OP184/OP284/OP484

Rev. J | Page 17 of 24

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

( )

2

4

2

1

3

C

R

dB

f

π

=

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.

VOUT

5

6

7

3V

A1, A2 = 1/2 OP284

GAIN = 1 +

R4

R3

SET R2 = R3

R1 + P1 = R4

8

4

C2

RP1

1k

Ω

RP2

1k

Ω

R1

9.53k

Ω

R2

1.1k

Ω

R3

1.1k

Ω

R4

10k

Ω

P1

500

Ω

3

2

1

VIN

A1

+

–

A2

C1

AC CMRR

TRIM

5pF TO 40pF

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.

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

3

2

1

3V

8

4

R3

100k

Ω

R2

100k

Ω

P1

5k

Ω

R1

17.4k

Ω

3V

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, 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.

3

2

1

5V

5V

8

4

R3

232

Ω

1%

R2

32.4

Ω

1%

R1

17.8k

Ω

R4

100k

Ω

1%

AD589

GND CLK SR1

LD

VREF

RRB

VDD

IOUT

1.23V

4

8

2

1

3

DAC8043

DIGITAL

CONTROL

7

6

5

1/2

OP284

VOUT =

D

4096

(5V)

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.