Analog Devices AN573-a Application Notes

AN-573

a

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: [phone] • FAX: [phone] • www.analog.com

OP07 Is Still Evolving

by Reza Moghimi

INTRODUCTION

The OP07 has been tinkered with over the years, and ver­sions of it are still available in plastic packages.

This application note highlights some of the major features
that the OP17x7 brings into new designs. A number of
applications using these features are presented.

SINGLE-SUPPLY OPERATION

One of the biggest problems with the part in today’s
environment is that the OP07 requires dual supplies. A
new family of amplifiers from Analog Devices addresses
this problem while still giving a close replica of the
original specifications. The OP777 single, OP727 dual,
and OP747 quad operational amplifiers allow supplies
from ±15 V down to ±1.35 V with split rails, and from
+30 V down to +2.7 V with single rail operation. The data
sheet characterizes the parts with rails of +5 V and ±15 V.
The OP7x7 family’s true single-supply capability enables
designers to operate down to the negative supply or
ground in both single- and dual-supply applications.

5V

R4

26.7k

V2

R7
100

V1

R12

1M

GAIN = 100 (V2 – V1)

R4

10.1k

U4

1/4

OP747

R14

10.1k

U3

1/4

OP747

R15

1M

AD589

R3

37.4k

D1N

V+

3

U1

2

V–

2.55M

1

1/4

OP747

R8

R2
200

RTD

100

R5

26.7k

6.19k

R9

V

OUT

Figure 1 shows that the gain of the instrumentation am­plifier (made up of U3 and U4) is set for 100. The AD589
establishes 1.235 V while the U1 amplifier servos the
bridge while maintaining the voltage across the parallel
combination of 2.55 MΩ and 6.19 kΩ to generate a 200 µA
current source. This current splits evenly and flows into
both halves of the bridge, eventually through RTD, and
establishes an output voltage based upon its value.

As shown in Figure 2, the circuit floats up from the
single-supply (12 V to 30 V) return. It consumes only

1.5 mA, leaving 2.5 mA available to the user for power­ing other signal conditioning circuitry.

VIN 0V TO 3V

R23
10k

R21
182k

HP5082-2800

R24
100k

R20

1.21M

V+

OP777

3

1

2

R22

1k

D2

2

1

V–

C2

220pF

R28

100k

R25

220

R26
100

Q1
2N1711

REF-02A/D

R27
10k

2

V

IN

3

TRIM

V

GND

OUT

4

5

6

T1

TWIST

PAIR

R29

100

12V TO

30V

420

mA

Figure 2. Self-Powered 4–20 mA Current Loop
Transmitter

Figure 1. Low Power Single-Supply RTD Amplifier

REV. A

AN-573

+V

S

2

V

IN

REF192

V

GND

4

OUT

+V

S

6

C7

0.1F

V+

3

1

2

1/4

R1

V–

OP747

R1(1+)

R2

R1(1+)

R1

REF192

+V

2

V

IN

GND

4

A = 300

AR1V

REF

=

V

OUT

S

R84

1M

6

V

OUT

2R2

OP747

R82

10.1k

1/4

 +2.5V

R85

10k

R91

10.1k

1/4

OP747

R83

1M

V

OUT

Figure 3. Single-Supply Linear Response Bridge

The OP7x7 is very useful in many bridge applications.
Figure 3 shows a single-supply bridge circuit whose out­put is linearly proportional to the fractional deviation ()
of the bridge.

R

Note that

 =

R

To process ac signals in single-supply systems, it is
often best to use a false-ground biasing scheme. This
is shown in Figure 4, done by amplifier A3. The user
should replace the 2.67 kΩ Twin-T section with a 3.16 kΩ
resistor to reject 50 Hz. Sensitivity is due to the
relative matching of the capacitors and resistors in
the Twin-T section. Use Mylar (5%) and 1% resistors
for satisfactory results.

100k

1F

V

IN

+3V

3

2

1M

1M

+3V

V+

V–

2.67k

1

1/4

OP747

0.01F

1/4

OP747

2.67k

100k

2F

1F

499

2.67k

1F

2.67k

1.33k

1k

1F

1/4

OP747

1k

V

OUT

Figure 4. 3 V Single-Supply 50 Hz/60 Hz Active Notch
Filter with False Ground

MUCH LOWER SUPPLY CURRENTS

The OP07 has a quiescent current that is higher than
desired in today’s portable applications. The quiescent
current of the OP777 in-amplifier is less than 350 µA,
while the old OP07 required 4 mA for ±15 V operation. In
terms of power consumption, the new part wins hands
down. This allows the part to be designed into many
portable applications.

V1

R12

1M

3

2

5V

V+

U4

V–

R14

10.1k

1

1/2

OP727

V2

R13

10.1k

U3

R15

1M

1/2

OP727

V

OUT

Figure 5. Single-Supply Micropower In-Amp

OP727 can be used to build an instrumentation amplifier
(IA) with two op amps. A single-supply instrumentation
amplifier using one OP727 amplifier is shown in Figure

5. For true difference, R14/R12 = R15/R13. The formula
for the CMRR of the circuit at dc is CMRR = 20 × log (100/
(1 – (R15 × R14)/(R13 × R12)). It is common to specify the
accuracy of the resistor network in terms of resistor-to­resistor percentage mismatch. The CMRR equation can
be rewritten to reflect this CMRR = 20 × log (10000/%
mismatch). The key to high CMRR is a network of resis­tors that is 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 a 0.1% mismatched resistor network. To
maximize CMRR, one of the resistors such as R12
should be trimmed. Tighter matching of two op amps
in one package (OP727) offers a significant boost in
performance over the triple op amp configuration. For

–2–

REV. A

AN-573

this circuit, V
≤ 290 mV, 2 mV ≤ V

= 100 (V2 – V1) for 0.02 mV ≤ (V1 – V2)

O

≤ 29 V.

OUT

Due to its great dc accuracy and specification, the OP747
can be used to create a multiple output tracking voltage
reference from a single source, as shown in Figure 6.

+15V

22k

AD680AD

2

V

IN

TEMP

GND

4

IN4002

1F

+V

R48

R49

10k

6

V

OUT

3

10k

2F

C8

1F

3

2

V–

R50

10k

S

V+

OP747

1

1/4

OP747

10k

10k

1/4

10k

OP747

10k

1/4

1/4

OP747

10V

7.5V

5V

2.5V

Figure 6. Multiple Output Tracking Voltage Reference

Figure 7 shows an example of a 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 resistor, R

SENSE

creates a very small voltage drop. The voltage at the
inverting terminal becomes equal to the voltage at the
noninverting terminal through the feedback of Q1,
which is a 2N2222 or equivalent NPN transistor. This
makes the voltage drop across R1 equal to the voltage
drop across R

. Therefore, the current through Q1

SENSE

becomes directly proportional to the current through
R

, and the output voltage is given by: V

SENSE

(R2/R3) × R
with I

increasing, so V

L

× IL). The voltage drop across R2 increases

SENSE

decreases with higher supply

OUT

OUT

= 5 V –

current being sensed. For the element values shown, the
V

is 2.5 V for a return current of 1 A.

OUT

Figure 8 shows the OP777 configured as a simple
summing amplifier. The output will be the sum of V1
and V2.

+15V

3.3k

10k

V1

10k

V2

V+

3

OP777

1

V

2

V–

–15V

10k

OUT

Figure 8. Summing Amplifier

ABSENCE OF CLAMPING DIODES AT THE INPUTS

The large differential voltage capability allows for opera­tion of the parts in both rectifier circuits and precision
comparator applications. The need for external clamp­ing diodes (on-board in the OP07) is eliminated; such
diodes are often needed on precision op amps and are
the bane of many comparator designs.

The simple oscillator shown in Figure 9 creates a square
wave output of ±V

at 1 kHz for the values shown.

S

Other oscillation frequencies can be derived using
f = 1/(2R3 × C10 × ln ((R61 + R60)/R61).

R61

100k

+V

R60

100k

,

C10

0.01F

S

3

V+

1

V

2

OP777

V–

–V

S

R3

68k

OUT

V

= (VS) @ 1kHz

OUT

Figure 9. Free-Running Square Wave Amplifier

The programmable window comparator is capable of
12-bit accuracy. DAC8212 is used in the voltage for set­ting the upper and lower thresholds.

5V

R

R3
100

SENSE

0.1

R2

2.49k

Q1

OUT

2N2222A/ZTX

V

Figure 7. Low-Side Current Sensing Circuit

REV. A

RETURN TO

GROUND

V+

3

U1

2

V–

1

OP777

–3–