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LT1990

1990f

APPLICATIO S I FOR ATIO

Primary Features

The LT1990 is a complete gain-block solution for high

input common mode voltage applications, incorporating a

low power precision operational amplifier providing rail-

to-rail output swing along with on-chip precision thin-film

resistors for high accuracy. The Block Diagram shows the

internal architecture of the part. The on-chip resistors

form a modified difference amplifier including a reference

port for introducing offset or other additive waveforms.

With pin-strapping alone either unity gain or gain of 10 is

produced with high precision. The resistor network is

designed to produce internal common-mode voltage divi-

sion of 27 so that a very large input range is available

compared to the power supply voltage(s) used by the

LT1990 itself. The LT1990 is ideally suited to situations

where relatively small signals need to be extracted from

high voltage circuits, as is the case in many current

monitoring instrumentation applications for example. With

the ability to accept a range of input voltages well outside

the limits of the local power rails and its greater than 1MΩ

input impedances, development of precision low power

over-the-top and under-the-bottom instrumentation de-

signs is greatly simplified with the LT1990 single chip

solution over conventional discrete implementations.

Classic Difference Amplifier

Used in the basic difference amplifier topology where the

gain G is pin-strap configurable to be unity or ten, the

following relationship is realized:

VO = G • (V+IN – V–IN) + VREF

To operate in unity gain, the GAIN1 and GAIN2 pins are left

disconnected. For G = 10 operation, the GAIN1 and GAIN2

pins are simply connected together or tied to a common

potential such as ground or V–.

The input common mode range capability is up to ±250V,

governed by the following relationships:

For G = 1 and G = 10 where GAIN1 and GAIN2 are only tied

together (not grounded,etc):

VCM+ ≤ 27 • V+ – 26 • VREF – 23

VCM– ≥ 27 • V– – 26 • VREF + 27

For G = 10 where GAIN1 and GAIN2 are tied to a common

potential VGAIN:

VCM+ ≤ 27 • V+ – 26 • VREF – 23 – VGAIN

VCM– ≥ 27 • V– – 26 • VREF + 27 – VGAIN

For split supplies over about ±11V, the full ±250V common

mode range is normally available (with VREF a small

fraction of the supply). With lower supply voltages, an

appropriate selection of VREF can tailor the input common

mode range to a specific requirement. As an example, the

following low supply voltage scenarios are readily imple-

mented with the LT1990:

Supply

VREF

VCM Range

+3V

1.25V

–5V to 25V (e.g. 12V automotive environment)

+5V

1.25V

–5V to 80V (e.g. 42V automotive environment)

+5V

4.00V

–77V to 8V (e.g. telecom environment;

use downward signaling)

Configuring Other Gains

An intermediate gain G ranging between 1 and 10 may be

produced by placing an adjustable resistance between the

GAIN1 and GAIN2 pins according to the following nominal

relationship:

RGAIN ≈ (180k/(G – 1)) – 20k

While the expression is exact, the value is approximate

because the absolute resistance of the internal network

could vary on a unit-to-unit basis by as much as ±30%

from the nominal figures and the external gain resistance

is required to accommodate that deviation. Once ad-

justed, however, the gain stability is excellent by virtue of

the –30ppm/°C typical temperature coefficient offered by

the on-chip thin-film resistor process.

Preserving and Enhancing Common Mode Rejection

The basic difference amplifier topology of the LT1990

requires that source impedances seen by the input pins

+IN and –IN, should be matched to within a few tens of

ohms to avoid increasing common mode induced errors

beyond the basic production limits of the part. Known

source imbalances beyond that level should be compen-

sated for by the addition of series resistance to the lower-

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