REV. 0

–13–

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.

2

3

1

+5V

1/2

OP284

8

4

R3

9k

Ω

R1

10k

Ω

R2

10k

Ω

10V STEP

–5V

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 an two op amp instrumentation amplifier as

shown in Figure 49. The circuit utilizes 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. Resistors R2 and R3 should be closely

matched to each other as well as 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)

=

1

2

π R4 C2

R3

1.1k

Ω

5

6

7

+3V

A2

A1, A2 = 1/2 OP284

GAIN = 1 + –––

R4

R3

SET R2 = R3

R1 + P1 = R4

8

4

R4

10k

Ω

C2

RP1

1k

Ω

RP2

1k

Ω

R2

1.1k

Ω

R1

9.53k

Ω

P1

500

Ω

3

2

1

C1

AC CMRR

TRIM

5pF–40pF

A1

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

µV/°C helps

to maintain an output voltage temperature coefficient which 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

capability 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.

P1

5k

Ω

3

2

1

+3V

1/2

OP284

8

4

R2

100k

Ω

R3

100k

Ω

+3V

0.1µF

R1

17.4k

Ω

AD589

RESISTORS = 1%, 100ppm/

°C

POTENTIOMETER = 10 TURN, 100ppm/

°C

Figure 50. A +2.5 V Reference that Operates on a Single

+3 V Supply