Can anyone enlighten me as to how to calculate the propagation delays for a series of opamps driving a video signal?
I have a video signal path through three op-amps, and an analog switch, but I do not know which parameters to look up on each device, and how to calculate the delay time introduced by each op-amp circuit. Is it dependent on the feedback and filters, or will it be a simple delay spec like those found in digital gates?
Any pointers or kindly comments would be appreciated.
John
If the signal spectrum is well within the bandwidth of the individual stages, and they are well behaved near their upper passband limit, the delay will be approximated by
1/BWR where BWR is the bandwidth in radians/Second.If you want to get an accurate picture of delay, you can simulate or analyze the circuit's group and phase delay. Most simulators will plot group delay.
Beware that the delay will vary, just as bandwidth does.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
The delay will be dependent on the upper frequency limit of each opamp, together with the external circuitry. Lets say we have a gain stage with 6dB gain and an opamp with 80MHz GWB, which now has 40MHz -3dB. This lowpass will induce roughly 1/3 wavelength delay at lower frequencies. 40MHz make
25ns wavelength, so the delay will be around 8.3ns. Usually the delay increases with gain peaking to extend the frequency response, so this is a bad thing to do. Better to take higher bandwidth amplifiers and use Bessel filters, which have a constant delay in their passband.-- ciao Ban Bordighera, Italy
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