OPA820

V

OUT

402Ω

335Ω

Video

Input

75Ω

75Ω

75Ω Transmission Line

V

OUT

75Ω

75Ω

V

OUT

75Ω

75Ω

75Ω

High output current drive capability allows three

back−terminated 75Ωtransmission lines to be simultaneously driven.

OPA820-HT

www.ti.com

SBOS587

– DECEMBER 2011

> 90-dB (< 1MHz) SFDR for the OPA820 in this configuration implies a < 3-dB degradation (for the system) from

the converter

’s specification. For further SFDR improvement with the OPA820, a differential configuration is

suggested.

Successful application of the OPA820 for ADC driving requires careful selection of the series resistor at the

amplifier output, along with the additional shunt capacitor at the ADC input. To some extent, selection of this RC

network will be determined empirically for each converter. Many high performance CMOS ADCs, such as the

ADS850, perform better with the shunt capacitor at the input pin. This capacitor provides low source impedance

for the transient currents produced by the sampling process. Improved SFDR is often obtained by adding this

external capacitor, whose value is often recommended in this converter data sheet. The external capacitor, in

combination with the built-in capacitance of the ADC input, presents a significant capacitive load to the OPA820.

Without a series isolation resistor, an undesirable peaking or loss of stability in the amplifier may result.

Since the DC bias current of the CMOS ADC input is negligible, the resistor has no effect on overall gain or

series resistor. This will ensure flat frequency response to the ADC input. Increasing the external capacitor value

will allow the series resistor to be reduced. Intentionally bandlimiting using this RC network can also be used to

limit noise at the converter input.

Video Line Driving

Most video distribution systems are designed with 75-

Ω series resistors to drive a matched 75-Ω cable. In order

to deliver a net gain of 1 to the 75-

Ω matched load, the amplifier is typically set up for a voltage gain of +2,

compensating for the 6-dB attenuation of the voltage divider formed by the series and shunt 75-

Ω resistors at

either end of the cable.

The circuit of Figure 2 applies to this requirement if all references to 50-

Ω resistors are replaced by 75-Ω values.

Often, the amplifier gain is further increased to 2.2, which recovers the additional DC loss of a typical long cable

case, both the gain flatness and the differential gain/phase performance of the OPA820 will 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 (3.58 MHz in NTSC systems) versus changes in the

large-signal output level (which represents luminance information in a composite video signal). The OPA820, with

the typical 150-

Ω load of a single matched video cable, shows less than 0.01%/0.01° differential gain/phase

errors over the standard luminance range for a positive video (negative sync) signal. Similar performance would

be observed for multiple video signals, as shown in Figure 6.

Figure 6. Video Distribution Amplifier

Copyright

© 2011, Texas Instruments Incorporated

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Product Folder Link(s): OPA820-HT