LT1168 Linear Technology, LT1168 Datasheet - Page 10

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

LT1168
THEORY OF OPERATIO
The LT1168 is a modified version of the three op amp
instrumentation amplifier. Laser trimming and monolithic
construction allow tight matching and tracking of circuit
parameters over the specified temperature range. Refer to
the block diagram (Figure 1) to understand the following
circuit description. The collector currents in Q1 and Q2 are
trimmed to minimize offset voltage drift, thus assuring a
high level of performance. R1 and R2 are trimmed to an
absolute value of 24.7k to assure that the gain can be set
accurately (0.6% at G = 100) with only one external
resistor R
determines the transconductance of the preamp stage. As
R
ductance of the input preamp stage increases to that of the
input transistors Q1 and Q2. This increases the open-loop
gain when the programmed gain is increased, reducing
the input referred gain related errors and noise. The input
voltage noise at gains greater than 50 is determined only
by Q1 and Q2. At lower gains the noise of the difference
amplifier and preamp gain setting resistors increase the
noise. The gain bandwidth product is determined by C1,
C2 and the preamp transconductance which increases
with programmed gain. Therefore, the bandwidth does not
drop proportionally with gain.
The input transistors Q1 and Q2 offer excellent matching,
which is inherent in NPN bipolar transistors, as well as
picoampere input bias current due to superbeta process-
ing. The collector currents in Q1 and Q2 are held constant
due to the feedback through the Q1-A1-R1 loop and
Q2-A2-R2 loop which in turn impresses the differential
input voltage across the external gain set resistor R
Since the current that flows through R
R1 and R2, the ratios provide a gained-up differential volt-
age, G = (R1 + R2)/R
A3. The common mode voltage is removed by A3, result-
ing in a single-ended output voltage referenced to the
voltage on the REF pin. The resulting gain equation is:
solving for the gain set resistor gives:
Table 1 shows appropriate 1% resistor values for a variety
of gains.
10
G
G = (49.4k / R
R
is reduced for larger programmed gains, the transcon-
G
= 49.4k /(G – 1)
G
. The value of R
G
G
) + 1
, to the unity-gain difference amplifier
G
in parallel with R1 (R2)
G
U
also flows through
G
.
Table 1
DESIRED GAIN
1
2
5
10
20
50
100
200
500
1000
Input and Output Offset Voltage
The offset voltage of the LT1168 has two components: the
output offset and the input offset. The total offset voltage
referred to the input (RTI) is found by dividing the output
offset by the programmed gain (G) and adding it to the
input offset. At high gains the input offset voltage domi-
nates, whereas at low gains the output offset voltage
dominates. The total offset voltage is:
Reference Terminal
The reference terminal is one end of one of the four 30k
resistors around the difference amplifier. The output volt-
age of the LT1168 (Pin 6) is referenced to the voltage on
the reference terminal (Pin 5). Resistance in series with
the REF pin must be minimized for best common mode
rejection. For example, a 6 resistance from the REF pin
to ground will not only increase the gain error by 0.02%
but will lower the CMRR to 80dB.
Single Supply Operation
For single supply operation, the REF pin can be at the same
potential as the negative supply (Pin 4) provided the
output of the instrumentation amplifier remains inside the
specified operating range and that one of the inputs is at
least 2.5V above ground. The barometer application later
Total input offset voltage (RTI)
= input offset + (output offset/G)
Total output offset voltage (RTO)
= (input offset • G) + output offset
5488.89
1008.16
498.99
248.24
49400
12350
49.95
Open
2600
R
99
G
CLOSEST 1% VALUE
49900
12400
Open
5490
2610
1000
49.4
499
249
100
RESULTANT GAIN
199.394
19.927
99.998
4.984
9.998
1001
1.99
50.4
495
1