LT1787HVCMS8 datasheet(13/16 Pages) LINER

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LT1787/LT1787HV

1787fc

APPLICATIONS INFORMATION

Adjusting Gain Setting

The LT1787 may be used in all operating modes with an

external resistor used in place of the internal 20k ROUT

+ – V

– (V)

0.30

0.25

0.20

0.15

0.10

0.05

0.005 0.010

IDEAL

0.015 0.020

1787 F09

0.025 0.030

Figure 9. Expanded Scale of Unidirectional Output

resistor. When an external resistor is used, leave the VBIAS

pin floating or connected to the VOUT pin. This will remove

the internal ROUT from the circuit.

The voltage gain will be gm • ROUT where gm is the

LT1787’s transconductance, 400

µA/V typical. A nominal

gain of 40 may be obtained with an external 100k resistor

used in place of the internal 20k ROUT:

AV = gm • ROUT = 400µA/V • 100k = 40

The transconductance gm is set by on-chip resistors on

the LT1787. These resistors match well but have loose

absolute tolerance. This will normally require that the

external gain setting resistor be trimmed for initial accu-

racy. After trimming, the temperature stability of the gm

and therefore gain will be –200ppm/

°C.

The only limitations placed upon the resistor choice is care

must be taken not to saturate the internal circuitry by

violating the VOMAX specification of VS + –0.75V.

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