13
LT1787/LT1787HV
1787fc
APPLICATIONS INFORMATION
WUU
U
Adjusting Gain Setting
The LT1787 may be used in all operating modes with an
external resistor used in place of the internal 20k R
OUT
V
S
+
– 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 V
BIAS
pin floating or connected to the V
OUT
pin. This will remove
the internal R
OUT
from the circuit.
The voltage gain will be gm • R
OUT
where gm is the
LT1787’s transconductance, 400µA/V typical. A nominal
gain of 40 may be obtained with an external 100k resistor
used in place of the internal 20k R
OUT
:
A
V
= gm • R
OUT
= 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 V
OMAX
specification of V
S
+ –0.75V.
