Biomedical Engineering Reference
In-Depth Information
FIGURE 5-11
Op
amp circuit for a
buffered input
differential amplifier.
two buffer circuits together, as shown in Figure 5-12. Consider all resistors to be of equal
value except for
R
gain
. The negative feedback of the upper-left op amp causes the voltage
at node 1 to be equal to
V
1
. Likewise, the voltage at node 2 is held to a value equal to
V
2
.
This establishes a voltage drop across
R
gain
equal to the voltage difference between
V
1
and
V
2
. That voltage drop causes a current through
R
gain
, and since the feedback loops of
the two input op amps draw no current an identical current to that flowing through
R
gain
must pass through the two
R
resistors above and below it. This produces a voltage drop
between nodes 3 and 4 equal to
1
+
2
R
R
gain
V
3
−
4
=
(
V
2
−
V
1
)
(5.17)
FIGURE 5-12
Practical differential
input
instrumentation
amplifier.
