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AMC7836: SAR ADC Driver Circuit verification

Valerio Giampa
Prodigy 145 points
Part Number: AMC7836
Other Parts Discussed in Thread: ADS8028, TINA-TI

I has to evaluate if the bipolar an unipolar AMC7836 input channels are correctly driven in my design (output resistance and GBW of the driver opamp, Rf and Cf correctly defined).

For that reason, I am looking on the AMC7836 documentation in order to establish the taq and Csh.

Taq can be established by means of the equation defined in the datasheet on page 77, while Csh is not declared or at least is 34pF for unipolar channels (input capacitance) and not declared for bipolar channels.

In addition, the bipolar channels of the AMC7836 have an internal resistor network used to scale the input signal (probably it is composed by two resistors with a value equal to 175k).

The value of Rmus is not defined.

The user guide of the AMC7836 Evaluation Module has inside the schematic a Csh for each analogue input of 470pF.

It is not available a PSPICE model of the ADC7836 or equivalent to be used to perform a simulation of the ADC Driver Circuit.

Have someone used this ADC?

How do you have established and verified that the driver circuit is correctly designed?

Thanks

Valerio

  • 0
    TI__Expert 5530 points

    Hi Valerio,

    Here are some answers to your questions:

    1) the bipolar channels will have the same Csh as the unipolar channels. I'm asking a designer to find out the actual sampling capacitance, but it will likely be no more than 40pF.

    2) The EVM uses external capacitors of 470pF to help with fast ADC sampling time. 

    I'll need to check to see if we have any models of this part or similar ADCs that you can use to test. In general, when working with SAR ADCs here's what you want to do:

    -Low Input impedance. SAR ADCs require input current to charge the internal sampling capacitor. If you have a resistor divider or something similar, it will need to be comprised of low ohm resistors. If there is a concern about current draw into the ADC, an external buffer can be used instead.

    -External capacitor. Like I said earlier, an external capacitor is helpful for quickly charging the internal sampling capacitor. This helps increase accuracy at high sampling speeds.

  • +1
    TI__Expert 5530 points

    Hi Valerio,

    I have some more information. The internal sampling capacitor is typically 40pF. 

    The ADC is based on the ADS8028 https://www.ti.com/product/ADS8028. There is a Tina-TI Reference Design that you can use to simulate the ADC. The only difference is that there is a resistor divider in the AMC7836 to bring the voltage down to the ADS8028 values. 

    Let me know if you have any other questions.

    Thanks,
    Erin