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  • TI Thinks Resolved

Starterware/DAC1282: DAC1282+ADS1282 to test detector leakage

Part Number: DAC1282

Tool/software: Starterware

Hi, I am trying to use DAC1282 and ADS1282 to test the detector’s leakage, which has been written in the datasheet for seismic applications. However, I am still not sure about the official suggested circuit for the detector leakage test. Can I use the following circuit? If so, what should I do?

  • Hi,

    Which leakage current are your trying to measure?

    I don't believe we have a recommended circuit for measuring leakage current in this application. However, if you help me understand the type of leakage current you interested in, I might be able to provide some suggestions.

    Best regards,
    Chris


    Christopher Hall
    Applications Engineer | Precision Delta-Sigma Converters

    Check out our helpful resources:
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  • In reply to Christopher Hall:

    Firstly I was trying to measure the leakage impedance.

  • In reply to user4687076:

    Hi,

    I think this could be accomplished with this circuit:

    The basic concept is that by comparing the voltage reading(s) on either side of the resistors you know the voltage drop across them. Form there you can calculate the current, knowing the values of the resistors.

    Best regards,
    Chris


    Christopher Hall
    Applications Engineer | Precision Delta-Sigma Converters

    Check out our helpful resources:
    TI Precision Data Converters | TI Precision Labs - ADCs | Analog Engineer's Calculator | Data Converters Learning Center | Selection Guide

  • In reply to Christopher Hall:

    Hi Chris:

     Thank you for your reply. However, what you proposed is the circuit to know the actual built-in resistance of the geophone, while I was asking to test the LEAKAGE resistance.

    Kenn

  • In reply to user4687076:

    Hi Kenn,

    I'm afraid I need a bit more more information... Is the geophone earth-ground referenced? What other input circuitry (filter, protection, etc.) or components are you using? What is the main leakage current path that you're trying to measure?

    Likely, you could still use something similar to the above circuit concept; however, you will certainly need to tweak it to your specific application.  

    Best regards,
    Chris


    Christopher Hall
    Applications Engineer | Precision Delta-Sigma Converters

    Check out our helpful resources:
    TI Precision Data Converters | TI Precision Labs - ADCs | Analog Engineer's Calculator | Data Converters Learning Center | Selection Guide

  • In reply to Christopher Hall:

    We use the protection circuit recommended in the ADS1282 data sheet. The leakage current path is from the geophone to earth ground. The circuit between earth ground and analogue ground is not decided yet, which I think should be part of the leakage resistance test circuit.


    We can manage to test the leakage resistance using several external switches to build up a test circuit. Since TI provides ADS1282 and DAC1282 with internal switches and mux for AD and geophone test, I wonder if you have any suggestion for testing geophone leakage resistance with ONLY INTERNAL SWITCHES AND MUX, saving us use of EXTRA EXTERNAL SWITCHES.
  • In reply to user4687076:

    Hi Ken,

    Sorry for the delay. I think you could adapt the above circuit to do one of the following measurements:

    Perhaps you could use the second ADC channel to measure the I3 current directly (across the resistor). If you wanted to measure I1 and I2, then you would probably need to use some external switches to measure make the connections across each of the input series resistors.

    Note that whatever source you use to provide these currents will need to be connected to earth ground.

    Best Regards,
    Chris


    Christopher Hall
    Applications Engineer | Precision Delta-Sigma Converters

    Check out our helpful resources:
    TI Precision Data Converters | TI Precision Labs - ADCs | Analog Engineer's Calculator | Data Converters Learning Center | Selection Guide

  • In reply to Christopher Hall:

    Dear Chris

    Could you please provide a precise equation of the group delay of the digital filter chain? The 31/fdata one on the datasheet is too rough for us.

    Best Regards,

    Ken

  • In reply to user4687076:

    Hi Ken,

    Correct me if I'm wrong, but I don't think you can get a group delay much more accurate than 31/fdata. Group delay here is the amount of time it takes for a step input to propagate through the digital filter, so that the the amplitude change is seen in the output (but not settled). Here the conversion period (1/fdata) is the minimum step size in time.

    If it would help...the coefficients for the ADS1282's digital filter are given in an appendix at the end of the datasheet, if you wanted to try to plot the filter response in a program like MATLAB. I'm familiar with with finding the frequency response from the coefficients, but not as familiar with plotting the phase response and/or the group delay.

    Best Regards,
    Chris


    Christopher Hall
    Applications Engineer | Precision Delta-Sigma Converters

    Check out our helpful resources:
    TI Precision Data Converters | TI Precision Labs - ADCs | Analog Engineer's Calculator | Data Converters Learning Center | Selection Guide

  • In reply to Christopher Hall:

    Hi Chris,

    I think the group delay is much more accurate than 1/fdata. The digital filters are driven by a clock much faster than fdata. The actual group delay is related to clock frequency rather than fdata. Same applies to tdr (time to data ready), which has a very accurate equation. Actually we measured the group delay at 1k SPS. The result is 31.269 us to the accuracy of less than 1us. But we still need the theoretical equation to confirm our measurement.

    We did calculate the group delay using coefficients in the appendix. But this group delay is a purely theoretical. A more accurate equation should consider the actual digital circuit, such as the timing of registers.

    Best Regards,

    Ken

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