OP191 datasheet(17/20 Pages) AD

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OP191/OP291/OP491

REV. 0

–17–

A +3 V, 50 Hz/60 Hz Active Notch Filter with False Ground

To process ac signals in a single-supply system, it is often best to

use a false-ground biasing scheme. A circuit that uses this

approach is illustrated in Figure 66. In this circuit, a false-

ground circuit biases an active notch filter used to reject 50 Hz/

60 Hz power line interference in portable patient monitoring

equipment. Notch filters are quite commonly used to reject

power line frequency interference which often obscures low

frequency physiological signals, such as heart rates, blood

pressure readings, EEGs, EKGs, etcetera. This notch filter

effectively squelches 60 Hz pickup at a filter Q of 0.75. Substi-

tuting 3.16 k

Ω resistors for the 2.67 kΩ resistors in the twin-T

section (R1 through R5) configures the active filter to reject

50 Hz interference.

R11

100k

Ω

OUT

2.67k

Ω

2.67k

Ω

1/4

OP491

+3V

100k

Ω

µF

µFx2)

1/4

OP491

0.01

µF

R12

499

Ω

1.5V

µF

+3V

Ω

R10

Ω

µF

1/4

OP491

µF

2.67k

Ω

1.33k

Ω

(2.67k

Ω÷2)

Ω

µF

2.67k

Ω

Figure 66. A +3 V Single-Supply, 50 Hz/60 Hz Active Notch

Filter with False Ground

Amplifier A3 is the heart of the false-ground bias circuit. It

simply buffers the voltage developed by R9 and R10 and is the

reference for the active notch filter. Since the OP491 exhibits a

rail-to-rail input common-mode range, R9 and R10 are chosen

to split the +3 V supply symmetrically. An in-the-loop compen-

sation scheme is used around the OP491 that allows the op amp

to drive C6, a 1

µF capacitor, without oscillation. C6 maintains

a low impedance ac ground over the operating frequency range

of the filter.

The filter section uses a pair of OP491s in a twin-T configura-

tion whose frequency selectivity is very sensitive to the relative

matching of the capacitors and resistors in the twin-T section.

Mylar is the material of choice for the capacitors, and the

relative matching of the capacitors and resistors determines the

filter’s passband symmetry. Using 1% resistors and 5% capaci-

tors produces satisfactory results.

Single-Supply Half-Wave and Full-Wave Rectifiers

An OP191 family configured as a voltage follower operating on

a single supply can be used as a simple half-wave rectifier in

low-frequency (<2 kHz) applications. A full-wave rectifier can

be configured with a pair of OP291s as illustrated in Figure 67.

The circuit works in the following way: When the input signal is

above 0 V, the output of amplifier A1 follows the input signal.

Since the noninverting input of amplifier A2 is connected to

A1’s output, op amp loop control forces the A2’s inverting input

to the same potential. The result is that both terminals of R1

are equipotential; i.e., no current flows. Since there is no

current flow in R1, the same condition exists upon R2; thus, the

output of the circuit tracks the input signal. When the input

signal is below 0 V, the output voltage of A1 is forced to 0 V.

This condition now forces A2 to operate as an inverting voltage

follower because the noninverting terminal of A2 is at 0 V as

well. The output voltage at VOUTA is then a full-wave rectified

version of the input signal. If needed, a buffered, half-wave

rectified version of the input signal is available at VOUTB.

100

200

µs

500mV

(1V/DIV)

OUT B

(0.5V/DIV)

OUT A

(0.5V/DIV)

TIME – 200

µs/DIV

100k

Ω

1/2

OP291

+5V

2Vpp

<2kHz

100k

Ω

1/2

OP291

OUT A

OUT B

FULL-WAVE

RECTIFIED

OUTPUT

HALF-WAVE

RECTIFIED

OUTPUT

500mV

Figure 67. Single-Supply Half-Wave and Full-Wave

Rectifiers Using an OP291

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