OPA3690ID Datasheet, PDF (21/39 Pages) Texas Instruments – Triple, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3690
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INVERTING AMPLIFIER OPERATION
Since the OPA3690 is a general-purpose, wideband
voltage-feedback op amp, all of the familiar op amp
application circuits are available to the designer.
Inverting operation is one of the more common
requirements and offers several performance
benefits. Figure 47 shows a typical inverting
configuration where the I/O impedances and signal
gain from Figure 36 are retained in an inverting circuit
configuration.
+5V
+
0.1mF
6.8mF
0.1mF
R_B
145W
1/3
OPA3690
DIS R_O
50W
50W Load
50W
Source
R_G
200W
R_F
402W
R_M
66.5W
0.1mF ₊ 6.8mF
-5V
Figure 47. Gain of –2 Example Circuit
In the inverting configuration, three key design
considerations must be noted. The first is that the
gain resistor (R_G) becomes part of the signal channel
input impedance. If input impedance matching is
desired (which is beneficial whenever the signal is
coupled through a cable, twisted-pair, long PCB
trace, or other transmission line conductor), R_G may
be set equal to the required termination value and R_F
adjusted to give the desired gain. This is the simplest
approach and results in optimum bandwidth and
noise performance. However, at low inverting gains,
the resultant feedback resistor value can present a
significant load to the amplifier output. For an
inverting gain of –2, setting R_G to 54Ω for input
matching eliminates the need for R_M but requires a
SBOS237G – MARCH 2002 – REVISED MARCH 2010
100Ω feedback resistor. This has the interesting
advantage that the noise gain becomes equal to 2 for
a 50Ω source impedance—the same as the
noninverting circuits considered in the previous
section. The amplifier output, however, will now see
the 100Ω feedback resistor in parallel with the
external load. In general, the feedback resistor should
be limited to the 100Ω to 1.5kΩ range. In this case, it
is preferable to increase both the R_F and R_G values,
as shown in Figure 47, and then achieve the input
matching impedance with a third resistor (R_M) to
ground. The total input impedance becomes the
parallel combination of R_G and R_M.
The second major consideration, touched on in the
previous paragraph, is that the signal source
impedance becomes part of the noise gain equation
and influences the bandwidth. For the example in
Figure 47, the R_M value combines in parallel with the
external 50Ω source impedance, yielding an effective
driving impedance of 50Ω || 66.5Ω = 28.5Ω. This
impedance is added in series with R_G for calculating
the noise gain (NG). The resultant NG is 2.76 for
Figure 47, as opposed to only 2 if R_M could be
eliminated as discussed above. The bandwidth will
therefore be slightly lower for the gain of –2 circuit of
Figure 47 than for the gain of +2 circuit of Figure 36.
The third important consideration in inverting amplifier
design is setting the bias current cancellation resistor
on the noninverting input (R_B). If this resistor is set
equal to the total dc resistance looking out of the
inverting node, the output dc error, due to the input
bias currents, will be reduced to (Input Offset Current)
× R_F. If the 50Ω source impedance is dc-coupled in
Figure 47, the total resistance to ground on the
inverting input will be 228Ω. Combining this in parallel
with the feedback resistor gives the R_B = 145Ω used
in this example. To reduce the additional
high-frequency noise introduced by this resistor, it is
sometimes bypassed with a capacitor. As long as R_B
< 350Ω, the capacitor is not required because the
total noise contribution of all other terms will be less
than that of the op amp input noise voltage. As a
minimum, the OPA3690 requires an R_B value of 50Ω
to damp out parasitic-induced peaking—a direct short
to ground on the noninverting input runs the risk of a
very high-frequency instability in the input stage.
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