Datasheet
14
LT1168
1168fa
The LT1168 is a low power precision instrumentation
amplifier that requires only one external resistor to accu-
rately set the gain anywhere from 1 to 1000. The LT1168
is trimmed for critical DC parameters such as gain error
(0.04%, G = 10), input offset voltage (40µV, RTI), CMRR
(90dB min, G = 1) and PSRR (103dB min, G = 1). These
trims allow the amplifier to achieve very high DC accuracy.
The LT1168 achieves low input bias current of just 250pA
(max) through the use of superbeta processing. The
output can handle capacitive loads up to 1000pF in any
gain configuration and the inputs are protected against
ESD strikes up to ±13kV (human body).
Input Protection
The LT1168 can safely handle up to ±20mA of input
current in an overload condition. Adding an external 5k
input resistor in series with each input allows DC input
fault voltage up to ±100V and improves the ESD immunity
to ±8kV (contact) and ±15kV (air discharge), which is the
IEC 1000-4-2 level 4 specification. If lower value input
resistors must be used, a clamp diode from the positive
supply to each input will maintain the IEC 1000-4-2
specification to level 4 for both air and contact discharge.
A 2N4393 drain/source to gate is a good low leakage diode
for use with resistors between 1k and 20k, see Figure 6.
The input resistors should be carbon and not metal film or
carbon film in order to withstand the fault conditions.
path. The first example is of a purely differential signal
source with a 10kΩ input current path to ground. Since the
impedance of the signal source is low, only one resistor is
needed. Two matching resistors are needed for higher
impedance signal sources as shown in the second
example. Balancing the input impedance improves both
common mode rejection and DC offset.
Figure 5. Providing an Input Common Mode Current Path
10k
1168 F05
THERMOCOUPLE
200k
MICROPHONE,
HYDROPHONE,
ETC
200k
CENTER-TAP PROVIDES
BIAS CURRENT RETURN
–
+
LT1168
–
+
LT1168
–
+
LT1168
Figure 6. Input Protection
APPLICATIO S I FOR ATIO
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–V
S
1168 F06
+V
S
J2
2N4393
J1
2N4393
OUT
OPTIONAL FOR
R
IN
< 20k
R
G
R
IN
R
IN
–
+
LT1168
REF
RFI Reduction
In many industrial and data acquisition applications,
instrumentation amplifiers are used to accurately amplify
small signals in the presence of large common mode
THEORY OF OPERATIO
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