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ADS7953: Charge Injection

Danilo A.
Mastermind 34110 points

Part Number: ADS7953
Other Parts Discussed in Thread: OPA192

Hi Team,

We would like to ask your help regarding our customer's inquiry below.

So this ADC includes a multiplexer on the front end, and in general any mux will inject a certain amount of charge on each input as it is selected. This is due to the FETs used to switch each channel to the common node. Charge injection at a certain repetition rate produces a known current. This charge that is injected needs to be known to design the drive circuit..

We are using the suggested op-amp for buffering OPA192 between the mux section and the ADC itself however we would like to have an input impedance of 10k and 44 microfarads into some of the channels.

Does the curve in figure 67 of the datasheet imply that the mux channel input cap is 150 picofarads? And the rc needs to be short enough to allow recovery of the injection voltage error. So would a 44uf cap produce a constant voltage offset?

Regards,

Danilo

  • 0
    TI__Mastermind 22950 points

    Hi Danilo,

    Yes, I believe that Figure 67 implies Cfilt = 150pF. 44uF seems rather large and the concern here is that the 44uF cap will not allow the input to settle to 0.5LSB during the acquisition phase and will affect the accuracy of the converter. 

    What is the particular reason for picking 10kohms and 44uF?

    Regards,
    Aaron

  • 0 in reply to Aaron Estrada
    TI__Mastermind 34110 points

    Hi Aaron,

    Thank you for your response. According to our customer,

    We need a low pass filter before the a/d mux.

    If we knew the charge injection coming out of the input of the a/d we could estimate the voltage drop across the 10k resistor. The 44uf will keep the input voltage constant and the charge injection will produce current and therefore an offset across the 10k source resistance. We realize the charge injection will be dependent on the mux input voltage. It would be useful to know worst case current.

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Mastermind 22950 points

    Hi Danilo,

    I do not have information on the magnitude of the charge injection but this might be something I can simulate. Let me see if I can figure something out. 


    Regards,
    Aaron

  • 0 in reply to Aaron Estrada
    TI__Mastermind 34110 points

    Hi Aaron,

    We will be waiting for your feedback.

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Mastermind 22950 points

    Hi Danilo,

    It's a bit complicated to simulate after discussing this with the team, however, I was able to simulate the LPF at the input and check the device settling. With the OPA connected between the MUX out and ADC in there should not be any issues with settling and that was my main concern. I understand that the customer is worried about the voltage drop across the 10kohm resistor but I don't think that will be as much of a concern. 

    The MUX change happens after the input command is received. Then there is a relatively long time for the input to settle, ~11 clock cycles. Now the issue is that the ADC sample cap comes into play and that is when we need to take care of the charge injection spike. Adding the buffer between MUX out and ADC in should help with this.


    Regards,
    Aaron