I am trying to use an ADA4430 to buffer the baseband analogue video O/P from an old B/W video camera. The motivation is that the video camera's O/P impedance is not low enough to drive a 75R load properly and so needs something to act like a unity-gain buffer with suitably high input and low output impedances. The video standard is US TV System-M (525 lines, 60Hz, RS-170).
Viewing the video O/P on an old analogue monitor produces a darker than expected (and as as I/P) picture with tearing on the RHS. A similar result is obtained using an analogue video capture USB stick, displaying on a PC. Moreover the USB capture stick's S/W struggles to recognise the signal and tends to blank out accordingly.
Scoping the I/P and O/P signals reveals the problem. The video content in the O/P looks fine but the line sync pulses are inverted! Here is an illustration,
Trace 1/yellow is the video input to the ADA4430 and trace 2/cyan is the O/P. The circuit is the same as Figure 30 in the "ADA4430-1" datasheet -- that is a 220uF decoupling capacitor and 75R resistor in the O/P line which drives the analogue monitor or USB video capture stick. The ADA4430's !DIS signal is tied up to Vs (+5.0V) so the chip is permanently enabled. Both scope channels are DC coupled.
Is there something I am missing or doing wrong? Are there any other chips that would better suit my application?
Many thanks
Tom Crane
