OP191_15 Datasheet, PDF (22/24 Pages) Analog Devices – Micropower Single-Supply Rail-to-Rail Input/Output Op Amps

OP191/OP291/OP491
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. 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.
R2
2.67kΩ
3V
2 ¹¹
V_IN
1/4
OP491
1
3 _{4 A1}
R1
2.67kΩ
C1
1μF
R3
2.67kΩ
R6
100kΩ
C3
2μF
(1μF × 2)
R11
100kΩ
C5
C2
1μF
R4
2.67kΩ
5
1/4
OP491
V_OUT
7
6
A2
R5
1.33kΩ
(2.67kΩ ÷ 2)
R8
1kΩ
R7
1kΩ
3V
0.01μF
R9
1MΩ
9
R12
1/4
499Ω
OP491 ⁸
C6
C4
10
A3
1.5V
1μF
1μF
R10
1MΩ
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.
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.
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.
R1
100kΩ
R2
100kΩ
V_IN
2V p-p
<2kHz
5V
3
8
1/2
OP291 ¹
2
4 A1
1V
V_IN
(1V/DIV) 100
90
6
1/2
OP291 ⁷
5
A2
500mV
V_OUTA
FULL-WAVE
RECTIFIED
OUTPUT
V_OUTB
HALF-WAVE
RECTIFIED
OUTPUT
V_OUTA
(0.5V/DIV)
10
0%
V_OUTB
(0.5V/DIV)
500mV
200μs
TIME (200μs/DIV)
Figure 73. Single-Supply, Half-Wave, and Full-Wave Rectifiers
Using an OP291
Rev. E | Page 22 of 24