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AD8307 - Detection of short burst with preceding filter delay

Hi,

Please see attached filter output display using LTspice, with AD8616 opamps. The filter is a pair of cascaded 8th order butterworth bessel bandpass filters, total 30dB gain. (Need very strong rejection out of band)  Unfortunately, even with minimal delay filters, delay *is* happening with a 16th order filter.

So...the question is this: With this filter/gain stage ahead of a AD8307 (yes, gain has been accounted for in output) what will the impact be when detecting a very short signal burst of, let's say duration 60ms? 100ms?

By T = 200ms, the filter has reached almost full magnitude. Does this mean that for any signals bursts under approximately 125ms, will not be seen at all by the AD8307 ? Or are they just delayed and will not reach the output (and be of significant amplitude) for a time period > approximately 125ms?

Thanks for your help !!

Parents
  • It might not even respond at that low of frequency (it looks to be near 30 Hz).  The AD8307 will only have a couple of cycles to respond to in the time periods you are quoting above. What is your frequency(s) of operation?  I may be able to measure the AD8307 on the bench to see if it'll respond fast enough at the low frequency.

    I've attached an app note detailing how to use detectors at low frequency. 

    AN-691.pdf
Reply
  • It might not even respond at that low of frequency (it looks to be near 30 Hz).  The AD8307 will only have a couple of cycles to respond to in the time periods you are quoting above. What is your frequency(s) of operation?  I may be able to measure the AD8307 on the bench to see if it'll respond fast enough at the low frequency.

    I've attached an app note detailing how to use detectors at low frequency. 

    AN-691.pdf
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