Datasheet

+5V
-5V
R
2
50W
Power-supply decoupling not shown.
V
2
R
1
V
1
R
F
R
G
V =
O
(V V )
1 2
-
R
F
R
G
when
=
R
2
R
1
R
F
R
G
OPA842
Power-supply decoupling not shown.
V
O
V
1
R
G
500W
V
2
+5V
+5V
-5V
-5V
500W
500W
R
F1
500W
R
F1
500W
500W 500W
OPA842
OPA2822
OPA2822
OPA842
www.ti.com SBOS267D –NOVEMBER 2002–REVISED SEPTEMBER 2010
SINGLE OP AMP DIFFERENTIAL AMPLIFIER THREE OP AMP DIFFERENCING
(Instrumentation Topology)
The voltage-feedback architecture of the OPA842,
with its high Common-Mode Rejection Ratio (CMRR), The primary drawback of the single op amp
will provide exceptional performance in differential differential amplifier is its relatively low input
amplifier configurations. Figure 39 shows a typical impedance. Where high impedance is required at the
configuration. The starting point for this design is the differential input, a standard instrumentation amplifier
selection of the R
F
value in the range of 200Ω to 2kΩ. (INA) topology may be built using the OPA842 as the
Lower values reduce the required R
G
, increasing the differencing stage. Figure 40 shows an example of
load on the V
2
source and on the OPA842 output. this, in which the two input amplifiers are packaged
Higher values increase output noise and exacerbate together as a dual voltage-feedback op amp, the
the effects of parasitic board and device OPA2822. This approach saves board space, cost,
capacitances. Following the selection of R
F
, R
G
must and power compared to using two additional OPA842
be set to achieve the desired inverting gain for V
2
. devices, and still achieves very good noise and
Remember that the bandwidth will be set distortion performance due to the moderate loading
approximately by the Gain Bandwidth Product (GBP) on the input amplifiers.
divided by the noise gain (1 + R
F
/R
G
). For accurate
differential operation (that is, good CMRR), the ratio
R
2
/R
1
must be set equal to R
F
/R
G
.
Figure 39. High-Speed, Single Differential
Amplifier
Figure 40. Wideband Three-Op Amp Differencing
Amplifier
Usually, it is best to set the absolute values of R
2
and
R
1
equal to R
F
and R
G
, respectively; this equalizes
the divider resistances and cancels the effect of input
In this circuit, the common-mode gain to the output is
bias currents. However, it is sometimes useful to
always 1, due to the four matched 500Ω resistors,
scale the values of R
2
and R
1
in order to adjust the
whereas the differential gain is set by (1 + 2R
F1
/R
G
),
loading on the driving source V
1
. In most cases, the
which is equal to 2 using the values in Figure 40. The
achievable low-frequency CMRR will be limited by the
differential to single-ended conversion is still
accuracy of the resistor values. The 85dB CMRR of
performed by the OPA842 output stage. The
the OPA842 itself will not determine the overall circuit
high-impedance inputs allow the V
1
and V
2
sources to
CMRR unless the resistor ratios are matched to
be terminated or impedance matched as required. If
better than 0.003%. If it is necessary to trim the
the V
1
and V
2
inputs are already truly differential,
CMRR, then R
2
is the suggested adjustment point.
such as the output from a signal transformer, 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
1
and V
2
inputs; for the transformer case, a center-tapped
secondary connected to ground would provide an
optimum dc operating point.
Copyright © 2002–2010, Texas Instruments Incorporated 13
Product Folder Link(s): OPA842