OP484ESZ datasheet(17/24 Pages) AD

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OP184/OP284/OP484

Rev. D | Page 17 of 24

The low TCV

SINGLE-SUPPLY, 3 V INSTRUMENTATION

AMPLIFIER

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.

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

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.

2.5VREF

100kΩ

5kΩ

17.4kΩ

0.1µF

AD589

1/2

OP284

–

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, I

()

2.5 V REFERENCE FROM A 3 V SUPPLY

OUT

, 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.

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

232Ω

32.4Ω

17.8kΩ

100kΩ

AD589

GND CLK SR1 LD

VREF

RRB

VDD

IOUT

1.23V

DAC8043

DIGITAL

CONTROL

765

1/2

OP284

VOUT =

4096 (5V)

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.

VOUT

A1, A2 = 1/2 OP284

GAIN = 1 +

SET R2 = R3

R1 + P1 = R4

RP1

1kΩ

RP2

1kΩ

9.53kΩ

1.1kΩ

10kΩ

500Ω

VIN

–

AC CMRR

TRIM

5pF TO 40pF

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

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