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when choose DC-Coupled Differential input mode,hwo to protect ADS2807's input side?

Menglin Sun
Prodigy 50 points
Other Parts Discussed in Thread: ADS2807

Hello, I use a high speed ADC ADS2807 in my anolog & digital system and I choose DC-Coupled Differential input mode as the image shows. According to my calculation, the Vin range is only -0.375~+0.375 in this circuit. Obviously, this input voltage range is so narrow that  it can  be exceeded easily and do damage to ADS2807. So I'm looking for a way to protect the chip. Thank you !

 

  • 0
    TI__Mastermind 29220 points

    Hi,

    Are you saying that with the gain and common mode biasing of the circuit as shown that the Vin  must be between -0.375V and 0.375V in order for the signal at the ADC pins to remain within the full scale range?  And that if you have an input to the amp circuit that is outside this range that you worry about damaging the ADC?

    Please refer to the absolute max ratings for the analog input of the ADS2807 on page two of the datasheet.  The analog inputs to the ADC must not go below -0.3V or above the supply voltage plus 0.3V.  With the supply voltage at 5V, the inputs must remain between -0.3V and 5.3V.  If the amp is also powered off of 5V, then the amp circuit would not be able to supply a signal out of the absolute max range of the ADC anyway.  so i don't think you need to worry about damage to the data converter.  The amp will be limited in what it can output.  The data converter will not be damaged if the input voltage is larger than full scale, only if it is much larger than the absolute max ratings for an extended period of time and I don't believe this circuit can do that.

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    Prodigy 50 points

    Hi, Richard P

    Thanks very much for your reply. Your analysis is absolutely right. However,since my amp is powered by +/- 5V, it is possible that the AD's input voltage exceed -0.3V. In fact ,when Vin>1.4V or Vin<-1.4V, AD's input voltage will exceed -0.3V.  So I intend to use two diods to limit Vin range,