Datasheet
R
11
= V
1
- V
2
V
V
O1
- V
O2
= (2R
1
+
) I
R
11
R
11
= (2a + 1) V
R
11
= (2a + 1)
R
11
x
I
R
11
GIVEN: I
R
1
= I
R
11
+
-
+
-
-
+
R
2
KR
2
R
1
R
1
R
2
KR
2
V
OUT
V
1
V
2
V
01
V
02
R
1
a
R
11
=
SM73308
SNOSB90B –JUNE 2011–REVISED APRIL 2013
www.ti.com
APPLICATION NOTE
SM73308
The SM73308 is a precision amplifier with very low noise and ultra low offset voltage. SM73308's extended
temperature range of −40°C to 125°C enables the user to design a variety of applications including automotive.
The SM73308 has a maximum offset voltage of 1mV over the extended temperature range. This makes the
SM73308 ideal for applications where precision is important.
INSTRUMENTATION AMPLIFIER
Measurement of very small signals with an amplifier requires close attention to the input impedance of the
amplifier, gain of the overall signal on the inputs, and the gain on each input since we are only interested in the
difference of the two inputs and the common signal is considered noise. A classic solution is an instrumentation
amplifier. Instrumentation amplifiers have a finite, accurate, and stable gain. Also they have extremely high input
impedances and very low output impedances. Finally they have an extremely high CMRR so that the amplifier
can only respond to the differential signal. A typical instrumentation amplifier is shown in Figure 42.
Figure 42. Instrumentation Amplifier
There are two stages in this amplifier. The last stage, output stage, is a differential amplifier. In an ideal case the
two amplifiers of the first stage, input stage, would be set up as buffers to isolate the inputs. However they
cannot be connected as followers because of real amplifier's mismatch. That is why there is a balancing resistor
between the two. The product of the two stages of gain will give the gain of the instrumentation amplifier. Ideally,
the CMRR should be infinite. However the output stage has a small non-zero common mode gain which results
from resistor mismatch.
In the input stage of the circuit, current is the same across all resistors. This is due to the high input impedance
and low input bias current of the SM73308. With the node equations we have:
(2)
By Ohm’s Law:
(3)
However:
(4)
So we have:
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