OP-184 Datasheet, PDF (13/20 Pages) Analog Devices – Precision Rail-to-Rail Input & Output Operational Amplifiers

OP184/OP284/OP484
Overdrive Recovery
The overdrive recovery time of an operational amplifier is the
time required for the output voltage to recover to its linear re-
gion from a saturated condition. The recovery time is important
in applications where the amplifier must recover quickly after a
large transient event. The circuit shown in Figure 48 was used
to evaluate the OP284’s overload recovery time. The OP284
takes approximately 2 µs to recover from positive saturation and
approximately 1 µs to recover from negative saturation.
R1
10kΩ
R2
10kΩ
V_IN
10V STEP
+5V
2
8
R3
1/2 ¹
9kΩ
3 ^OP284
4
– 5V
V_OUT
Figure 48. Output Overload Recovery Test Circuit
A Single-Supply, +3 V Instrumentation Amplifier
The OP284’s low noise, wide bandwidth, and rail-to-rail input/
output operation makes it ideal for low supply voltage applica-
tions such as in a two op amp instrumentation amplifier as
shown in Figure 49. The circuit uses the classic two op amp in-
strumentation amplifier topology with four resistors to set the
gain. The transfer equation of the circuit is identical to that of a
noninverting amplifier. Resistors R2 and R3 should be closely
matched to each other as well as to resistors (R1 + P1) and R4
to ensure good common-mode rejection performance. Resistor
networks should be used in this circuit for R2 and R3 because
they exhibit the necessary relative tolerance matching for good
performance. Matched networks also exhibit tight relative resis-
tor temperature coefficients for good circuit temperature stabil-
ity. Trimming potentiometer P1 is used for optimum dc CMR
adjustment, and C1 is used to optimize ac CMR. With the cir-
cuit values as shown, circuit CMR is better than 80 dB over the
frequency range of 20 Hz to 20 kHz. Circuit RTI (Referred-to-
Input) noise in the 0.1 Hz to 10 Hz band is an impressively low
0.45 µV p-p. Resistors RP1 and RP2 serve to protect the
OP284’s inputs against input overvoltage abuse. Capacitor C2
can be included to the limit circuit bandwidth and, therefore,
wide bandwidth noise in sensitive applications. The value of
this capacitor should be adjusted depending on the required
closed-loop bandwidth of the circuit. The R4-C2 time constant
creates a pole at a frequency equal to:
f
(3
dB )
=
2
π
1
R4
C2
RP1
1kΩ
V_IN
RP2
1kΩ
C1
AC CMRR
TRIM
5pF–40pF
+3V
5
3
R3
1
1.1kΩ
6
A1
2
R2
1.1kΩ
R1
9.53kΩ
P1
500Ω
A1, A2 = 1/2 OP284
GAIN = 1 + –R–4–
R3
SET R2 = R3
R1 + P1 = R4
8
7
A2
4
R4
10kΩ
C2
V_OUT
Figure 49. A Single Supply, +3 V Low Noise Instrumenta-
tion Amplifier
A +2.5 V Reference from a +3 V Supply
In many single-supply applications, the need for a 2.5 V refer-
ence often arises. Many commercially available monolithic
2.5 V references require at least a minimum operating supply of
4 V. The problem is exacerbated when the minimum operating
supply voltage is +3 V. The circuit illustrated in Figure 50 is an
example of a +2.5 V reference that operates from a single +3 V
supply. The circuit takes advantage of the OP284’s rail-to-rail
input/output voltage ranges to amplify an AD589’s 1.235 V
output to +2.5 V. The OP284’s low TCV_OS of 1.5 µV/°C helps
maintain an output voltage temperature coefficient that is domi-
nated by the temperature coefficients of R2 and R3. In this
circuit with 100 ppm/°C TCR resistors, the output voltage
exhibits a temperature coefficient of 200 ppm/°C. Lower tempco
resistors are recommended for more accurate performance over
temperature.
One measure of the performance of a voltage reference is its
capacity to recover from sudden changes in load current. While
sourcing a steady-state load current of 1 mA, this circuit recov-
ers to 0.01% of the programmed output voltage in 1.5 µs for a
total change in load current of ± 1 mA.
+3V
R1
17.4kΩ
AD589
+3V
3
8
1/2 1
2 ^OP284
4
0.1µF
+2.5V_REF
R3
100kΩ
R2
P1
100kΩ 5kΩ
RESISTORS = 1%, 100ppm/°C
POTENTIOMETER = 10 TURN, 100ppm/°C
Figure 50. A +2.5 V Reference that Operates on a Single
+3 V Supply
REV. 0
–13–