OP-184 Datasheet, PDF (16/20 Pages) Analog Devices – Precision Rail-to-Rail Input & Output Operational Amplifiers

OP184/OP284/OP484
Obviously, it is desirable to keep this comparison voltage small,
since it becomes a significant portion of the overall dropout
voltage. Here, the 20 mV reference is higher than the typical
offset of the OP284 but still reasonably low as a percentage of
V_OUT (< 0.5%). In adapting the limiter for other I_LIMIT levels,
sense resistor R_S should be adjusted along with R7-R8, to main-
tain this threshold voltage between 20 mV and 50 mV.
Performance of the circuit is excellent. For the 4.5 V output
version, the measured dc output change for a 225 mA load
change was on the order of a few microvolts while the dropout
voltage at this same current level was about 30 mV. The current
limit as shown is 400 mA, which allows the circuit to be used at
levels up to 300 mA or more. While the Q1 device can actually
support currents of several amperes, a practical current rating
takes into account the SO-8 device’s 2.5 W, 25°C dissipation.
Because a short circuit current of 400 mA at an input level of 5
V will cause a 2 W dissipation in Q1, other input conditions
should be considered carefully in terms of Q1’s potential over-
heating. Of course, if higher powered devices are used for Q1,
this circuit can support outputs of tens of amperes as well as the
higher V_OUT levels noted above.
The circuit shown can be used either as a standard low dropout
regulator, or it can be used with ON/OFF control. By
driving Pin 3 of U1 with the optional logic control signal V_C, the
output is switched between ON and OFF. Note that when the
output is OFF in this circuit, it is still active (i.e., not an open cir-
cuit). This is because the OFF state simply reduces the voltage
input to R1, leaving the U1A/B amplifiers and Q1 still active.
When ON/OFF control is used, resistor R10 should be used
with U1 to speed ON-OFF switching and to allow the output of
the circuit to settle to a nominal zero voltage. Components D3
and R11 also aid in speeding up the ON-OFF transition by pro-
viding a dynamic discharge path for C2. OFF-ON transition
time is less than 1 ms, while the ON-OFF transition is longer
but under 10 ms.
A +3 V, 50 Hz/60 Hz Active Notch Filter with False Ground
To process 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 56. 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 equip-
ment. 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 read-
ings, EEGs, EKGs, etc. This notch filter effectively squelches
60 Hz pickup at a filter Q of 0.75. Substituting 3.16 kΩ resis-
tors for the 2.67 kΩ in the twin-T section (R1 through R5)
configures the active filter to reject 50 Hz interference.
R2
2.67kΩ
+3V
R1
2.67kΩ
C1
C2
2
4
1µF
1µF
A1 1
V_IN
3
11
R3
2.67kΩ
R4
2.67kΩ
5
A2 7
VO
6
R6
10kΩ
C3
2µF
(1µF x 2)
R5
1.33kΩ
R7
R8
1kΩ
(2.67kΩ ÷ 2) 1kΩ
R11
10kΩ
C5
Q = 0.75
0.03µF
+3V
NOTE: FOR 50Hz APPLICATIONS
CHANGE R1–R4 TO 3.1kΩ
R9
9
R12
150Ω
AND R5 TO 1.58kΩ (3.16kΩ ÷ 2).
20kΩ
A3 8
10
1.5V
C6
C4
R10
1µF
1µF
20kΩ
A1, A2, A3 = OP484
Figure 56. A +3 V Single Supply, 50/60 Hz Active Notch
Filter with False Ground
Amplifier A3 is the heart of the false-ground bias circuit. It
simply buffers the voltage developed at R9 and R10 and is the
reference for the active notch filter. Since the OP484 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 OP484 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 OP484 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 filter’s pass band
symmetry. Using 1% resistors and 5% capacitors produces
satisfactory results.
–16–
REV. 0