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ADS131B26-Q1: ADS131B26-q1

Joachim Sekinger
Prodigy 20 points
Part Number: ADS131B26-Q1
Other Parts Discussed in Thread: ADS131B26Q1EVM-PDK,

Tool/software:

Hello,

i am in trouble to read adc voltages for example at adc input V2A.

If i connect a high impedance resistor divider 12,5MOhm - 25K || 25K at V2A, the adc value has an offset and varies.

If i connect a lower resistor divider equal to at V2A ADS131B26Q1EVM-PDK all is fine.

Also when i connect the high impedance resistor divider 12,5MOhm - 25K || 25K at VPA all is fine.

Is the impedance for VA MUX limited?  If yes what is the maximium impedance?

Best regards,

Joachim

  • 0
    TI__Mastermind 23275 points

    Hi Joachim,

    welcome to our e2e forum and thanks a lot for your interest in our ADS131B26-Q1.

    We don't have any limitation on the maximum input impedance that we can measure with the various ADCs of the ADS131B26-Q1. However, you certainly need to keep the input impedance of the ADC inputs in mind as specified in the "Electrical Characteristics" table. The ADC input impedance interacts with the impedance that you want to measure and introduces measurement error.
    Depending on the data rates (OSR settings), ADC3y has higher input impedance compared to ADC2y. However in all cases the differential input impedance is >=15MOhm (typical). I am therefore surprised you see a lot of error when your bottom resistor is effectively only 12.5kOhm.

    Maybe this is more a "settling issue" of the RC filter. Are you only measuring a single channel with ADC2A constantly or are you multiplexing between multiple inputs?
    Can you try to increase the MUX2A_DELAY[2:0] setting to see if that makes any difference?
    Alternatively you could also try to increase the OSR2A[1:0] setting.

    Regards,
    Joachim Würker

  • 0
    Prodigy 30 points

    Hello Joachim Wuerker, 

    Joachim is a colleague of mine. For a test a 10k resistor was connected to the V2A pin against AGNDA, no external voltage source applied to this pin, no other components connected to this pin. Voltage drop on the resistor was measured to be 80mV (current sink/source of the V2A are disabled). If 4uA source is enabled, than the voltage drop on the resistor rises to circa 120mV, if 4uA sink is enabled than the voltage drop falls to circa 60mV. For the test we measured V2A constantly (no multiplexer sequencing).  

    Is there a way that the V2A pin of the ADC131B26-Q1 provides 8uA? Thank You

    Best regards,

    Petr

  • 0 in reply to Petr Nemec
    TI__Mastermind 23275 points

    Hello Petr,

    the ADS131B26-Q1 only offers four different settings for the burnout current sources/sinks: 4uA, 40uA and 240uA.

    But I am actually also a little surprised that you measure 80mV via the 10k resistor in this case to be honest.
    Could you do me a favor and measure the same signal with OSR2A[1:0] = 11b and then also a test with MUX2A_DELAY[2:0] = 111b? Just to see if that makes any difference?

    Regards,
    Joachim Wuerker

  • 0 in reply to Joachim Wuerker
    Prodigy 30 points

    Hello Joachim, 

    the problem appears to be a 10k pull-up resistor between +5V and the V7A (an intended NTC thermistor on the V7A against AGNDA was not connected at the time). The voltage on the V7A was therefore above absolute maximum ratings. After unsoldering the resistor, the problem is gone. The issue is solved for us. Thank You, Joachim, for your support.

  • 0 in reply to Petr Nemec
    TI__Mastermind 23275 points

    Thanks for letting me know Petr.

    Sorry that I didn't catch that myself.

    Regards,
    Joachim Wuerker