ZXFV4089

Issue 4 - December 2008

8

www.zetex.com

© Diodes Incorporated 2008

www.diodes.com

Figure 3 shows a portion of a typical video waveform, where the sample pulse is synchronised to

fall within the back porch interval. This can for example be achieved using the Zetex ZXFV4583

sync separator to derive the pulse as in the evaluation circuit described in the data sheet for that

part. Again, during the logic LOW period of the HOLD input, the waveform is driven towards VREF.

Eventually, after a few line scans, the video waveform is stabilised with the back porch level equal

In the video application, the HOLD input state will be HIGH during the picture line sweep and a

negative-going sampling pulse of typically 1.2µs duration will be applied during a central portion

ground).

Figure 3 Response to typical video signal

If desired, by changing the external pulse timing the signal may be restored such that the sync tip

In either case, for each line scan, this gives a brightness level consistent with that of the original

camera signal, despite the AC coupling. The value of the coupling capacitor affects two main

characteristics of the circuit:

1. DC level acquisition change

2. DC level droop

DC level acquisition change

In the restore mode the available charging current, together with the capacitor value, determines

the maximum DC voltage correction which can be applied at each sample. For a charging current

limit of 300µA applied for 1.2µs, the charge injected is:

Qmax = 300µA x 1.2µs = 360pC

Then the maximum voltage shift correction is:

Vmax = Qmax/C = 360pC / 0.01µF

= 36mV