Datasheet

10
100 1k 10k
FREQUENCY (Hz)
-140
-120
-100
-80
-60
-40
-20
0
CMRR (dB)
V
S
= ±2.5V
V
CM
= 0V
V
IN
= 3V
PP
V
O
= -K (2a + 1) (V
1
- V
2
)
V
O
=
KR
2
R
2
(V
O2
- V
O1
)
= -K (V
O1
- V
O2
)
SM73308
www.ti.com SNOSB90B –JUNE 2011–REVISED APRIL 2013
(5)
Now looking at the output of the instrumentation amplifier:
(6)
Substituting from Equation 5:
(7)
This shows the gain of the instrumentation amplifier to be:
−K(2a+1) (8)
Typical values for this circuit can be obtained by setting: a = 12 and K= 4. This results in an overall gain of −100.
Figure 43 shows typical CMRR characteristics of this Instrumentation amplifier over frequency. Three SM73308
amplifiers are used along with 1% resistors to minimize resistor mismatch. Resistors used to build the circuit are:
R
1
= 21.6kΩ, R
11
= 1.8kΩ, R
2
= 2.5kΩ with K = 40 and a = 12. This results in an overall gain of −1000, −K(2a+1)
= −1000.
Figure 43. CMRR vs. Frequency
ACTIVE FILTER
Active filters are circuits with amplifiers, resistors, and capacitors. The use of amplifiers instead of inductors,
which are used in passive filters, enhances the circuit performance while reducing the size and complexity of the
filter.
The simplest active filters are designed using an inverting op amp configuration where at least one reactive
element has been added to the configuration. This means that the op amp will provide "frequency-dependent"
amplification, since reactive elements are frequency dependent devices.
LOW PASS FILTER
The following shows a very simple low pass filter.
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