LTC1921 datasheet(12/16 Pages) AD

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LT1990-10

Rev 0

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APPLICATIONS INFORMATION

Primary Features

The LT1990-10 is a complete gain-block solution for

high input common mode voltage applications. The part

combines a low-power precision operational amplifier

with thin-film resistors trimmed to produce a gain of 10

with high accuracy. The Block Diagram shows the inter-

nal architecture of the part. The on-chip resistors form a

modified difference-amplifier including a reference port for

introducingoffsetorotheradditivewaveforms.Theresistor

network is structured to produce internal common mode

voltage division of 27, enabling a very large input range.

The input range can far exceed the power supply voltage(s)

used by the LT1990-10 itself. Standard ESD clamp diodes

are included on all the I/O except the –IN and +IN pins.

The inputs are rated to ±250V and protected to ±500V. The

LT1990-10 is ideally suited to situations where relatively

smallsignalsneedtobeextractedfromhighvoltagecircuits,

asisthecaseinmanyinstrumentationapplications.Withits

wide input voltage range and greater than 1 megohm input

impedances, development of instrumentation designs is

greatly simplified with the LT1990-10 single-chip solution

over conventional discrete methods.

Classic Difference Amplifier

The basic gain of ten difference amplifier topology has the

following dc transfer function:

VO = 10·(V+IN – V–IN) + VREF

By including the internal common mode division by 27,

the input common mode range capability is extended up

to ±250V according to the following relationships:

VCM+ ≤ 27 • V+ – 26 • VREF – 23

VCM– ≥ 27 • V– – 26 • VREF + 27

Forsplitsuppliesoverabout±11V,thefull±250Vcommon

moderangeisnormallyavailable(withVREFasmallfraction

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

selection of VREF can tailor the input common mode range

to a specific requirement. For single supply circuits, VREF

should be greater than V– to allow bidirectional output

swing and to keep the inputs of the internal op amp within

their operating region. Note: the differential input voltage

range is reduced as VCM approaches its limits. The follow-

ing low supply-voltage scenarios are readily implemented

with the LT1990-10:

Table 1.

Supply

VREF

VCM Range

1.25V

–5V to 25V (e.g. 12V Automotive Environment)

1.25V

–5V to 80V (e.g. 42V Automotive Environment)

4.00V

–77V to 8V (e.g. Telecom Environment;

Use Downward Signaling)

Preserving and Enhancing Common Mode Rejection

ThebasicdifferenceamplifiertopologyoftheLT1990-10is

sensitive to the external resistances of circuits driving the

part. To preserve the high accuracy of the LT1990-10, the

source impedance of any signal connected to the REF pin

must be on the order of a few ohms or less, such as from

a Reference or op-amp output. The difference inputs have

nominal 1 megohm internal resistances that are matched

to within a few hundred ohms, so source resistances

should also be kept low to maximize accuracy and CMRR.

WhileeveryLT1990-10isfactorytrimmed,someprecision

applications with a large applied common mode voltage

may benefit from a trim method to further minimize com-

mon mode error. This is easily accomplished as shown in

Figure 1. A series resistance is added to each input: a fixed

1kΩ in series with one of the inputs and a 2kΩ trimmer

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