11
LT1920
APPLICATIONS INFORMATION
WUU
U
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 LT1920. 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)
= (2)(R
S
)(C
XD
)
t
CM(LPF)
= (R
S1, 2
)(C
XCM1, 2
)
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.
Set the common mode time constants such that they do
not degrade the LT1920’s inherent AC CMR. Then the
differential mode time constant can be set for the band-
width required for the application. Setting the differential
mode time constant close to the sensor’s BW also mini-
mizes any noise pickup along the leads. To avoid any
possibility of inadvertently affecting the signal to be pro-
cessed, set the common mode time constant an order of
magnitude (or more) larger than the differential mode time
constant. 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, then the series resistors
R
S1, 2
can be omitted, if the sensor is in proximity to the
instrumentation amplifier.
N8 1197
0.100 ± 0.010
(2.540 ± 0.254)
0.065
(1.651)
TYP
0.045 – 0.065
(1.143 – 1.651)
0.130 ± 0.005
(3.302 ± 0.127)
0.020
(0.508)
MIN
0.018 ± 0.003
(0.457 ± 0.076)
0.125
(3.175)
MIN
12
3
4
876
5
0.255 ± 0.015*
(6.477 ± 0.381)
0.400*
(10.160)
MAX
0.009 – 0.015
(0.229 – 0.381)
0.300 – 0.325
(7.620 – 8.255)
0.325
+0.035
–0.015
+0.889
–0.381
8.255
()
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
N8 Package
8-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
Dimensions in inches (millimeters) unless otherwise noted.
PACKAGE DESCRIPTION
U
Figure 5. Adding a Simple RC Filter at the Inputs to an
Instrumentation Amplifier is Effective in Reducing Rectification
of High Frequency Out-of-Band Signals
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
V
–
V
+
IN
+
IN
–
1920 F05
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
–
+
LT1920
f(–3dB) ≈ 500Hz
