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ADS1294: What is the AC input impedance

user4725373
Expert 1645 points
Part Number: ADS1294

Hi,

The datasheet of the ADS1294 says that the DC input impedance is >500 MOhms when using the current source lead-off detection, but are there measurements that exist for a signal of 10 Hz?

Thank you,

Fred

  • 0
    TI__Mastermind 47625 points

    Hi Fred,

    Thanks for your post! 

    I do not have measurements to provide, however this has been tested within the group before and the change to the impedance is minimal. 

  • 0 in reply to Alexander Smith
    Expert 1645 points

    Thanks for the reply, Alex.

    Can you tell me what the test conditions were, and what setup was used? I'm trying to measure it myself by applying 2 sine waves at IN1P and IN1N at 10Hz off-phased by 120 degrees with 1Vpp each, so the resulting amplitude should be v_in = vpp * sqrt(3) = 1.73 Vpp (I measure each channel input to calculate the actual amplitude that should be read by the ADC). By knowing this amplitude and the fact that we have 100k resistors at the inputs, I'm computing the input impedance by assuming a simple voltage divider with:

    z = v_adc * z_i / (v_in - v_adc)

    Is that correct? Because using this, I calculate ~10MOhms input impedance, but we are configuring the ADC to use lead-off detection with current source, so it should be ~500M. And yes, an error of measurement of v_in will cause a great difference, but some mV offset won't change the 10M to 500M.

    Thank you,

    Fred

  • 0 in reply to Alexander Smith
    Expert 1645 points

    Hi Alex,

    Any update on this?

  • 0 in reply to user4725373
    TI__Mastermind 47625 points

    Hi Fred,

    Unfortunately I do not have any information to provide as far as test setup goes. 

    For our own curiosity, why is this spec important to your design?

    There are capacitors (as well as parasitic capacitance) connected to GND on the input pins, so it is not surprising that you see a reduction in impedance at 10Hz. 

  • 0 in reply to Alexander Smith
    Expert 1645 points

    Hi Alex,

    The standard IEC60601-2-47:2012 subclause 201.12.4.4.102 requires an input impedance greater than 10 MOhms for a 10 Hz sinewave.

    As the ADS1294's datasheet only specifies the DC input impedance, I need to test the AC input impedance.

    What do you think of the test setup that I described in my previous post?

  • 0 in reply to user4725373
    TI__Mastermind 47625 points

    Hi Fred,

    I think it's an interesting approach. I am not familiar with that standard so I'm not sure if there is a test procedure that must be followed. I would probably do something a little more simple:

    Connect the 100k resistor in parallel with the input pins impedance. Set the negative pin and one side of the resistor to mid-supply, (GND for bipolar 2.5V or 2.5V for a unipolar 5V supply, etc) Connect the other side of the resistor to the positive input pin. Apply the 10Hz sine wave to the positive input with respect to the negative input, record what the resulting ADC output magnitude is. Remove resistor, repeat the test so that you can quantify the affect of adding the resistor.