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ADS1120 input impedance

Susanna Polino
Prodigy 30 points
Other Parts Discussed in Thread: ADS1120, ADS1220

Hi,

I would like to know what is the input impedance of the ADS1120. I need an ADC with at least 1 MOhm of input impedance but I was not able to find this information in the ADS1120 datasheet. It is only specified that this ADC has a "high input impedance"....

Does someone know where I can find the exact value?

Thanks.

Susanna

  • 0
    TI__Guru** 112935 points

    Hi Susanna,

    Welcome to the forum!

    I can't give you an exact value as the impedance will change depending on what mode of operation your are using the ADS1120, which inputs are selected and the temperature range of operation.  If the PGA is enabled, you will be looking at an impedance similar to an op amp or INA which is in the gigaohm range.  If the PGA is bypassed, the impedance will lower somewhat, but will be much greater than 1M.

    The best thing to look at is the absolute/differential current graphs in the datasheet starting with Figure 15. You will notice that the current is not static relative to applied voltage or with temperature and that is why it is difficult to give an exact number.  In the end you should have no issues.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points

    Hi Bob,

    Thank you for your answer. I will use the ADS1120 with PGA enabled, then now I know that it will work fine.

    Susanna

  • 0 in reply to Susanna Polino
    TI__Guru** 112935 points

    Hi Susanna,

    When using the PGA you must make sure that the measurement is within the input common-mode range.  Make sure that you read and understand the information in the datasheet relative to the common-mode range for the ADS1120.  What is your measurement and what kind of sensor are you using?

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points
    Thanks for making me double check this parameter. I think the common mode range will be OK for my application.
    I use a sensor which is made of a wheatstone bridge, with a supply voltage of 5V, which is also used as reference voltage. And I set the gain of the PGA to 64. Then according to my computations:
    - VCM(min) >= 2.367 V
    - VCM(max) <= 2.624 V
    Correct me if I am wrong, but I think it's good. (The maximum differential output the sensor is supposed to have is 38 mV.)

    Susanna
  • 0 in reply to Susanna Polino
    TI__Guru** 112935 points

    Hi Susanna,

    Yes, your sensor should work in this configuration with no problem.  If you need a little more resolution, you could also try the ADS1220 which is a 24-bit device that is pin for pin compatible with the ADS1120.

    Best regards,

    Bob B