OP90 Datasheet, PDF (9/12 Pages) Analog Devices – Precision Low-Voltage Micropower Operational Amplifier

OP90
Single Op Amp Full-Wave Rectifier
Figure 7 shows a full-wave rectifier circuit that provides the
absolute value of input signals up to ±2.5 V even though operated
from a single 5 V supply. For negative inputs, the amplifier acts
as a unity-gain inverter. Positive signals force the op amp output
to ground. The 1N914 diode becomes reversed-biased and the
signal passes through R1 and R2 to the output. Since output
impedance is dependent on input polarity, load impedances
cause an asymmetric output. For constant load impedances, this
can be corrected by reducing R2. Varying or heavy loads can be
buffered by a second OP90. Figure 8 shows the output of the
full-wave rectifier with a 4 V_p-p, 10 Hz input signal.
R2
10k⍀
+5V
R1
V_IN
10k⍀
HP5082-2800
2
7
6
OP90FZ
3
4
1N914
R3
100k⍀
V_OUT
Figure 7. Single Op Amp Full-Wave Rectifier
2-WIRE 4 mA TO 20 mA CURRENT TRANSMITTER
The current transmitter of Figure 9 provides an output of 4 mA
to 20 mA that is linearly proportional to the input voltage.
Linearity of the transmitter exceeds 0.004% and line rejection is
0.0005%/volt.
Biasing for the current transmitter is provided by the REF-02EZ.
The OP90EZ regulates the output current to satisfy the current
summation at the noninverting node:
I_OUT
=
1
R6
V_INR2R5
+
5V R5
R1 
For the values shown in Figure 9,
I_OUT
=


16
100 Ω
_VIN
+ 4 mA
giving a full-scale output of 20 mA with a 100 mV input.
Adjustment of R2 will provide an offset trim and adjustment of
R1 will provide a gain trim. These trims do not interact since
the noninverting input of the OP90 is at virtual ground. The
Schottky diode, D1, prevents input voltage spikes from pulling
the noninverting input more than 300 mV below the inverting
input. Without the diode, such spikes could cause phase reversal of
the OP90 and possible latch-up of the transmitter. Compliance of
this circuit is from 10 V to 40 V. The voltage reference output
can provide up to 2 mA for transducer excitation.
Figure 8. Output of Full-Wave Rectifier with 4 V_p-p,
10 Hz Input
+5V
REFERENCE
2mA MAX
+
V_IN
R2
5k⍀
–
R1
1M⍀
D1
HP
5082-
2800
2
7
6
OP90EZ
3
4
R3
4.7k⍀
6 _REF-02EZ 2
4
V+
(10V TO 40V)
2N1711
R4
100k⍀
R5
80k⍀
I_OUT = ^16VIN + 4mA
100⍀
Figure 9. 2-Wire 4 mA to 20mA Transmitter
R6
100⍀
I_OUT
R_L
REV. A
–9–