LT1168 Linear Technology, LT1168 Datasheet - Page 11

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

in this data sheet is an example that satisfies these
conditions. The resistance R
to ground sets the operating current for the bridge and
also has the effect of raising the input common mode
voltage. The output of the LT1168 is always inside the
specified range since the barometric pressure rarely goes
low enough to cause the output to rail (30.00 inches of Hg
corresponds to 3.000V). For applications that require the
output to swing at or below the REF potential, the voltage
on the REF pin can be level shifted. An op amp is used to
buffer the voltage on the REF pin since a parasitic series
resistance will degrade the CMRR. The application in the
front of this data sheet, Single Supply Pressure Monitor,
is an example.
Output Offset Trimming
The LT1168 is laser trimmed for low offset voltage so that
no external offset trimming is required for most applica-
tions. In the event that the offset needs to be adjusted, the
circuit in Figure 2 is an example of an optional offset adjust
circuit. The op amp buffer provides a low impedance to the
REF pin where resistance must be kept to minimum for
best CMRR and lowest gain error.
Input Bias Current Return Path
The low input bias current of the LT1168 (250pA) and the
high input impedance (200G ) allow the use of high
impedance sources without introducing additional offset
voltage errors, even when the full common mode range is
THEORY OF OPERATIO
THERMOCOUPLE
10k
–
+
LT1168
b
from the bridge transducer
Figure 3. Providing an Input Common Mode Current Path
U
HYDROPHONE,
MICROPHONE,
ETC
200k
200k
required. However, a path must be provided for the input
bias currents of both inputs when a purely differential
signal is being amplified. Without this path the inputs will
float to either rail and exceed the input common mode
range of the LT1168, resulting in a saturated input stage.
Figure 3 shows three examples of an input bias current
path. The first example is of a purely differential signal
source with a 10k input current path to ground. Since the
impedance of the signal source is low, only one resistor is
needed. Two matching resistors are needed for higher
impedance signal sources as shown in the second
example. Balancing the input impedance improves both
common mode rejection and DC offset.
–IN
+IN
–
+
Figure 2. Optional Trimming of Output Offset Voltage
LT1168
R
G
2
1
8
3
–
+
LT1168
ADJUSTMENT RANGE
REF
5
10mV
6
BIAS CURRENT RETURN
CENTER-TAP PROVIDES
LT1112
OUTPUT
–
+
–
+
LT1168
LT1167
10k
LT1168
V
V
+
–
10mV
100
100
–10mV
11
1168 F03
1168 F02