OP191 datasheet(22/24 Pages) AD

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OP191/OP291/OP491

Rev. E | Page 22 of 24

3 V, 50 HZ/60 HZ ACTIVE NOTCH FILTER WITH

FALSE GROUND

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

configuration 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 pass band symmetry of the filter. Using

1% resistors and 5% capacitors produces satisfactory results.

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

to use a false ground biasing scheme. Figure 72 illustrates a

circuit that uses this approach. 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 that often obscures low

frequency physiological signals, such as heart rates, blood

pressure readings, EEGs, and EKGs. This notch filter effectively

squelches 60 Hz pickup at a filter Q of 0.75. Substituting

3.16 kΩ resistors for the 2.67 kΩ resistors in the twin-T section

(R1 through R5) configures the active filter to reject 50 Hz

interference.

SINGLE-SUPPLY, HALF-WAVE, AND FULL-WAVE

RECTIFIERS

An OPx91 device 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 73.

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

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

Because the noninverting input of Amplifier A2 is connected to

the output of A1, op amp loop control forces the inverting input

of the A2 to the same potential. The result is that both terminals

of R1 are equipotential; that is, no current flows. Because there

is no current flow in R1, the same condition exists for 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 also at 0 V.

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.

R11

100kΩ

VOUT

2.67kΩ

1/4

OP491

VIN

100kΩ

0.01μF

R12

499Ω

1.5V

1μF

1MΩ

R10

1MΩ

1μF

2.67kΩ

1.33kΩ

(2.67kΩ ÷ 2)

1kΩ

1μF1μF

2.67kΩ

1/4

OP491

1/4

OP491

2μF

(1μF × 2)

(1V/DIV)

(0.5V/DIV)

TIME (200μs/DIV)

100kΩ

VIN

2V p-p

<2kHz

100kΩ

1/2

OP291

VOUTA

VOUTB

FULL-WAVE

RECTIFIED

OUTPUT

HALF-WAVE

RECTIFIED

OUTPUT

1/2

OP291

500mV

VOUTA

VIN

VOUTB

100

500mV

200μs

Figure 72. 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 buffers the voltage developed by R9 and R10 and is the

reference for the active notch filter. Because 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

compensation scheme used around the OP491 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.

Figure 73. Single-Supply, Half-Wave, and Full-Wave Rectifiers

Using an OP291

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