Datasheet OP184, OP284, OP484 (Analog Devices) - 20

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OP184/OP284/OP484
Obviously, it is desirable to keep this comparison voltage small Notch filters are commonly used to reject power line frequency because it becomes a significant portion of the overal dropout interference that often obscures low frequency physiological voltage. Here, the 20 mV reference is higher than the typical signals, such as heart rates, blood pressure readings, EEGs, and offset of the OP284 but is still reasonably low as a percentage EKGs. This notch filter effectively squelches 60 Hz pickup at a of VOUT (<0.5%). In adapting the limiter for other ILIMIT levels, Filter Q of 0.75. Substituting 3.16 kΩ resistors for the 2.67 kΩ Sense Resistor RS should be adjusted along with R7 to R8, to resistor in the twin-T section (R1 through R5) configures the maintain this threshold voltage between 20 mV and 50 mV. active filter to reject 50 Hz interference. Performance of the circuit is excellent. For the 4.5 V output
R2 3V
version, the measured dc output change for a 225 mA load
2.67k R1 Ω 2.67kΩ C1 C2
change was on the order of a few microvolts, while the dropout
2 4 1µF 1µF A1 1 5
voltage at this same current level was about 30 mV. The current
V 3 A2 7 IN 11 VO 6
limit, as shown in Figure 58, is 400 mA, allowing the circuit to
R3 R4 2.67kΩ 2.67kΩ
be used at levels up to 300 mA or more. While the Q1 device can
R6 10kΩ R7 C3 R5
actually support currents of several amperes, a practical current
1kΩ R8 2µF 1.33kΩ 1k (1µF × 2) (2.68kΩ ÷ 2) Ω
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
R11 10kΩ
at an input level of 5 V causes a 2 W dissipation in Q1, other input
C5 Q = 0.75
conditions must be considered careful y in terms of potential
0.03µF NOTE: FOR 50Hz APPLICATIONS 3V
overheating of Q1. Of course, if higher powered devices are used
CHANGE R1, R2, R3, AND R4 TO 3.1kΩ AND R5 TO 1.58kΩ (3.16kΩ ÷ 2). R12
for Q1, this circuit can support outputs of tens of amperes as
R9 9 150Ω 20kΩ A3 8 C6
wel as the higher V
10
OUT levels already noted.
1µF C4 R10 1.5V
059 The circuit shown can either be used as a standard low dropout
1µF 20kΩ A1, A2, A3 = OP484
00293- regulator, or it can be used with on/off control. By driving Pin 3 Figure 59. A 3 V Single-Supply, 50Hz to 60 Hz Active Notch Filter of U2 with the optional logic control signal, V with False Ground C, the output is switched between on and off. Note that when the output is off Amplifier A3 is the heart of the false ground bias circuit. It buffers in this circuit, it is still active (that is, not an open circuit). This the voltage developed at R9 and R10 and is the reference for the is because the off state simply reduces the voltage input to R1, active notch filter. Because the OP484 exhibits a rail-to-rail input leaving the U1A/U1B amplifiers and Q1 still active. common-mode range, R9 and R10 are chosen to split the 3 V When the on/off control is used, Resistor R10 should be used supply symmetrically. An in-the-loop compensation scheme is with U2 to speed on/off switching and to al ow the output of the used around the OP484 that al ows the op amp to drive C6, a circuit to settle to a nominal zero voltage. Component D3 and 1 μF capacitor, without oscillation. C6 maintains a low impedance Component R11 also aid in speeding up the on/off transition by ac ground over the operating frequency range of the filter. providing a dynamic discharge path for C2. Off/on transition The filter section uses an OP484 in a Twin-T configuration whose time is less than 1 ms, while the on/off transition is longer, but frequency selectivity is very sensitive to the relative matching of less than 10 ms. the capacitors and resistors in the twin-T section. Mylar is the
3 V, 50 HZ/60 HZ ACTIVE NOTCH FILTER WITH
material of choice for the capacitors, and the relative matching
FALSE GROUND
of the capacitors and resistors determines the pass band symmetry of the filter. Using 1% resistors and 5% capacitors produces satis- To process signals in a single-supply system, it is often best to use factory results. 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. Rev. J | Page 20 of 24 Document Outline Features Applications General Description Pin Configurations Table of Contents Revision History Specifications Electrical Characteristics Absolute Maximum Ratings Thermal Resistance ESD Caution Typical Performance Characteristics Applications Information Functional Description Input Overvoltage Protection Output Phase Reversal Designing Low Noise Circuits in Single-Supply Applications Overdrive Recovery Single-Supply, 3 V Instrumentation Amplifier 2.5 V Reference from a 3 V Supply 5 V Only, 12-Bit DAC Swings Rail-to-Rail High-Side Current Monitor Capacitive Load Drive Capability Low Dropout Regulator with Current Limiting 3 V, 50 Hz/60 Hz Active Notch Filter with False Ground Outline Dimensions Ordering Guide