LT1167
15
1167fc
APPLICATIONS INFORMATION
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 interference
is coupled (inductively, capacitively or via radiation) and
rectified by the instrumentation amplifier’s input transis-
tors. These transistors act as high frequency signal detec-
tors, 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 appears 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 am-
plifiers, simple lowpass filters can be used at the inputs.
These filters 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 LT1167. Capacitors C
XCM1
and C
XCM2
form
lowpass filters with the external series resistors 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 capacitors 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. To
avoid any possibility of inadvertently affecting the signal
to be processed, set the common mode time constant an
order of magnitude (or more) larger than the differential
mode time constant. Set the common mode time constants
such that they do not degrade the LT1167’s inherent AC
CMR. Then the differential mode time constant can be set
for the bandwidth required for the application. 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, then the series resistors R
S1, 2
can be omitted, if the
sensor is in proximity to the instrumentation amplifier.
“Roll Your Own”—Discrete vs Monolithic LT1167
Error Budget Analysis
The LT1167 offers performance superior to that of “roll
your own” three op amp discrete designs. A typical ap-
plication that amplifies and buffers a bridge transducer’s
differential output is shown in Figure 6. The amplifier, with
its gain set to 100, amplifies a differential, full-scale output
voltage of 20mV over the industrial temperature range. To
make the comparison challenging, the low cost version of
the LT1167 will be compared to a discrete instrumentation
amp made with the A grade of one of the best precision
quad op amps, the LT1114A. The LT1167C outperforms
the discrete amplifier that has lower V
OS
, lower I
B
and
comparable V
OS
drift. The error budget comparison in
Table 1 shows how various errors are calculated and how
each error affects the total error budget. The table shows
the greatest differences between the discrete solution and
V
–
V
+
IN
+
IN
–
1167 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
–
+
LT1167
f
–3dB
≈ 500Hz
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
