LT1997-2 datasheet(13/26 Pages) AD

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LT1997-2

Rev 0

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APPLICATIONS INFORMATION

Figure 1. Difference Amplifier with Dual-Supply

Operation (Gain = 0.1)

Introduction

The LT1997-2 is a precision, high voltage funnel ampli-

fier combined with a highly-matched resistor network. It

can easily be configured into many different gain circuits

without adding external components, as it will be shown

in this data sheet. The LT1997-2 provides the resistors

and op amp together in a small package in order to save

board space and reduce complexity. Highly accurate

measurement circuits can be easily constructed with the

LT1997-2. The circuits can be tailored to specific measure-

ment applications.

Common Mode Voltage Range

The common mode voltage range of the LT1997-2 is set

by the voltage range allowed on the LT1997-2’s input pins

and by the input voltage range of the internal op amp.

The internal op amp of LT1997-2 has 2 operating regions:

a) if the common-mode voltage at the inputs of the

internal op amp (VCMOP) is between V– and V+ – 1.75V,

the op amp operates in its normal region;

b) If VCMOP is between V+ – 1.75V and V– + 76V, the op

amp continues to operate, but in its Over-The-Top (OTT)

region with degraded performance (see Over-The-Top

Operation section of this data sheet for more detail).

The LT1997-2 will not operate correctly if the common-

mode voltage at the inputs of the internal op amp (VCMOP)

is below V–, but the part will not be damaged as long as

VCMOP is greater than V– – 25V and the junction tempera-

ture of the LT1997-2 does not exceed 150ºC.

The voltage on LT1997-2’s input pins should never be

higher than V– + 270V or lower than V– – 270V under any

circumstances.

The common-mode voltage at the inputs of the internal

op amp (VCMOP) is determined by the voltages on pins

+INA, +INB, +INC and REF (see the Calculating Input Volt-

age Range section). This condition is true provided that

the internal op amp’s output is not clipped and feedback

maintains the internal op amp’s inputs at the same voltage.

In addition to the limits mentioned above, the common

mode input voltage of the amplifier should be chosen so

that the input resistors do not dissipate too much power.

The power dissipated in a 250k resistor must be less than

1.8W. It must be less than 0.9W for the 125k resistor and

less than 0.72W for the 100k resistor. For most applica-

tions, the pin voltage limitations will be reached before

the resistor power limitation is reached.

Calculating Input Voltage Range

Figure 2 shows the LT1997-2 in the generalized case

of a difference amplifier, with the inputs shorted for the

common mode calculation. The values of RF and RG are

dictated by how the positive inputs (+INA, +INB, +INC)

and REF pin are connected.

By superposition we can write:

VCMOP = VEXT •

RF + RG

+ VREF •

RF + RG

Or, solving for VEXT:

VEXT = VCMOP • 1+

⎛

⎝⎜

⎞

⎠⎟

– VREF •

But valid VCMOP voltages are limited to VS+ – 1.75V (or

VS– + 76V for OTT) on the high side and VS– on the low

side, so:

MAX VEXT = VS+ –1.75

(

) • 1+

⎛

⎝⎜

⎞

⎠⎟

– VREF •

19972 F01

OUT

REF

250k

100k

125k

25k

125k

100k

25k

–INA –INB –INC

+INA +INB +INC

V–

V+

SHDN

VS+

VS–

VOUT

VREF

V–IN

V+IN

LT1997-2

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