REV. C

OP777/OP727/OP747

–12–

and limiting device power dissipation is of prime importance in

these designs. Figure 7 shows an example of 5 V, single-supply

current monitor that can be incorporated into the design of a voltage

regulator with foldback current limiting or a high current power

supply with crowbar protection. The design capitalizes on the

OP777’s common-mode range that extends to ground. Current

is monitored in the power supply return where a 0.1

Ω shunt

inverting terminal becomes equal to the voltage at the noninverting

terminal through the feedback of Q1, which is a 2N2222 or equiva-

lent NPN transistor. This makes the voltage drop across R1 equal to

the output voltage is given by:

VV

R

R

RI

OUT

SENSE

L

=−

×

×









5

2

1

decreases with higher supply current being sensed. For the element

5V

R2 = 2.49k

OP777

5V

R1 = 100

Q1

RETURN TO

GROUND

0.1

Figure 7. A Low-Side Load Current Monitor

The OP777/OP727/OP747 is very useful in many bridge applica-

tions. Figure 8 shows a single-supply bridge circuit in which its

output is linearly proportional to the fractional deviation ( ) of

the bridge. Note that

=

∆R/R.

REF

192

15V

1M

R1(1+ )

R1

1/4 OP747

15V

15V

1M

1/4 OP747

10.1k

0.1 F

2.5V

1/4 OP747

R2

V2

V1

3

4

REF

192

2

2

10.1k

RG = 10k

R1(1+ )

R1

3

4

6

+ 2.5V

2R2

=

R1

R1

= 300

Figure 8. Linear Response Bridge, Single Supply

In systems where dual supplies are available, the circuit of Figure

9 could be used to detect bridge outputs that are linearly related

to the fractional deviation of the bridge.

REF

192

+15V

15V

R1

R2

=

R

R

R2

R1

R

R1

+15V

15V

1/4 OP747

1/4 OP747

12k

15V

1k

3

2N2222

R(1+ )

1/4 OP747

20k

4

Figure 9. Linear Response Bridge

A single-supply current source is shown in Figure 10 . Large resistors

are used to maintain micropower operation. Output current can be

adjusted by changing the R2B resistor. Compliance voltage is:

VV

V

L

SAT

S

≤−

R1

R2B

R2

= 1mA

11mA

100k

OP777

R2A

97.3k

2.7V TO 30V

10pF

10pF

100k

R2B

2.7k

+

R1 = 100k

R2 = R2A + R2B

Figure 10. Single-Supply Current Source

A single-supply instrumentation amplifier using one OP727

amplifier is shown in Figure 11. For true difference R3/R4 =

R1/R2. The formula for the CMRR of the circuit at dc is CMRR =

20

× log (100/(1–(R2 × R3)/(R1× R4)). It is common to specify t he

accuracy of the resistor network in terms of resistor-to-resistor

percentage mismatch. We can rewrite the CMRR equation to

reflect this CMRR = 20

× log (10000/% Mismatch). The key to

high CMRR is a network of resistors that are well matched from

the perspective of both resistive ratio and relative drift. It should

be noted that the absolute value of the resistors and their absolute

drift are of no consequence. Matching is the key. CMRR is 100 dB

with 0.1% mismatched resistor network. To maximize CMRR,

one of the resistors such as R4 should be trimmed. Tighter match-

ing of two op amps in one package (OP727) offers a significant

boost in performance over the triple op amp configuration.

2.7V TO 30V

R2 = 1M

1/2 OP727

2.7V TO 30V

R3 = 10.1k

1/2 OP727

R1 = 10.1k

R4 = 1M

V1

V2

V1)

0.02mV

V1

V2

290mV

2mV

29V

USE MATCHED RESISTORS

Figure 11. Single-Supply Micropower Instrumentation

Amplifier