LT1167 Linear Technology, LT1167 Datasheet - Page 14

• Manufacturer Part Number: LT1167
• Description: Single Resistor Gain Programmable/ Precision Instrumentation Amplifier
• Manufacturer: Linear Technology

LT1167
APPLICATIONS
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 LT1167. 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.
To avoid any possibility of inadvertently affecting the
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
14
IN
IN
XCM2
+
–
t
t
DM(LPF)
CM(LPF)
S1, 2
1.6k
1.6k
R
R
form lowpass filters with the external series resis-
S1
S2
EXTERNAL RFI
to any out-of-band signal appearing on each of
C
C
100pF
100pF
= (R
= (2)(R
XCM1
XCM2
10pF
FILTER
C
XD
S1, 2
S
U
)(C
)(C
XD
XCM1, 2
INFORMATION
)
U
R
G
XD
XD
)
forms a filter to reduce any
+
–
is that the circuit’s AC
LT1167
V
V
W
+
–
U
XCM1
1167 F05
V
OUT
and
signal to be processed, 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
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 appli-
cation 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 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
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 the LT1167
are input offset voltage and CMRR. Note that for the
discrete solution, the noise voltage specification is multi-
plied by 2 which is the RMS sum of the uncorelated noise
of the two input amplifiers. Each of the amplifier errors is
referenced to a full-scale bridge differential voltage of
20mV. The common mode range of the bridge is 5V. The
LT1114 data sheet provides offset voltage, offset voltage
drift and offset current specifications for the matched op
amp pairs used in the error-budget table. Even with an
excellent matching op amp like the LT1114, the discrete
solution’s total error is significantly higher than the
LT1167’s total error. The LT1167 has additional advan-
tages over the discrete design, including lower compo-
nent cost and smaller size.
S1, 2
can be omitted, if the sensor is in
OS
, lower I
B
and comparable V
OS