OP90

–9–

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

+5V

2

3

4

6

OP90

7

R3

100k

HP5082-2800

R1

10k

R2

10k

1N914

Figure 7. Single Op Amp Full-Wave Rectifier

10 Hz Input

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 OP90

regulates the output current to satisfy the current

summation at the noninverting node:

I

R

VR

R

VR

R

OUT

IN

=+









1

6

5

2

55

1

For the values shown in Figure 9,

IV

mA

OUT

IN

= 







+

16

100

4

Ω

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.

2

3

4

6

7

100

+ 4mA

R2

5k

6

4

2

R6

100

R3

4.7k

R4

100k

R1

1M

D1

HP

5082-

2800

R5

80k

+5V

REFERENCE

2mA MAX

–

+

2N1711

V+

(10V TO 40V)

OP90

REF-02EZ

Figure 9. 2-Wire 4 mA to 20mA Transmitter

REV. C