Most video distribution systems are designed with 75Ω series resistors to drive a matched 75Ω cable. To deliver a net gain of 1 to the 75Ω matched load, the amplifier is typically set up for a voltage gain of +2V/V, compensating for the 6dB attenuation of the voltage divider formed by the series and shunt 75Ω resistors at either end of the cable.

The circuit of Figure 8-1 applies to this requirement if all references to 50Ω resistors are replaced by 75Ω values. Often, the amplifier gain is further increased to 2.2, which recovers the additional dc loss of a typical long cable run. This change can require the gain resistor (R_G) in Figure 8-1 to be reduced from 750Ω to 625Ω. In either case, both the gain flatness and the differential gain/phase performance of the OPA4830 provide exceptional results in video distribution applications. Differential gain and phase measure the change in overall small-signal gain and phase for the color sub-carrier frequency (3.58MHz in NTSC systems) versus changes in the large-signal output level (which represents luminance information in a composite video signal). The OPA4830, with the typical 150Ω load of a single matched video cable, shows less than 0.07%/0.17° differential gain/phase errors over the standard luminance range for a positive video (negative sync) signal. Similar performance is observed for multiple video signals (see Figure 8-15).

Figure 8-14 Single-Supply Differential ADC Driver (1 of 2 channels)

Figure 8-15 Video Distribution Amplifier