
3–419
Motorola Sensor Device Data
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Figure 2. Circuit Schematic
V
offset
is the reference voltage for the first op-amp and is
pre-set with a voltage divider from the supply voltage. This
value is set to be 6.7 percent of the supply voltage. It is
important to keep this value relatively small simply because it
too is amplified by the second gain stage. It is also desirable
to have resistors R7 and R8 sufficiently large to reduce power
consumption.
The second gain stage takes the signal from the first gain
stage, V, and feeds it into the non-inverting input of a single
op-amp. This op-amp is also configured with standard
non-inverting feedback, resulting in a gain of
G2=R5/R6+1
.
The default value is set to 2, but can easily be changed.
The signal produced at the output of the second stage
amplifier, V (op-amp pin 8) is the fully amplified signal. This is
calculated as
V2 = G2* V1.
…
.Equation (2)
From this point, there are two possible output types
available. One is a simple follower circuit, as shown in Figure
3, in which the circuit output, Vout (op-amp pin 14), is
essentially a buffered V signal. This analog output option is
available for applications in which the real time nature of the
pressure signal needs to be measured. This option is selected
by connecting jumpers J5 and J6. J4 and J7 are not connected
for analog output.
The second output choice, a switch output as shown in
Figure 4, is accomplished by setting jumpers J4 and J7, and
leaving J5 and J6 unconnected. This is appropriate for
applications in which a switching function is desired. In this
case, the fourth op-amp is configured as a comparator, which
will invert V2, high or low, depending on whether V2 is larger
or smaller than the preset reference signal, set by trim-pot R9.
This signal can be used to simulate a real world threshold.
Figure 3. Analog Output Jumper Settings
Figure 4. Switch Output Jumper Settings
F
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n
.