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 ex-
tremely 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 1.
30155536
FIGURE 1. Instrumentation Amplifier
There are two stages in this amplifier. The last stage, output
stage, is a differential amplifier. In an ideal case the two am-
plifiers of the first stage, input stage, would be set up as
buffers to isolate the inputs. However they cannot be con-
nected 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 in-
strumentation 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:
(1)
By Ohm’s Law:
(2)
However:
(3)
So we have:
(4)
Now looking at the output of the instrumentation amplifier:
(5)
Substituting from Equation 4:
(6)
This shows the gain of the instrumentation amplifier to be:
−K(2a+1)
Typical values for this circuit can be obtained by setting: a =
12 and K= 4. This results in an overall gain of −100.
Figure 2 shows typical CMRR characteristics of this Instru-
mentation amplifier over frequency. Three SM73308 ampli-
fiers are used along with 1% resistors to minimize resistor
mismatch. Resistors used to build the circuit are: R1 =
21.6kΩ, R11 = 1.8kΩ, R2 = 2.5kΩ with K = 40 and a = 12. This
results in an overall gain of −1000, −K(2a+1) = −1000.
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FIGURE 2. CMRR vs. Frequency
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SM73308