OPA2832 datasheet(23/37 Pages) NSC

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374

Ω

+5V

1/2

OPA2832

1/2

OPA2832

2.2nF

µF

2.2nF

374

Ω

750

Ω

S/2

750

Ω

400

Ω

400

Ω

−1

−2

−3

−4

−5

−6

−7

−8

−9

−10

−11

−12

Frequency (Hz)

100k

10k

10M

HIGH-PASS FILTERS

−3

−6

−9

−12

Frequency (MHz)

0.01

0.1

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

shows

the

frequency response for the filter of Figure 67.

Figure 69. 138kHz, 2nd-Order, High-Pass Filter

Results showing the frequency response for the

circuit of Figure 69 is shown in Figure 70.

Figure 68. 5MHz, 2nd-Order, Butterworth

Low-Pass Filter

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

Figure 70. Frequency Response for the Filter of

resistors

forming

the

divider

network,

but

Figure 69

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.

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