladimir,

    The ADC also has an input impedance which could cause an error in your measurement. You get best performance when the ADC is driven by a low impedance (see section 7.3.3. in the datasheet). You mention though that you only 10 - 15% accuracy, so it may be ok without the buffer. I would give it a try without the buffer, if it doesn't give you the accuracy you need then put in the buffer.

    Mike