OP184_06 Datasheet, PDF (17/24 Pages) Analog Devices – Precision Rail-to-Rail Input and Output Operational Amplifiers

OP184/OP284/OP484
SINGLE-SUPPLY, 3 V INSTRUMENTATION
AMPLIFIER
The low noise, wide bandwidth, and rail-to-rail input/output
operation of the OP284 make it ideal for low supply voltage
applications such as in the two op amp instrumentation
amplifier shown in Figure 52. The circuit uses the classic two
op amp instrumentation amplifier topology with four resistors
to set the gain. The transfer equation of the circuit is identical to
that of a noninverting amplifier. Resistor R2 and Resistor R3
should be closely matched to each other, as well as to Resistors
(R1 + P1) and Resistor 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 resistor temperature coefficients for
good circuit temperature stability. Trimming Potentiometer P1
is used for optimum dc CMR adjustment, and C1 is used to
optimize ac CMR. With the circuit 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. Resistor RP1 and
Resistor RP2 serve to protect the OP284 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 to C2 time constant creates a pole at a
frequency equal to
f (3 dB) = 1
2π R4 C2
2.5 V REFERENCE FROM A 3 V SUPPLY
In many single-supply applications, the need for a 2.5 V
reference 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 53 is an
example of a 2.5 V reference that operates from a single 3 V
supply. The circuit takes advantage of the OP284 rail-to-rail
input/output voltage ranges to amplify an AD589 1.235 V
output to 2.5 V.
+
V_IN
–
RP1
1kΩ
RP2
1kΩ
C1
AC CMRR
TRIM
5pF TO 40pF
3V
5
8
3
1
A1
2
R2
1.1kΩ
R3
1.1kΩ
7
A2
6
4
R4
10kΩ
R1
9.53kΩ
C2
A1, A2 = 1/2 OP284
P1
500Ω
GAIN
=
1
+
R4
R3
SET R2 = R3
R1 + P1 = R4
V_OUT
The low TCVOS of the OP284 at 1.5 μV/°C helps maintain an
output voltage temperature coefficient that is dominated 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 recovers
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
OP284
1
2
4
0.1µF
2.5V_REF
R3
100kΩ
R2
P1
100kΩ
5kΩ
RESISTORS = 1%, 100ppm/°C
POTENTIOMETER = 10 TURN, 100ppm/°C
Figure 53. 2.5 V Reference That Operates on a Single 3 V Supply
5 V ONLY, 12-BIT DAC SWINGS RAIL-TO-RAIL
The OP284 is ideal for use with a CMOS DAC to generate a
digitally controlled voltage with a wide output range. Figure 54
shows a DAC8043 used in conjunction with the AD589 to gen-
erate a voltage output from 0 V to 1.23 V. The DAC is actually
operating in voltage switching mode, where the reference is
connected to the current output, IOUT, and the output voltage is
taken from the VREF pin. This topology is inherently noninverting,
as opposed to the classic current output mode, which is
inverting and not usable in single-supply applications.
5V
R1
17.8kΩ
1.23V
AD589
8
V_DD
R_RB ²
3
I_OUT
DAC8043 ^VREF ¹
GND CLK SR1 LD
4765
DIGITAL
CONTROL
5V
3
8
1/2
2 ^OP284
1
4
D
V_OUT = ₄₀₉₆ (5V)
R3
R2
232Ω 32.4Ω
1% 1%
R4
100kΩ
1%
Figure 54. 5 V Only, 12-Bit DAC Swings Rail-to-Rail
In this application the OP284 serves two functions. First, it
buffers the high output impedance of the DAC’s VREF pin, which
is on the order of 10 kΩ. The op amp provides a low impedance
output to drive any following circuitry.
Figure 52. Single Supply, 3 V Low Noise Instrumentation Amplifier
Rev. D | Page 17 of 24