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Analog Analysis
Figure 1: Analog Circuit
The analog circuit had
to be analyzed and its function understood before any
work could be done on the replacement. In order to
determine the input and output ranges of the circuit,
its schematic was entered in PSpice. Additional tests
were used to determine the power used by the circuit and
its accuracy. The schematic of the analog circuit can
be found in Figure 1.
The circuit was
simulated by grounding the low input and varying the
high input from 0 – 3V using 0.25V increments. The
output voltage was recorded for each of the inputs.
Using the equation Gain = Vout/Vin, the gain for
each of the input voltages was calculated. The following
table shows the gain for the input voltages that we used
for our simulation. A graph of Vout vs. Vin in Graph 1 shows that this is a linear circuit with a gain of
2.49.
|
Vin |
Vout |
Gain |
|
|
|
|
|
0 |
0.001913 |
0 |
|
0.25 |
0.619751 |
2.479004 |
|
0.5 |
1.2479 |
2.4958 |
|
0.75 |
1.873 |
2.497333333 |
|
1 |
2.4948 |
2.4948 |
|
1.25 |
3.1052 |
2.48416 |
|
1.5 |
3.7215 |
2.481 |
|
1.75 |
4.3403 |
2.480171429 |
|
2 |
4.976 |
2.488 |
|
2.25 |
5.5877 |
2.483422222 |
|
2.5 |
6.2362 |
2.49448 |
|
2.75 |
6.8779 |
2.501054545 |
|
3 |
7.5156 |
2.5052 |
Table 1: Gain
The total power
consumption of the circuit was also found during
simulation to be 432.66mW. This total was calculated by
summing the power dissipated in each op-amp, the switch,
and all of the resistors and capacitors. The table which
shows the total power consumed by the analog circuit is
found in Table 4.
Since the top priority
of the company was board space reduction without an
increase in price, a board space analysis and a price
analysis was conducted. The board space was found by
looking up the footprint of the existing parts and using
a standard safety factor of 1.3 to allot for spacing on
the board. These values can be found in the table
below. The total board space of the analog circuit was
83.58 mm2 so our goal was to create a digital
circuit with a smaller footprint than this.
|
Part |
Quantity |
Dimensions per package (mm x mm) |
Total board space (mm2) |
Board space times safety factor (mm2) |
|
LT1112 |
1(2 op amps) |
5.004 x 6.197 |
31.01 |
40.313 |
|
ADG201 |
¼ (1 switch) |
6.2 x 10.0 |
15.5 |
20.15 |
|
Resistors |
8 |
1.6 x .8 |
10.24 |
13.312 |
|
Capacitors |
3 |
2.01 x 1.25 |
7.54 |
9.802 |
|
Total |
|
|
64.29 |
83.577 |
Table 2: Board Space
Cost analysis was
conducted by researching the cost of the individual
parts used in the analog circuit. The circuit was found
to cost $4.69 so the digital circuit must cost less than
or equal to that amount.
|
Part |
Quantity |
Cost per Part |
Total Price |
|
LT1112 |
1(2 op amps) |
$2.55 |
$2.55 |
|
ADG201 |
¼ (1 switch) |
$1.91 |
$0.4775 |
|
Resistors |
8 |
$0.166 |
$1.328 |
|
Capacitors |
3 |
$0.11 |
$0.33 |
|
Total |
|
|
$4.69 |
Table 3: Cost
The amplifier
information was found using
Linear using the S8
package.
Analog was used to find the switch data
(SO1C-16A). The resistors and capacitors were
researched using
DigiKey and
AVXCorp respectively.
Graph 1: Vout vs. Vin Base Case
|
Part |
Power Dissipation (W) |
|
|
|
|
U1 |
0.013382 |
|
U2 |
0.011898 |
|
U3 |
0.404544 |
|
R1 |
0.000116774 |
|
R2 |
0.000007097 |
|
R3 |
0.00047369 |
|
R4 |
0.000028788 |
|
R5 |
0.0020048 |
|
R6 |
0.000016992 |
|
R7 |
8.832E-18 |
|
R8 |
2.03586E-07 |
|
C1 |
0.000044674 |
|
C2 |
0.000105415 |
|
C3 |
0.000033049 |
|
|
|
|
|
|
|
TOTAL |
0.432655483 |
Table 4: Power
Dissipation
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