AC-coupled ADC inputs... why?

[dougy83]

Hi,

Does anyone know why certain video ADCs require AC-coupled input signals? What would be the effect of DC-coupling the input signal?

Here is an example video ADC: https://www.ti.com/lit/ds/symlink/tvp7002.pdf

Thanks for any insight.
Doug

 

[KeepItSimpleStupid]

That IC says: Support for DC- and AC-Coupled Input Signals

This:

Analog Input Clamping
The TVP7002 provides dc restoration for all analog video inputs including the SOG slicer inputs. The dc
restoration circuit (a.k.a. clamp circuit) restores the ac-coupled video signal to a fixed dc level. One dc restoration
circuit is implemented prior to each of the three ADCs, and a fourth one is located prior to the SOG slicer. The dc
restoration circuit can be programmed to operate as either a sync-tip clamp (a.k.a. coarse clamp) or a back-
porch clamp (a.k.a. fine clamp). The sync-tip clamp always clamps the video sync-tip level near the bottom of the
ADC range. The back-porch type clamp supports two clamping levels (bottom level and mid level) that are
selectable using bits 0, 1, and 2 of register 10h. When using the fine bottom-level clamp, an optional 300-mV
common-mode offset may be selected using bit 7 of register 2Ah.
In general, the analog video input being used for horizontal synchronization purposes should always use the
sync-tip clamp; all other analog video inputs should use the back-porch clamp. The advantage of the back-porch
clamp is that it has negligible video droop or tilt across a video line.

sort of says why AC is preferred. It can DC level the inputs.

Then within the datasheet it says AC coupling necessary.

Then see: https://electronicdesign.com/analog/get-grip-clamps-bias-and-ac-coupled-video-signals

The short answer is it's required by government regulations.

 

[dougy83]

Great! Thanks for the reply; it was actually quite interesting reading that reference about clamps and DC-level restoration. I'd never thought about it before.

I saw the statements in the datasheet of the requirement that all inputs be AC-coupled, and previously didn't notice any about DC-coupling. So if the clamps are disabled, DC-coupled inputs can possibly be used without being interfered with by the clamps... maybe.

Thanks again.