OP291GBC AD [Analog Devices], OP291GBC Datasheet - Page 17

• Manufacturer Part Number: OP291GBC
• Description: Micropower Single-Supply Rail-to-Rail Input/Output Op Amps
• Manufacturer: AD [Analog Devices]

REV. 0
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 resistors for the 2.67 k resistors in the twin-T
section (R1 through R5) configures the active filter to reject
50 Hz interference.
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 F 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.
V
1 F
C4
IN
1M
R9
2
3
R6
100k
+3V
OP491
1/4
R10
1M
+3V
10
11
4
9
100k
A1
R11
OP491
1
1/4
0.01 F
(1 Fx2)
C5
A3
R1
2.67k
2.67k
R3
2 F
C3
8
1 F
C1
499
R12
2.67k
R5
1.33k
(2.67k
R2
C6
1.5V
1 F
1 F
C2
2.67k
R4
2)
5
6
R8
1k
OP491
1/4
1k
R7
A2
7
V
OUT
–17–
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 V
version of the input signal. If needed, a buffered, half-wave
rectified version of the input signal is available at V
Figure 67. Single-Supply Half-Wave and Full-Wave
Rectifiers Using an OP291
<2kHz
(0.5V/DIV)
(0.5V/DIV)
2Vpp
(1V/DIV)
V
V
V
IN
OUT
OUT
V
IN
B
A
100
90
10
0%
3
2
OP291
1V
1/2
+5V
8
4
500mV
A1
100k
R1
1
TIME – 200 s/DIV
OP191/OP291/OP491
500mV
OUT
6
5
A is then a full-wave rectified
OP291
1/2
100k
R2
200 s
A2
7
V
V
OUT
OUT
OUT
A
FULL-WAVE
RECTIFIED
OUTPUT
HALF-WAVE
RECTIFIED
OUTPUT
B
B.