OPA2836-Q1 Datasheet, PDF (23/44 Pages) Texas Instruments – Very-Low Power, Rail-to-Rail Out, Negative-Rail In, Voltage-Feedback Operational Amplifier

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OPA2836-Q1
SBOS794 – DECEMBER 2016
Feature Description (continued)
Figure 53 shows the small-signal frequency response for a noninverting gain of 2 with R_F and R_G equal to 1 kΩ,
10 kΩ, and 100 kΩ. The test was done with R_L = 1 kΩ. Lower values can reduce the peaking because of loading
effects of R_L, but higher values do not have a significant effect.
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21
V_S = 5 V,
V_OUT = 100
mVpp,
18 ^{G = 2,}
R_L = 1 kW
15
R_F = 100 kW
12
R_F = 100 kW
9
C_F = 1 pF
R_F = 10 kW
R_F = 10 kW
C_F = 1 pF
6
R_F = 1 kW
3
0
-3
-6
-9
0
1
10
100
1000
f - Frequency - MHz
Figure 53. Frequency Response With Various Gain-Setting Resistor Values
As expected, larger value gain resistors cause lower bandwidth and peaking in the response (peaking in the
frequency response is synonymous with overshoot and ringing in pulse response). Adding 1-pF capacitors in
parallel with R_F helps compensate the phase margin and restores flat frequency response. Figure 54 shows the
test circuit used.
V_IN
R_G
OPA2836-Q1
V_OUT
1 kW
R_F
C_F
Copyright © 2016, Texas Instruments Incorporated
Figure 54. G = 2 Test Circuit for Various Gain-Setting Resistor Values
8.3.4 Driving Capacitive Loads
The OPA2836-Q1 can drive up to a nominal capacitive load of 2.2 pF on the output with no special
consideration. When driving capacitive loads greater than this load, using a small resister (R_O) in series with the
output as close to the device as possible is recommended. Without R_O, the capacitance on the output interacts
with the output impedance of the amplifier causing phase shift in the loop gain of the amplifier that reduces the
phase margin. This scenario causes peaking in the frequency response and overshoot and ringing in the pulse
responses. Interaction with other parasitic elements can lead to instability or oscillation. Inserting R_O isolates the
phase shift from the loop-gain path and restores the phase margin; however, the bandwidth is also limited.
Copyright © 2016, Texas Instruments Incorporated
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