OP284_15 Datasheet, PDF (17/24 Pages) Analog Devices – Precision Rail-to-Rail Input and Output Operational Amplifiers

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 referred-to-input
(RTI) 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)
=
2π
1
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.
RP1
1kΩ
+
3V
V_IN
–
RP2
1kΩ
C1
AC CMRR
TRIM
5pF TO 40pF
5
8
3
R3
1
1.1kΩ
A2
6
7
A1
2
4
R2
1.1kΩ
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
Figure 52. Single Supply, 3 V Low Noise Instrumentation Amplifier
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 tempera-
ture coefficient of 200 ppm/°C. Lower tempco resistors are
recommended for more accurate performance over temperature.
OP184/OP284/OP484
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
1
2 ^OP284
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 VREF pin, which
is on the order of 10 kΩ. The op amp provides a low impedance
output to drive any following circuitry.
Second, the op amp amplifies the output signal to provide a rail-
to-rail output swing. In this particular case, the gain is set to 4.1
so that the circuit generates a 5 V output when the DAC output
is at full scale. If other output voltage ranges are needed, such as
0 V ≤ VOUT ≤ 4.095 V, the gain can be easily changed by adjusting
the values of R2 and R3.
Rev. J | Page 17 of 24