LINEAR TECHNOLOGY LTC1992 Technical data

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Easy-to-Use Differential Amplifiers Simplify Balanced
Signal Designs – Design Note 333

Jon Munson

Introduction

The LTC®1992 product family provides simple amplifica­tion or level translation solutions for amplifying signals
that are intrinsically differential or need to be made
differential. In addition to the uncommitted configuration
of the base LTC1992, fixed gain versions with space­saving on-chip factory-trimmed resistors are available as
the LTC1992-1, LTC1992-2, LTC1992-5 and LTC1992-10,
where the nominal gain is indicated by the suffix dash
number. Figure 1 shows a typical gain-of-ten application
where all gain setting components are included in the tiny
8-lead MSOP package.

Easy-to-Use Circuit Topology

The block diagram in Figure 2 shows the general configu­ration of the differential-in/differential-out CMOS ampli­fier core, along with an output common mode servo. The
values of the on-chip gain resistors depend on the dash
suffix of the device as indicated. A convenient on-chip
voltage-divider resistor network is also provided to sup­port applications where a source of mid-supply potential
(V

) is needed.

MID

The LTC1992 is easy to use. Any signal difference at the
inputs (within the input common mode range) are ampli­fied and presented as a voltage difference at the output

5V

3

+V

LTC1992-10

+1V

V

IN

–1V

15k

1

V

OCM

8

15k

V

MID

7

S

150k

–

+

–

+

150k

–V

S

2

–5V

DN333 F01

6

+5V

4

–5V
+5V

5

–5V

pins with a gain bandwidth product of about 4MHz. The
differential gain, A, is set by resistor values:

A = R

F/RG

Any input common mode induced errors, primarily caused
by small mismatches of resistor values, appear at the
output as differential error. The common mode (shared
offset) of the output pair is (V

OUT

+

+ V
pendently governed to track the user-supplied V
put common mode control voltage (V
strapped to V

if desired). The uncommitted LTC1992

MID

–

)/2 and inde-

OUT

may be simply

OCM

OCM

out-

(no dash suffix) may be user configured for any desired
differential gain by selection of external resistors, or
configured specifically for other specialized uses.

, LTC and LT are registered trademarks of Linear Technology Corporation.

+V

S

3

–V

S

1

–IN

V

MID

V

OCM

+IN

+V

S

–V

–V

S

DN333 F02

200k

200k

S

7

2

8

+V

S

R

F

+

R

G

–
+

R

G

Σ

+

30k

–

A2

+

30k

+

Σ

R

F

–

+V

S

6

–V

S

SUFFIX

-10

-5

-2

-1

NONE

+OUT

4

–OUT

5

R

R

G

F

15k

150k

30k

150k

30k

60k

30k

30k

0

∞

Figure 1. Typical Single-Ended to Differential Conversion Figure 2. LTC1992 Functional Block Diagram

03/04/333

Common Mode Range Considerations

For a given input common mode voltage (V
output common mode voltage (V

), the designer must

OCM

INCM

) and

verify that voltage appearing at the internal amplifier
inputs (V
–V

– 0.1V to +VS – 1.3V. With a standard differential

S

) is within the specified operating range of

ICM

amplifier topology having a closed loop gain of A, the
following relationship holds:

= (A/(A+1)) • V

V

ICM

+ (1/(A+1)) • V

INCM

OCM

For example, assume an LTC1992 (no dash) is powered
from 5V, configured for a gain of 2.5 with V
V

(i.e. 2.5V), and driven from a source with common

MID

mode of 0V. From the relation above, V

ICM

tied to

OCM

is (2.5/3.5) •
0 + (1/3.5) • 2.5 = 0.71V, which is well within the
performance range of the device. In this example, the
outputs can swing ±2.5V around the 2.5V V

OCM

level.
Therefore, the differential inputs can swing 1V below
ground without clipping effects or the need for a minus
rail. The dash suffix versions have an additional input
limitation due to the possibility of forward biasing the
ESD input protection diodes (shown in the Figure 2),
which limit the maximum allowable signal swings to 0.3V
beyond the supply voltages (while the base LTC1992 also
includes the ESD diodes, conduction can only occur
outside the usable V

ICM

range).

Common Mode Input Range Extension

Use of the non-committed LTC1992 provides the possi­bility of extending input common mode capability well
outside the supply range by operating with a gain below

+

LTC1992

–

m =

m • V

A + m

R

F

–

V

+

R

F

RG + R

OCM

OUT

DN333 F03

R

S

S

= 2.5V

OCM

< 38V

INCM

R

G

R

S

V

V

IN

GAIN: A =

COMMON MODE SCALING:

V

ICM

EXAMPLE: ASSUME A = 1, m = 0.1, 5V SUPPLY, V
THUS: R
FOR 0.1% RESISTORS, CMRR ≥ 48dB

OCM

V

MID

R

S

R

G

R

F

R

G

A • m • V

INCM

=+

A + m

= RG = 30.1k, RS = 3.32k, –3.6V < V

F

Figure 3. Extending Input Common Mode Range

unity and/or introducing common mode shunt resistors
(see R
method is that component tolerances of R

in Figure 3). The drawback to the shunt resistor

S

and R

G

S

become magnified by the common mode improvement of
the circuit (approximately), leading to reduced CMRR
performance for a given resistor tolerance. For low gain
operation, common mode extension of 10× is realizable
with the use of high accuracy resistor networks.

Versatile Functional Block

The LTC1992 family is especially useful for making con­versions to or from differential signaling. Analog-to­digital converters (ADCs) are often optimized for differential
inputs with a specific common mode input voltage. Use of
an LTC1992 amplifier simplifies the ADC interface by
using the V

control feature to establish the requisite

OCM

offset. In many cases, the mid-scale potential is provided
by the ADC and can be tied directly to the V

OCM

input. In
addition, the source signal input may then be differential
or single ended (by grounding the unused input) or have
inverted polarity.

Since it is not necessary to connect to both outputs, one
can treat the part as single ended which provides the
useful feature that the V

input represents a third

OCM

algebraic input term (see Figure 4). This capability is
useful in performing analog addition or simple translation
functions.

V

INA

V

INB

V1 = V

+ V

INA

V2 = –V

+ V

INA

Figure 4. Single Ended Adder/Subtractor

+

V

OCM

LTC1992-2

V

–

INC

– V

INB

INC

+ V

INB

INC

–

+

DN333 F04

V2

V1

Conclusion

The LTC1992 family of differential amplifiers offers easy­to-use building blocks that provide simple, minimum
component-count solutions to balanced-signal designs.
These parts are useful in a wide range of applications,
including simple methods of transforming signals to/from
differential form to providing component-free gain or DC
offset functions.

Data Sheet Download

http://www.linear.com/go/dnLTC1992

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone] ● www.linear.com

For applications help,

call [phone], Ext. 2156

dn333f LT/TP 0304 305K • PRINTED IN THE USA

 LINEAR TECHNOLOGY CORPORATION 2004