OP90 Datasheet, PDF (8/12 Pages) Analog Devices – Precision Low-Voltage Micropower Operational Amplifier

OP90
4
3
2
1
0
0 1000 2000 3000 4000 5000 6000 7000
HOURS
Figure 3. Lithium Sulphur Dioxide Cell Discharge
Characteristic with OP90 and 100 kΩ 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 kΩ 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
affect TCV_OS performance.
TEST CIRCUITS
V+
2
7
6
OP90
3
4
5
1
100k⍀
V–
Figure 4. Offset Nulling Circuit
V+
2
7
6
OP90
3
4
5
1
100k⍀
100k⍀
V–
Figure 5. High Resolution Offset Nulling Circuit
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 MΩ
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
and Q2. The resulting V_BE mismatch creates a temperature
proportional voltage across R3 which, in turn, produces a larger
temperature-proportional voltage across R4 and R5. This volt-
age appears at the output added to the V_BE of Q1, which has an
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.
C1
R1
1000pF 240k⍀
R2
1.5M⍀
2
7
6
OP90
3
5
4
V+
(2.5V TO 36V)
V_OUT
(1.23V @ 25؇C)
1 ^MAT-01AH
2
3
R3
68k⍀
R5
20k⍀
OUTPUT
ADJUST
R4
130k⍀
7
6
5
Figure 6. Battery-Powered Voltage Reference
–8–
REV. A