LT1168 Linear Technology, LT1168 Datasheet - Page 12

• Manufacturer Part Number: LT1168
• Description: Low Power/ Single Resistor Gain Programmable/ Micropower Precision Instrumentation Amplifier
• Manufacturer: Linear Technology

APPLICATIONS
LT1168
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 (40 V, 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 1k resistors, see Figure 4. The input resistors
should be carbon and not metal film or carbon film.
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
voltages or high levels of noise. Typically, the sources of
12
R
R
IN
IN
J1
2N4393
Figure 4. Input Protection
U
INFORMATION
J2
2N4393
U
R
G
OPTIONAL FOR
R
IN
+
–
< 20k
W
LT1168
V
V
CC
EE
REF
U
1168 F04
OUT
these very small signals (on the order of microvolts or
millivolts) are sensors that can be a significant distance
from the signal conditioning circuit. Although these sen-
sors may be connected to signal conditioning circuitry,
using shielded or unshielded twisted-pair cabling, the ca-
bling may act as antennae, conveying very high frequency
interference directly into the input stage of the LT1168.
The amplitude and frequency of the interference can have
an adverse effect on an instrumentation amplifier’s input
stage by causing an unwanted DC shift in the amplifier’s
input offset voltage. This well known effect is called RFI
rectification and is produced when out-of-band interfer-
ence is coupled (inductively, capacitively or via radiation)
and rectified by the instrumentation amplifier’s input tran-
sistors. These transistors act as high frequency signal
detectors, in the same way diodes were used as RF
envelope detectors in early radio designs. Regardless of
the type of interference or the method by which it is
coupled into the circuit, an out-of-band error signal ap-
pears in series with the instrumentation amplifier’s inputs.
To significantly reduce the effect of these out-of-band
signals on the input offset voltage of instrumentation
amplifiers, simple lowpass filters can be used at the
inputs. This filter should be located very close to the input
pins of the circuit. An effective filter configuration is
illustrated in Figure 5, where three capacitors have been
added to the inputs of the LT1168. Capacitors C
C
tors R
the input traces. Capacitor C
unwanted signal that would appear across the input traces.
An added benefit to using C
common mode rejection is not degraded due to common
mode capacitive imbalance. The differential mode and
common mode time constants associated with the capaci-
tors are:
Setting the time constants requires a knowledge of the
frequency, or frequencies of the interference. Once this
frequency is known, the common mode time constants
can be set followed by the differential mode time constant.
XCM2
t
t
DM(LPF)
CM(LPF)
S1, 2
form lowpass filters with the external series resis-
to any out-of-band signal appearing on each of
= (2)(R
= (R
S1, 2
S
)(C
)(C
XD
XCM1, 2
)
XD
XD
)
forms a filter to reduce any
is that the circuit’s AC
XCM1
and