®

OPA642

9

is typically set up for a voltage gain of +2, compensating for

the 6dB attenuation of the voltage divider formed by the

series and shunt 75

Ω resistors at either end of the cable. The

circuit of Figure 1 applies to this requirement if all refer-

ences to 50

Ω 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

reduced from 402

Ω to 335Ω. In either case, both the gain

flatness and the differential gain/phase performance of the

OPA642 will provide exceptional results in video distribu-

tion applications. Differential Gain and Phase measure the

change in overall small-signal gain and phase for the color

subcarrier frequency (3.58MHz in NTSC systems) vs changes

in the large-signal output level (which represents luminance

information in a composite video signal). The OPA642, 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 ob-

served for negative video signals. In practice, similar perfor-

mance is achieved even with two video loads due to the

linear high-frequency output impedance of the OPA642.

SINGLE OP-AMP DIFFERENTIAL AMPLIFIER

The voltage feedback architecture of the OPA642, with its

high CMR, will provide exceptional performance in differ-

ential amplifier configurations. Figure 2 shows a typical

configuration. The starting point for this design is the selec-

source and on the OPA642 output. Higher values increase

output noise and exacerbate the effects of parasitic board

must be set to achieve the desired inverting gain for V

Remember that the bandwidth will be set approximately by

the gain bandwidth product (GBP) divided by the noise gain

(1+ R

and R

and cancels the effect of input bias currents. However, it is

the achievable low frequency CMR will be limited by the

accuracy of the resistor values. The 90dB CMR of the

OPA642 itself will not determine the overall circuit CMR

unless the resistor ratios are matched to better than 0.003%.

adjustment point.

THREE OP AMP DIFFERENCING

(Instrumentation Topology)

The primary drawback of the single op-amp differential

amplifier is its relatively low input impedances. Where a

high impedance is required at the differential input, a stan-

dard instrumentation amplifier (INA) topology may be built

using the OPA642 as the differencing stage. Figure 3 shows

an example of this, in which the two input amplifiers are

packaged together as a dual voltage feedback op-amp—the

OPA2650. This approach saves board space, cost and power

compared to using two additional OPA642 devices, and still

achieves very good noise and distortion performance due to

the moderate loading on the input amplifiers. In this circuit,

the common mode gain to the output is always one due to the

four matched 1k

Ω resistors, while the differential gain is set

Figure 3. The differential to single-ended conversion is still

performed by the OPA642 output stage. The high imped-

impedance matched as required without further loading by

truly differential, such as the output from a signal trans-

former, then a single matching termination resistor may be

used between them. Remember, however, that a defined DC

the transformer case, a center-tapped secondary connected

to ground would provide an optimum DC operating point.

FIGURE 3. Wideband 3-Op Amp Differencing Amplifier.

1/2

OPA2650

1/2

OPA2650

OPA642

1k

Ω

1k

Ω

1k

Ω

R

F1

500

Ω

R

F1

500

Ω

V

1

V

2

V

O

V

Power Supplies and

De-coupling Not Shown

R

G

1k

Ω

1k

Ω

V

(V

when

=

R

F

R

G

R

2

R

1

R

F

R

G

R

F

–5V

+5V

Power Supply

De-coupling Not Shown

R

G

V

2

V

1

R

1

OPA642

R

2

FIGURE 2. High Speed, Single Amplifier Differential

Amplifier.