Datasheet

OP184/OP284/OP484
Rev. J | Page 20 of 24
Obviously, it is desirable to keep this comparison voltage small
because 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 is 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 to R8, to
maintain 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 in Figure 58, is 400 mA, allowing 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 2.5 W, 25°C dissipation of the
8-lead SOIC device. Because a short-circuit current of 400 mA
at an input level of 5 V causes a 2 W dissipation in Q1, other input
conditions must be considered carefully in terms of potential
overheating of Q1. 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 already noted.
The circuit shown can either be used as a standard low dropout
regulator, or it can be used with on/off control. By driving Pin 3
of U2 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 (that is, not an open circuit). This
is because the off state simply reduces the voltage input to R1,
leaving the U1A/U1B amplifiers and Q1 still active.
When the on/off control is used, Resistor R10 should be used
with U2 to speed on/off switching and to allow the output of the
circuit to settle to a nominal zero voltage. Component D3 and
Component R11 also aid in speeding up the on/off transition by
providing a dynamic discharge path for C2. Off/on transition
time is less than 1 ms, while the on/off transition is longer, but
less than 10 ms.
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
shown in Figure 59. 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 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
resistor in the twin-T section (R1 through R5) configures the
active filter to reject 50 Hz interference.
00293-059
R2
2.67kΩ
R6
10kΩ
R7
1kΩ
R8
1kΩ
R11
10kΩ
R9
20kΩ
R12
150Ω
R10
20kΩ
1
3
5
6
7
11
2
3V
V
O
V
IN
A2
A1
8
A3
4
10
9
3V
A1, A2, A3 = OP484
Q = 0.75
NOTE: FOR 50Hz APPLICATIONS
CHANGE R1, R2, R3, AND R4 TO 3.1kΩ
AND R5 TO 1.58kΩ (3.16kΩ ÷ 2).
R3
2.67kΩ
R1
2.67kΩ
R4
2.67kΩ
R5
1.33kΩ
(2.68kΩ ÷ 2)
C3
2µF
(1µF × 2)
C5
0.03µF
C1
1µF
C2
1µF
C4
1µF
C6
1µF
1.5V
Figure 59. A 3 V Single-Supply, 50Hz to 60 Hz Active Notch Filter
with False Ground
Amplifier A3 is the heart of the false ground bias circuit. It buffers
the voltage developed at R9 and R10 and is the reference for the
active notch filter. Because 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 compensation 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 an 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 pass band symmetry
of the filter. Using 1% resistors and 5% capacitors produces satis-
factory results.
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