AD7718 R-C Filtering on AIN/REF inputs

Hi,

I'm using the AD7718 ADC and I'm not sure the best way to filter the AIN inputs, REF+ inputs and AVDD/DVDD inputs. My issue is that I'm going to be using both Single Ended and Differential type sensors but I need to have the flexibility of using either/or on any input. So, I'm not sure the best way to apply and RC Filter, if any. Same question about the REFIN+ input. The attached diagram shows essentially what types of sensors I'm dealing with and the voltages. I was thinking of keeping it simple and just putting a 1k resistor on each AIN input followed by a 0.1uF cap to ground, but my concern was that the caps will be mismatched creating issues for the differential inputs. Any help is much appreciated.

AD7718RC.pdf
  • 0
    •  Analog Employees 
    on Feb 28, 2018 12:08 AM

    Hi,

    Usually, for sigma delta ADC a single pole (possibly a 2 pole) RC filter is all that is required, that is a 1KΩ resistor in series with each analog input, a 0.1uF capacitor from AIN+ to AIN-, and a 0.01 uF capacitor from each analog input pin to GND is recommended when the analog input channel is buffered. However, when the the converter is operated in un buffered mode, the inputs look directly into the sampling capacitor of the modulator. The modulator is continually charging and discharging the sampling capacitor. If the time constant of the anti-aliasing filter is too large, the modulator may be unable to fully charge the sampling capacitor and gain errors will result. To prevent the R-C combination from introducing errors, the R and C values used must be limited. The same rule applied to the reference inputs. When the AD7718 is configured for differential inputs, the analog inputs are buffered. Therefore it can handle significant amount of input impedance. For reference, you can either provide a low output impedances reference voltage source or provide an external buffer reference. Our new product such as AD7124 have this internal reference buffer option. You may also want to consider using new products.

    Thanks,

    Jellenie

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:02 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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    EZ Admin