Video buffer/splitter circuit

Hi Chris,

The ideal set-up with the OPA693 operating as a composite video splitter would require +/-5V supplies.  Since composite video has a sync signal that goes below ground, having a bipolar supply set-up would allow you to DC-couple the inputs and outputs of the OPA693 and preserve the full voltage swing of the composite + sync signal.  In this case, the set-up shown in the first circuit (adjusted to have +/-5V) is appropriate.

The other option is to use an AC-coupled set-up.  This will allow you to operate with a single +5V supply, but it will also require you to add coupling capacitors and do some biasing on the input.  The example circuit for this is shown in Figure 4 of the datasheet and described on the same page.  AC-coupling allows you to have a simpler supply voltage set-up, but it generally comes with two main drawbacks:

1) In order to preserve the frequency response of the circuit, the high pass corner frequency of the pole formed by the coupling cap should be around 2.5 - 3 Hz.  This generally means that the capacitors will be large and expensive.

2) AC-coupling lead to a certain level of tilt or droop in your video signal, i.e. if one voltage level is held for a period of time, the transient response of the capacitor will cause this signal level to slightly taper off.  Generally this isn't going to be a significant issue if your pole is around 2.5 - 3 Hz as mentioned above, but it is still worth noting.

To conclude, if you have the means to add a -5V supply to your system, your video amp set-up will be simplified and not have any tilt or droop.  But if the +5V single supply is all you have to work with, use an AC-coupling configuration.

Regards,

Nick - TI High Speed Amps

Chris,

For the sake of completeness, I'll run another two options by you.  Both of these allow you to operate in a DC-coupled configuration with a single-supply set-up, thanks to internal level shifting in the parts.

1) OPA360 - Single-supply video amp with internal level shift.  It runs off of a 2.7 to 3.3V supply, so you'd also need to include an LDO to drop your 5V supply down to the required voltage.  It's still an additional part, but an LDO will be less noisy than adding a charge pump to create a negative rail.

2) THS7314 - Single-supply 3 channel video amp with internal level shift.  This will end up being the cheapest total cost solution, since it would not require any additional power supply work (it requires only +5V), and the internal level shift will allow a DC-coupling set-up.  Additionally, since it has three channels, you cold simply use two of the three channels to drive your signals, rather than using one load to drive two channels.  This will give you better performance, since driving two loads with one channel is always a little less than ideal.

Now you have plenty of options -- hopefully one will fit well into your system.

Regards,

Nick - TI High Speed Amps