OPA837 Datasheet, PDF (39/51 Pages) Texas Instruments – OPA837 Low-Power, Precision, 105-MHz, Voltage-Feedback Op Amp

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OPA837
SBOS673 – SEPTEMBER 2017
Typical Applications (continued)
8.2.3 1-Bit PGA Operation
Using the internal inverting input switch that operates along with the power disable function can also allow a
simple gain selection on a single input signal. Figure 87 shows an example gain select of either 1 V/V or 2 V/V
from a single input to a single output. The logic disables both channels before turning one of them on to avoid
high currents in both outputs to be active at the same time. If this approach is not possible, as in the simple
switch shown in Figure 87, insert 100-Ω resistors inside the loop of each op amp output. A bipolar supply is
shown in Figure 87, but any of the single-supply options are also possible. Any combination of gains can be
implemented, but wide gain ranges show a larger change in signal bandwidth. This approach can be expanded
to more than two gain settings. Testing with the circuit of Figure 87 showed no change in harmonic distortion;
see Figure 18 and Figure 36.
R5 2 k
R4 2 k
Å9S
1-Bit PGA
Gain of 2 or
Gain of 1
VIN
Å9S
OPA837
++
+VS
PD
R1 100 k
+VS
VOUT
This line is
actually
a logic
line
OPA837
Å9S
++
+VS
PD
R2 100 k
VS1 2.5
+VS
+VS
+
VS1 Å2.5
Å9S
+
Copyright © 2017, Texas Instruments Incorporated
Figure 87. 1-Bit PGA
8.2.3.1 Design Requirements
Configure two OPA837 device outputs in different gains when driving the noninverting input with the same input
signal. Select one the two channels using the disable control. Set one channel to a gain of 1 V/V and the second
channel to a gain of 2 V/V using the recommended 2-kΩ values from Table 2.
8.2.3.2 Detailed Design Procedure
The simple design of Figure 87 has several options and details to consider, which include:
• For split-supply operation, the disable control line must operate to within 0.55 V of the negative supply to
disable a channel. A logic level shift is required.
• Any combination of gains can be implemented. However, the signal bandwidths may vary widely through the
gain bandwidth product effect between the two channels if the gains are widely separated. If a more constant
bandwidth between gains is desired, consider adding a fixed RC filter after the combined outputs at a lower
cutoff frequency than the slowest gain setting.
Copyright © 2017, Texas Instruments Incorporated
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