TMPSNSRD-RTD2 Microchip Technology, TMPSNSRD-RTD2 Datasheet - Page 9

• Manufacturer Part Number: TMPSNSRD-RTD2
• Description: BOARD RTD REFERENCE DESIGN
• Manufacturer: Microchip Technology

Specifications of TMPSNSRD-RTD2

• Sensor Type: Temperature
• Interface: USB
• Embedded: No
• Utilized Ic / Part: MCP3551, MCP9804
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -
• Sensitivity: -
• Sensing Range: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

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 ele ments 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 perfor mance and then determine what
Example of Wheatstone Bridge Sensor Configuration with High Resolution Delta-Sigma ADC
Wheatstone Bridge
Altimeter Watch
NPP-301
R
1
∆V ~ [(∆R
With R
R
R
1
4
2
= R
2
2
R
+ ∆R
= R
3
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
out put, eliminating any need for external signal condition ing
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 specifi cation of 2.5 μV
The following equation is a fi rst order approximation of the
relation ship between pressure in pascals (P) and altitude (h),
in meters.
log(P) ≈ 5 –
Using 60 mV as the full scale range and 2.5 μV as the
resolution, the resulting resolution from direct digitiza tion 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 fi rst
order approximation must be included in fi nal sys tem design.
4
3
) - (∆R
= R
0.1 μF 1.0 μF
4
= R
1
+∆R
3
3
15500
)]/4R * V
2
V
h
V
IN-
IN+
MCP3551
V
1
V
REF
SS
DD
4
V
DD
SCK
SDO
CS
8
Signal Chain Design Guide
5, 6, 7
SPI
RMS
.
MCU
To V
DD
9