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Wheatstone Bridge
Bridge Sensor Circuit
Sensors for temperature, pressure, load or other phys-
ical excitation quantities are most often configured in a
Wheatstone bridge configuration. The bridge can have
anywhere from one to all four elements reacting to the
physical excitation, and should be used in a ratiometeric
configuration when possible, with the system reference
driving both the sensor and the ADC voltage reference. By
using the same reference for both the sensor excitation and
ADC, the variation in the reference can be cancelled out.
Furthermore, the output voltage from the bridge sensor is
proportional to the excitation voltage. Therefore, the ADC
that is using the external reference is more popular than
the ADC that is using internal reference, for the ratiometric
configuration.
One example sensor from GE NovaSensor is an absolute
pressure sensor, shown below, a four element varying
bridge. This example uses the MCP355X family of delta
sigma ADCs. When designing with the MCP355X family
of 22-bit delta-sigma ADCs, the initial step should be to
evaluate the sensor performance and then determine what
steps (if any) should be used to increase the overall system
resolution when using the MCP355X . In many situations, the
MCP355X devices can be used to directly digitize the sensor
output, eliminating any need for external signal conditioning
circuitry.
Using the absolute pressure sensor as our Wheatstone
bridge example, the NPP-301 device has a typical full
scale output of 60 mV when excited with a 3V battery. The
pressure range for this device is 100 kPa. The MCP3551
has a output noise speci?cation of 2.5 μV RMS .
The following equation is a ?rst order approximation of the
relationship between pressure in pascals (P) and altitude (h),
in meters.
h
log(P) ≈ 5 –
15500
Using 60 mV as the full scale range and 2.5 μV as the
resolution, the resulting resolution from direct digitization in
meters is 0.64 meters or approximately 2 feet.
It should be noted that this is only used as an example for
discussion; temperature effects and the error from a ?rst
order approximation must be included in ?nal system design.
Example of Wheatstone Bridge Sensor Configuration with High Resolution Delta-Sigma ADC
0.1 μF 1.0 μF
To V DD
NPP-301
R 1
R 2
R 3
2
3
1 8
V REF V DD
V IN+
SCK
MCP3551 SDO
V IN- CS
V SS
SPI
5, 6, 7
MCU
R 4
4
Altimeter Watch
?V ~ [(?R 2 + ?R 4 ) - (?R 1 +?R 3 )]/4R * V DD
With R 1 = R 2 = R 3 = R 4 = R
Signal Chain Design Guide
9
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