AD7691 Pulsar Differential ADC Common Mode Input Spec

AD7691 spec sheet says Common-Mode Range needs to be Vref/2 +-0.1V.

I have a system who's inputs to the ADC is In-=0.1V In+=1.4V(up to 1.8V), and I don't see any issues due to this.

Can someone help me understand when this specification is important.

My guess is that it has to due with the input voltage range and max input voltage.  Is this spec just to ensure a full range (+-Vref) signal stays within the absolute input voltage range?

Thanks in advance,

Shane

Parents
  • 0
    •  Analog Employees 
    on Nov 22, 2016 3:58 PM

    Hi Shane,

    The reference must be capable of supplying the average current needed to top up the reference capacitor without causing the reference voltage to droop significantly. Insufficient drive strength is an issue, especially if low-power references or micro power reference buffers are used, as these typically have much higher output impedances that increase dramatically with frequency. Have you consider using either the ADR4525 or ADR431 as an alternative to the ADR361? To avoid conversion errors, the average current required at a particular throughput should not cause the reference voltage to droop more than 1/2 LSB. It’s critical to place a 10uF (eg. 0805,  X7R) decoupling capacitor as close as possible to the REF pin of the ADC using wide traces to connect it.

     

    Which ADC driver are you using to drive the AD7691 inputs? The latest low power, low noise ADA4807-1 would be a good candidate to consider for the ADC driver. The RC filter between the ADC driver output and SAR ADC input does two-fold tasks: Attenuates kick-back coming from capacitive DAC input of the SAR ADC and also reduces noise coming from the upstream analog front end, so it's important to choose the right RC values (see figure 29) to achieve optimum performance. It is best to choose a capacitance value of 1nF to 3nF (NPO types) and a reasonable resistance value that will keep the ADC driver stable. Distortion increases with both input frequency and source resistance (see figure 31). The sensor common-mode should be set to Vref/2 as you have done.

     

    Refer to these Analog Dialogue articles for more details.

     

    Hope that helps.

    regards,

    LLoben

Reply
  • 0
    •  Analog Employees 
    on Nov 22, 2016 3:58 PM

    Hi Shane,

    The reference must be capable of supplying the average current needed to top up the reference capacitor without causing the reference voltage to droop significantly. Insufficient drive strength is an issue, especially if low-power references or micro power reference buffers are used, as these typically have much higher output impedances that increase dramatically with frequency. Have you consider using either the ADR4525 or ADR431 as an alternative to the ADR361? To avoid conversion errors, the average current required at a particular throughput should not cause the reference voltage to droop more than 1/2 LSB. It’s critical to place a 10uF (eg. 0805,  X7R) decoupling capacitor as close as possible to the REF pin of the ADC using wide traces to connect it.

     

    Which ADC driver are you using to drive the AD7691 inputs? The latest low power, low noise ADA4807-1 would be a good candidate to consider for the ADC driver. The RC filter between the ADC driver output and SAR ADC input does two-fold tasks: Attenuates kick-back coming from capacitive DAC input of the SAR ADC and also reduces noise coming from the upstream analog front end, so it's important to choose the right RC values (see figure 29) to achieve optimum performance. It is best to choose a capacitance value of 1nF to 3nF (NPO types) and a reasonable resistance value that will keep the ADC driver stable. Distortion increases with both input frequency and source resistance (see figure 31). The sensor common-mode should be set to Vref/2 as you have done.

     

    Refer to these Analog Dialogue articles for more details.

     

    Hope that helps.

    regards,

    LLoben

Children
No Data