3–324
Motorola Sensor Device Data
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Since the third op–amp contributes a gain of two to the
overall system, design the gain that the two op–amp circuit
contributes to the system to be one–half the desired sys-
tem gain. The gain term for the two op–amp circuit is:
G =
R4
R3
+
2R4
RG
+ 1
which is the same as presented in Equation 16.
Similarly, since the third op–amp also amplifies VREF by
two (refer to
Equation 22), the resistor divider that creates
VREF should be designed to provide one–half the desired
positive dc voltage level shift needed for the final output.
When designing the voltage divider for VREF, use the same
design constraints as were given in the section on Applica-
tion to Pressure Sensor Circuits.
With the above simplification of R5 = R6, the negative dc
level shift, V–shift, which is also created by a voltage divider, is
now amplified by a factor of unity. When designing the voltage
divider, use the same design constraints as were presented in
the section on Application to Pressure Sensor Circuits.
CONCLUSION
The amplifier circuits discussed in this paper apply to
pressure sensor applications, but the amplifier circuits can be
interfaced to low–level, differential–voltage output sensors, in
general. All of the circuits exhibit the desired instrumentation
amplifier characteristics of high input impedance, low output
impedance, high gain capability, and differential to
single–ended conversion of the sensor signal. Each
amplifier circuit provides positive dc level shift capability, while
the last two circuit topologies presented are also able to
provide a negative dc voltage level shift. This enables the user
to position the sensor’s dynamic output within a specified
range (e.g., within the high and low references of an A/D
converter). Also detailed is a method of using an additional
feedback resistor to adjust easily the differential voltage gain,
while not sacrificing common mode rejection. Combining the
appropriate sensor device and amplifier interface circuit
provides sensor users with a versatile system solution for
applications in which the ideal fully single–conditioned sensor
does not exist or in which such signal flexibility is warranted.
F
Freescale Semiconductor, Inc.
n
.