Hardware Reference
In-Depth Information
V
IN
+
V
OUT
O
P AMP
R
2
R
1
Figure 12.6
■
A voltage scaler
across
R
1
) would be the same as that at the noninverting terminal (
V
IN
). Therefore, the voltage gain of
this circuit is given by the following equation:
A
V
5
V
OUT
4
V
IN
5
(
R
1
1
R
2
)
4
R
1
5
1
1
R
2
/
R
1
(12.2)
Example 12.2
▼
Suppose the transducer output voltage ranges from 0 V to 200 mV. Choose the appropriate
values for
R
1
and
R
2
to scale this range to 0,5 V.
Solution:
5 V 4 200 mV 5 25; therefore
R
2
/
R
1
5 24.
By choosing 240 k
Ω
for
R
2
and 10 k
Ω
for
R
1
, we obtain a
R
2
/
R
1
ratio of 24 and achieve the
desired scaling goal.
▲
Some transducers have output voltage in the range of
V
1
,
V
2
(
V
1
can be negative and
V
2
can be
unequal to
V
DD
) instead of 0 V,
V
DD
. The accuracy of A/D conversion can be improved by using a
circuit that shifts and scales the transducer output so that it falls in the full range of 0 V,
V
DD
.
An OP AMP circuit that can shift and scale the transducer output is shown in Figure 12.7c.
This circuit consists of a summing circuit (Figure 12.7a) and an inverting circuit (Figure 12.7b).
The voltage
V
IN
comes from the transducer output;
V
1
is an adjusting voltage. By choosing appro-
priate values for
V
1
and resistors
R
0
,
R
1
,
R
2
, and
R
f
, the desired voltage shifting and scaling can be
achieved. Equation 12.5 shows that the resistance
R
0
is an independent variable and can be set to
a convenient value.
Example 12.3
▼
Choose appropriate resistor values and the adjusting voltage so that the circuit shown in
Figure 12.7c can shift the voltage from the range of 21.2 V,3.0 V to the range of 0 V,5 V.
Solution:
Applying Equation 12.5,
0
5
2
1.2
3
(
R
f
/
R
1
)
2
(
R
f
/
R
2
)
3
V
1
5
5
3.0
3
(
R
f
/
R
1
)
2
(
R
f
/
R
2
)
3
V
1
By choosing
R
0
5
R
1
5 10 k
, and
V
1
5 25 V, we can translate and
scale the voltage to the desired range. This example tells us that the selection of resistors and
the voltage
V
1
is a trial-and-error process at best.
Ω
,
R
2
5 50 k
Ω
,
R
f
5 12 k
Ω
▲
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