OP184_06 Datasheet, PDF (20/24 Pages) Analog Devices – Precision Rail-to-Rail Input and Output Operational Amplifiers

OP184/OP284/OP484
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 VOUT (<0.5%). In adapting the limiter for other ILIMIT levels,
Sense Resistor RS 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 micro volts, while the dropout
voltage at this same current level was about 30 mV. The current
limit, as shown, 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 the SOIC-8
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 should 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 VOUT levels already noted.
The circuit shown can be used as either 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, VC, 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 U1 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.
3V
R1
2.67kΩ
C1
2
4
1µF
R2
2.67kΩ
C2
1µF
A1
1
V_IN
3
11
R3
2.67kΩ
R4
2.67kΩ
5
A2 7
V_O
6
R6
10kΩ
C3
2µF
(1µF × 2)
R5
1.33kΩ
R7
R8
1kΩ
(2.68kΩ ÷ 2) ^1kΩ
R11
10kΩ
C5
Q = 0.75
0.03µF
NOTE: FOR 50Hz APPLICATIONS
3V
CHANGE R1, R2, R3, AND R4 TO 3.1kΩ
R9
9
20kΩ
A3
8
10
R12
150Ω
AND R5 TO 1.58kΩ (3.16kΩ ÷ 2).
C6
1µF
C4
R10
1.5V
1µF
20kΩ
A1, A2, A3 = OP484
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 satisfactory results.
Rev. D | Page 20 of 24