OPA642N BURR-BROWN [Burr-Brown Corporation], OPA642N Datasheet - Page 9

• Manufacturer Part Number: OPA642N
• Description: Wideband, Low Distortion, Low Gain OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

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 resistors at either end of the cable. The
circuit of Figure 1 applies to this requirement if all refer-
ences to 50
the amplifier gain is further increased to 2.2, which recovers
the additional DC loss of a typical long cable run. This
change would require the gain resistor (R
reduced from 402
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
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-
tion of the R
values reduce the required R
source and on the OPA642 output. Higher values increase
output noise and exacerbate the effects of parasitic board
and device capacitances. Following the selection of R
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
CMR), the ratio R
it is best to set the absolute values of R
and R
and cancels the effect of input bias currents. However, it is
sometimes useful to scale the values of R
to adjust the loading on the driving source V
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%.
If it is necessary to trim the CMR, then R
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
F
G
/R
respectively; this equalizes the divider resistances
G
). For accurate differential operation (i.e. good
F
resistors are replaced by 75
value in the range of 200
2
load of a single matched video cable,
/R
to 335 . In either case, both the gain
1
must be set equal to R
G
increasing the load on the V
2
G
and R
) in Figure 1 to be
2
2
1
is the suggested
and R
to 2k . Lower
. In most cases,
F
values. Often,
/R
1
equal to R
G
. Usually,
1
in order
F
, R
2
G
F
2
.
9
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 resistors, while the differential gain is set
by (1 + 2R
Figure 3. The differential to single-ended conversion is still
performed by the OPA642 output stage. The high imped-
ance inputs allow the V
impedance matched as required without further loading by
the differential amplifier. If the V
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
signal path must always exist for the V
the transformer case, a center-tapped secondary connected
to ground would provide an optimum DC operating point.
FIGURE 2. High Speed, Single Amplifier Differential
FIGURE 3. Wideband 3-Op Amp Differencing Amplifier.
V
V
V
V
1
2
1
2
1k
R
G
OPA2650
OPA2650
F1
R
R
/R
1/2
1/2
Amplifier.
G
1
500
500
G
R
R
)—which is equal to 2 using the values in
F1
F1
R
2
1
and V
OPA642
1k
1k
1k
+5V
–5V
2
OPA642
sources to be terminated or
1
R
and V
De-coupling Not Shown
F
De-coupling Not Shown
Power Supply
Power Supplies and
OPA642
1
V
2
O
and V
inputs are already
= 2 (V
1k
V
when
O
=
1
2
– V
R
R
inputs; for
R
R
F
G
2
1
2
(V
)
=
1
– V
R
R
V
2
F
G
O
)
®