OP90

–8–

Single-Supply Output Voltage Range

In single-supply operation, the OP90’s input and output ranges

include ground. This allows true “zero-in, zero-out” operation.

The output stage provides an active pull-down to around 0.8 V

above ground. Below this level, a load resistance of up to 1 M

Ω

to ground is required to pull the output down to zero.

In the region from ground to 0.8 V, the OP90 has voltage gain

equal to the data sheet specification. Output current source

capatibility is maintained over the entire voltage range includ-

ing ground.

APPLICATIONS

Battery-Powered Voltage Reference

The circuit of Figure 6 is a battery-powered voltage reference

that draws only 17

µA of supply current. At this level, two AA

cells can power this reference over 18 months. At an output voltage

of 1.23 V @ 25

°C, drift of the reference is only at 5.5 µV/°C over

the industrial temperature range. Load regulation is 85

µV/mA

with line regulation at 120

µV/V.

Design of the reference is based on the bandgap technique.

Scaling of resistors R1 and R2 produces unequal currents in Q1

proportional voltage across R3 which, in turn, produces a larger

temperature-proportional voltage across R4 and R5. This volt-

opposite temperature coefficient. Adjusting the output to l.23 V

at 25

°C produces minimum drift over temperature. Bandgap

references can have start-up problems. With no current in R1

and R2, the OP90 is beyond its positive input range limit and

has an undefined output state. Shorting Pin 5 (an offset adjust

pin) to ground, forces the output high under these conditions

and ensures reliable start-up without significantly degrading the

OP90’s offset drift.

4

2

3

5

6

7

OP90

R1

240k

R2

1.5M

C1

1000pF

V+

(2.5V TO 36V)

(1.23V @ 25 C)

6

5

7

3

2

1

R3

68k

R4

130k

R5

20k

OUTPUT

ADJUST

MAT-01AH

Figure 6. Battery-Powered Voltage Reference

HOURS

4

3

0

0

2000

7000

4000

2

1

1000

3000

6000

5000

Figure 3. Lithium Sulphur Dioxide Cell Discharge

Characteristic with OP90 and 100 k

Ω Load

Input Voltage Protection

The OP90 uses a PNP input stage with protection resistors in

series with the inverting and noninverting inputs. The high

breakdown of the PNP transistors coupled with the protection

resistors provides a large amount of input protection, allowing

the inputs to be taken 20 V beyond either supply without dam-

aging the amplifier.

Offset Nulling

The offset null circuit of Figure 4 provides 6 mV of offset adjust-

ment range. A 100 k

Ω resistor placed in a series with the wiper

of the offset null potentiometer, as shown in Figure 5, reduces

the offset adjustment range to 400

µV and is recommended for

applications requiring high null resolution. Offset nulling does not

TEST CIRCUITS

V+

1

2

3

5

6

7

OP90

4

100k

V–

Figure 4. Offset Nulling Circuit

V+

1

2

3

5

6

7

OP90

4

100k

V–

100k

Figure 5. High Resolution Offset Nulling Circuit

REV. C