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ADS112U04: Input impedance

S.Satoshi
Guru 21675 points
Part Number: ADS112U04
Other Parts Discussed in Thread: ADS1255

Hi,

Please let me ask about input impedance of ADS112U04. Now out out customer is considering ADS112U04 and asking this.

Can you share any example value about it or way to calculate (estimate)  it? I'm asking this because other delta-sigma ADC with MUX like ADS1255 has infomation of input impedance. 

Best Regards,

Satoshi / Japan Disty

  • +1
    TI__Genius 17857 points

    Satoshi, 

    The ADS112U04 has two different input modes: 1) PGA Bypass mode (this is a switched capacitor input with pre-charge buffers), and 2) PGA mode (this mode uses a linear amplifier with high input impedance).  

    The input impedance in PGA mode is very high.  It is not specified because what is really relevant is the leakage current and how this leakage current interacts with your sensor output impedance.  This leakage will depend on temperature and input signal.  Typical leakage is provided in all the plots (see figure 7 and 11).

    The pre-charge buffers substantially increase the input impedance (reduce input current) for the PGA bypass mode as compared to different devices that do not have pre-charge buffers.   However, when this approach is used the input impedance is not linear.  You can see this in the plots for input current.  In these plots, the current does not linearly increase with input voltage is it would with a linear impedance. See figure 6, 8, and 10.

    Because the input current does not linearly increase with input voltage for either the PGA bypass mode or the PGA mode, impedances are not specified.  You should look at the input current vs voltage curve to understand how the accuracy of your sensor is impacted by the input current, and not think about the input impedance.

    I hope this information is helpful to you.

    Art