13
LT1787/LT1787HV
1787fc
APPLICATIONS INFORMATION
WU
U
Adjusting Gain Setting
The LT1787 may be used in all operating modes with an
external resistor used in place of the internal 20k ROUT
VS
+ – V
S
– (V)
0
OUTPUT
VOLTAGE
(V)
0.30
0.25
0.20
0.15
0.10
0.05
0
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
A/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 = 400A/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.