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AD7124-8: What is the benefit of the post filters versus the standard sinc filters?

 

Q.         What is the benefit of the post filters versus the standard sinc filters?

 

A.         In industrial applications, many customers require 50/60 Hz rejection but require the minimum settling time. Ideally, customers would like a settling time of 20 ms i.e. the customer could sequence through multiple channels with each channel producing a valid conversion in 20 ms. This is difficult to achieve while still attaining good 50/60 Hz rejection.

The post filters offer a compromise between these 2 parameters. A standard sinc3 filter has a settling time of 60 ms when operating at an output data rate of 50 sps while offering sim 50/60 Hz rejection. The rejection at 50/60 Hz is 66 dB. The post filters can operate at an output data rate of 25 sps, with the settling time being approximately equal to 40 ms. The 50/60 Hz rejection is 62 dB. So, the rejection is a little worse than the sinc filter. However, it is still at an acceptable level for industrial applications and the settling time is 50% less.

 

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