OPA2832 Datasheet, PDF (23/37 Pages) National Semiconductor (TI) – Dual, Low-Power, High-Speed, Fixed-Gain Operational Amplifier

OPA2832
www.ti.com ............................................................................................................................................. SBOS327C – FEBRUARY 2005 – REVISED AUGUST 2008
Implementing the DC bias in this way also attenuates
the differential signal by half. This is recovered by
setting the amplifier gain at 2V/V to get a net
unity-gain filter characteristic from input to output. The
filter design shown here has also adjusted the
resistor values slightly from an ideal analysis to
account for the 100MHz bandwidth in the amplifier
stages. The filter capacitors at the noninverting inputs
are shown as two separate capacitors to ground.
While it is certainly correct to collapse these two
capacitors into a single capacitor across the two
inputs (which would be 50pF for this circuit) to get the
same differential filtering characteristic, tests have
shown two separate capacitors to a low impedance
point act to attenuate the common-mode feedback
present in this circuit giving more stable operation in
actual implementation. Figure 68 shows the
frequency response for the filter of Figure 67.
+V_S
+5V
374Ω
2.2nF 2.2nF
750Ω
2kΩ
1/2
OPA2832
400Ω
1µF
V_I
V_S/2
V_O
2kΩ
400Ω
750Ω
2.2nF 2.2nF
1/2
OPA2832
0
−1
−2
−3
−4
−5
−6
−7
−8
−9
−10
−11
−12
1k
10k
100k
1M
10M
Frequency (Hz)
Figure 68. 5MHz, 2nd-Order, Butterworth
Low-Pass Filter
HIGH-PASS FILTERS
Another approach to mid-supply biasing is shown in
Figure 69. This method uses a bypassed divider
network in place of the buffer used in Figure 67. The
impedance is set by the parallel combination of the
resistors forming the divider network, but as
frequency increases it looks more and more like a
short due to the capacitor. Generally, the capacitor
value needs to be two to three orders of magnitude
greater than the filter capacitors shown for the circuit
to work properly.
374Ω
Figure 69. 138kHz, 2nd-Order, High-Pass Filter
Results showing the frequency response for the
circuit of Figure 69 is shown in Figure 70.
3
0
−3
−6
−9
−12
0.01
0.1
1
10
Frequency (MHz)
Figure 70. Frequency Response for the Filter of
Figure 69
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