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OPA2835 Datasheet, PDF (33/53 Pages) National Semiconductor (TI) – Ultra Low-Power, Rail-to-Rail Out, Negative Rail In, VFB Op Amp
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OPA835
OPA2835
SLOS713E – JANUARY 2011 – REVISED JULY 2013
Low Power Applications and the Effects of Resistor Values on Bandwidth
The OPA835 and OPA2835 are designed for the nominal value of RF to be 2 kΩ in gains other than +1. This
gives excellent distortion performance, maximum bandwidth, best flatness, and best pulse response. It also loads
the amplifier. For example; in gain of 2 with RF = RG = 2 kΩ, RG to ground, and VOUT = 4 V, 1mA of current will
flow through the feedback path to ground. In gain of +1, RG is open and no current will flow to ground. In low
power applications, it is desirable to reduce this current by increasing the gain setting resistors values. Using
larger value gain resistors has two primary side effects (other than lower power) due to their interaction with
parasitic circuit capacitance.
1. Lowers the bandwidth.
2. Lowers the phase margin
(a) This will cause peaking in the frequency response.
(b) And will cause over shoot and ringing in the pulse response.
Figure 69 shows the small signal frequency response on OPA835EVM for non-inverting gain of 2 with RF and RG
equal to 2 kΩ, 10 kΩ, and 100 kΩ. The test was done with RL = 2 kΩ. Due to loading effects of RL, lower values
may reduce the peaking, but higher values will not have a significant effect.
21
VS = 5 V,
18 VOUT = 100 Vpp,
G = 2,
15 RL = 2 kW
12
RF = 100 kW
RF = 10 kW
9
RF = 2 kW
6
3
RF = 100 kW
CF = 1 pF
0
RF = 10 kW
-3
CF = 1 pF
-6
-9
0
1
10
100
f - Frequency - MHz
Figure 69. Frequency Response with Various Gain Setting Resistor Values
As expected, larger value gain resistors cause lower bandwidth and peaking in the response (peaking in
frequency response is synonymous with overshoot and ringing in pulse response). Adding 1 pF capacitors in
parallel with RF helps compensate the phase margin and restores flat frequency response. Figure 70 shows the
test circuit used.
VIN
RG
OPA 835
VOUT
2 kW
RF
CF
Figure 70. G=2 Test Circuit for Various Gain Setting Resistor Values
Copyright © 2011–2013, Texas Instruments Incorporated
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