OPA2631 Burr-Brown, OPA2631 Datasheet - Page 14

• Manufacturer Part Number: OPA2631
• Description: Dual / Low Power / Single-Supply OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

INVERTING AMPLIFIER OPERATION
Since the OPA2631 is a general purpose, wideband voltage
feedback op amp, all of the familiar op amp application
circuits are available to the designer. Figure 7 shows a
typical inverting configuration where the I/O impedances
and signal gain from Figure 1 are retained in an inverting
circuit configuration. Inverting operation is one of the more
common requirements and offers several performance ben-
efits. The inverting configuration shows improved slew rate
and distortion. It also biases the input at V
headroom. The output voltage can be independently moved
with bias adjustment resistors connected to the inverting
input.
FIGURE 7. Gain of –2 Example Circuit.
In the inverting configuration, three key design consider-
ation must be noted. The first is that the gain resistor (R
becomes part of the signal channel input impedance. If input
impedance matching is desired (which is beneficial when-
ever the signal is coupled through a cable, twisted pair, long
PC board trace or other transmission line conductor), R
may be set equal to the required termination value and R
adjusted to give the desired gain. This is the simplest
approach and results in optimum bandwidth and noise per-
formance. 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
50
requires a 100
advantage that the noise gain becomes equal to 2 for a 50
source impedance—the same as the non-inverting circuits
considered above. However, the amplifier output will now
see the 100
load. In general, the feedback resistor should be limited to
the 200
increase both the R
then achieve the input matching impedance with a third
resistor (R
the parallel combination of R
Source
50
for input matching eliminates the need for R
0.1 F
0.1 F
57.6
M
to 1.5k
R
) to ground. The total input impedance becomes
M
®
feedback resistor in parallel with the external
374
OPA2631
R
1.50k
1.50k
feedback resistor. This has the interesting
G
2R
2R
F
T
and R
T
range. In this case, it is preferable to
G
OPA2631
+5V
values as shown in Figure 7, and
1/2
G
and R
750
R
F
0.1 F
M
.
S
50
+
/2 for the best
R
O
6.8 F
50
Load
M
G
but
G
to
G
F
)
14
The second major consideration, touched on in the previous
paragraph, is that the signal source impedance becomes
part of the noise gain equation and hence influences the
bandwidth. For the example in Figure 7, the R
combines in parallel with the external 50
ance, yielding an effective driving impedance of 50
576
for calculating the noise gain. The resultant is 2.87 for
Figure 7, as opposed to only 2 if R
discussed above. The bandwidth will therefore be lower for
the gain of –2 circuit of Figure 7 (NG = +2.9) than for the
gain of +2 circuit of Figure 1.
The third important consideration in inverting amplifier
design is setting the bias current cancellation resistors on
the non-inverting input (a parallel combination of R
750 ). 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
through R
R
high frequency noise introduced by this R
power supply feedthrough, it is bypassed with a capacitor.
If we had R
minimal. As a minimum, the OPA2631 requires an R
value of 50
direct short to ground on the non-inverting input runs the
risk of a very high frequency instability in the input stage.
OUTPUT CURRENT AND VOLTAGE
The OPA2631 provides outstanding output voltage capabil-
ity. Under no-load conditions at +25 C, the output voltage
typically swings closer than 130mV to either supply rail; the
guaranteed swing limit is within 400mV of either rail (V
+5V).
The minimum specified output voltage and current specifi-
cations over temperature are set by worst-case simulations at
the cold temperature extreme. Only at cold start-up will the
output current and voltage decrease to the numbers shown in
the guaranteed tables. As the output transistors deliver power,
their junction temperatures will increase, decreasing their
V
increasing their current gains (increasing the available out-
put current). In steady-state operation, the available output
voltage and current will always be greater than that shown
in the over-temperature specifications since the output stage
junction temperatures will be higher than the minimum
specified operating ambient.
To maintain maximum output stage linearity, no output
short-circuit protection is provided. This will not normally
be a problem since most applications include a series match-
ing resistor at the output that will limit the internal power
dissipation if the output side of this resistor is shorted to
ground.
T
BE
= 750
’s (increasing the available output voltage swing) and
= 26.8 . This impedance is added in series with R
F
because of the 0.1 F capacitor. Thus, we need
F
= 1.50k ||1.50k
T
. The inverting input's bias current flows
to damp out parasitic-induced peaking—a
< 400 , its noise contribution would be
To reduce the additional
M
could be eliminated as
T
source imped-
resistor, and
M
value
T
S
G
=
=
||
T