15
LT1168
1168fa
voltages or high levels of noise. Typically, the sources of
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 7, where three capacitors have been
added to the inputs of the LT1168. Capacitors C
XCM1
and
C
XCM2
form lowpass filters with the external series resis-
tors R
S1, 2
to any out-of-band signal appearing on each of
the input traces. Capacitor C
XD
forms a filter to reduce any
unwanted signal that would appear across the input traces.
An added benefit to using C
XD
is that the circuit’s AC
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:
t
DM(LPF)
= (R
S1
+ R
S2
)(C
XD
+ C
XCM1
+ C
XCM2
)
t
CM(LPF)
= (R
S1
||
R
S2
)(C
XCM1
+
C
XCM2
)
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.
To avoid any possibility of inadvertently affecting the
signal to be processed, set the common mode time
constant an order of magnitude (or more) smaller than the
differential mode time constant. Set the common mode
time constants such that they do not degrade the LT1168
inherent AC CMR. Then the differential mode time con-
stant can be set for the bandwidth required for the appli-
cation. Setting the differential mode time constant close to
the sensor’s BW also minimizes any noise pickup along
the leads. To avoid any possibility of common mode to
differential mode signal conversion, match the common
mode time constants to 1% or better. If the sensor is an
RTD or a resistive strain gauge and is in proximity to the
instrumentation amplifier, then the series resistors R
S1, 2
can be omitted.
Figure 7. Adding a Simple RC Filter at the Inputs to an
Instrumentation Amplifier is Effective in Reducing Rectification
of High Frequency Out-of-Band Signals
APPLICATIO S I FOR ATIO
WUUU
–V
S
+V
S
IN
+
IN
–
1168 F07
V
OUT
R
G
C
XCM1
0.001µF
C
XCM2
0.001µF
C
XD
0.1µF
R
S1
1.6k
R
S2
1.6k
EXTERNAL RFI
FILTER
–
+
LT1168
f
–3dB
≈ 500Hz
Nerve Impulse Amplifier
The LT1168’s low current noise makes it ideal for EMG
monitors that have high source impedances. Demonstrat-
ing the LT1168’s ability to amplify low level signals, the
circuit in Figure 8 takes advantage of the amplifier’s high
gain and low noise operation. This circuit amplifies the low
level nerve impulse signals received from a patient at
Pins 2 and 3. R
G
and the parallel combination of R3 and R4
set a gain of ten. The potential on LT1112’s Pin 1 creates
