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ADS8881EVM-PDK: ADS8881-EVM Input Impedance

Part Number: ADS8881EVM-PDK

Hi,

I have an issue driving the inputs of the EVM properly. The front end THS4551 seems to have a very low input impedance (<10k) making it hard to drive with normal opamps.

For example I am using it in an optical acquisition system using the TI OPT101 to drive the positive input (as described in the EVM user guide for single ended applications). The result is that the OPT101 cannot drive the input to GND. If the OPT101 outputs the lowest possible value (7mV) when I connect it to the EVM it gest pulled up to 0.7V , effectively clipping any signal.

I only had success using a very low output impedance Buffer (BUF634P powered by +-5V to allow full voltage swing. 

Is this how this EVM is supposed to work? What are the input current drive requirements?

Thanks,

Petros

  • Hi Petros,

    The EVM is designed to get the best possible SNR and THD for a wide range of applications. Hence the EVM has 1-kohm resistor for setting the gain of the fully-differential-amplifier (FDA).

    In your application, the input being driven by a photo-transistor, you can use gain setting resistors of the FDA as 10-kohm. Doing this will help you reduce the output load seen by OPT101. You may chose to use even higher values (Rg, Rf > 10-kohm) of gain setting resistors.

    When you increase the gain setting resistors of the the FDA, you will get some increase in noise. This can be compensated by using a capacitor in parallel with the feedback resistor of the FDA.

    Regards,
    Rahul