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ADS1257: Typical Application Questions

Part Number: ADS1257

Tool/software:

Looking to use the typical application circuit (Fig 65) to sample a 4-20mA pressure transducer in an industrial environment.

The typical application samples at 10SPS for 50/60hz rejection. In the case where a higher sample rate is required (8ms update rate on transducer, 125hz -> 250SPS)

1) How drastically will a higher sampling rate affect 50/60hz noise rejection? 

2) If so, what modifications can be made to achieve this sample rate reliably without drastically affecting usable bits

3) If not, do passive components need to be modified to accommodate the higher sampling rate?

Thanks!

  • Hi Jeremy Styma,

    Answers to your questions:

    How drastically will a higher sampling rate affect 50/60hz noise rejection? 

    You can use the equation below (from the datasheet) to calculate the filter frequency response at different data rates, which will allow you to calculate the rejection of any specific frequency for a given data rate. Or you can use the filter bandwidth table (also shown below) that shows the -3dB point for each data rate. At 500 SPS (the slowest data rate that is >=250SPS), the 3dB bandwidth is at 221 Hz. So you can see that the rejection of 50/60Hz will be negligible at any data rate >60 SPS.

    If so, what modifications can be made to achieve this sample rate reliably without drastically affecting usable bits

    You would have to post process your data by applying a digital notch filter to remove these frequencies if you intend to sample at a data rate > 60 SPS

    -Bryan