TMPSNSRD-RTD2 Microchip Technology, TMPSNSRD-RTD2 Datasheet - Page 12

• 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

Thermistor Solutions
Typically, thermistors require an external resistor for biasing.
In addition, the inherent non-linearity of a thermistor is
improved by biasing the thermistor in a resistive ladder
circuit to linearize the temperature-to-voltage conversion.
Typically, the thermistor voltage is directly connected to an
ADC to digitize the voltage measurement. The measured
voltage is converted to temperature using a lookup table.
However, at hot and cold temperature extremes the non-
linearity of this approach is much greater with reduced
change in voltage, which results in lower accuracy. This
requires higher resolution and a more costly ADC.
The solution is to use Microchip’s Linear Active Thermistors,
the MCP9700 and MCP9701. These are low-cost voltage
output temperature sensors that replace almost any
Thermistor application solutions. Unlike resistive type
sensors such as Thermistors, the signal conditioning
and noise immunity circuit development overhead can be
avoided by using the low-cost Linear Active Thermistors.
These sensors output voltage is proportional to ambient
temperature with temperature coeffi cient of 10 mV/°C and
19.5 mV/°C. Unlike thermistors, these devices do not require
additional computation for temperature measurement.
The factory set coeffi cients provide linear interface to
measure ambient temperatures (refer to AN1001 for sensor
optimization).
MCP9700 and MCP9701 Key Features
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Applications
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RTD Instrumentation Circuit Block Diagram and Output Performance (see Application Note AN1154)
12
Thermistor and RTD Solutions
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SC70, TO92 packages
Operating temperature range: -40°C to +150°C
Temperature Coeffi cient: 10 mV/°C (MCP9700)
Temperature Coeffi cient: 19.5 mV/°C (MCP9701)
Low power: 6 μA (typ.)
Refrigeration equipment
Power supply over temperature protection
General purpose temperature monitoring
Signal Chain Design Guide
V
*See LDO Data Sheet at: www.microchip.com/LDO
DD
MCU
PIC
C*
®
LDO
SPI
3
1 μF
V
DD
MCP3551
C*
V
REF
+
–
V
V
LDO
REF
R
R
B
A
5%
1%
RTD
RTD Solution
Resistive Temperature Detectors (RTDs) are highly accurate
and repeatable temperature sensing elements. When
using these sensors a robust instrumentation circuit is
required and it is typically used in high performance thermal
management applications such as medical instrumentation.
This solution uses a high performance Delta-Sigma Analog-
to-Digital converter, and two resistors to measure RTD
resistance ratiometrically. A ±0.1°C accuracy and ±0.01°C
measurement resolution can be achieved across the RTD
temperature range of -200°C to +800°C with a single point
calibration.
This solution uses a common reference voltage to bias
the RTD and the ADC which provides a ratio-metric relation
between the ADC resolution and the RTD temperature
resolution. Only one biasing resistor, RA, is needed to set the
measurement resolution ratio (shown in equation below).
RTD Resistance
For instance, a 2V ADC reference voltage (V
a 1 μV/LSb (Least Signifi cant Bit) resolution. Setting RA =
RB = 6.8 kΩ provides 111.6 μV/°C temperature coeffi cient
(PT100 RTD with 0.385Ω/°C temperature coeffi cient). This
provides 0.008°C/LSb temperature measurement resolution
for the entire range of 20Ω to 320Ω or -200°C to +800°C. A
single point calibration with a 0.1% 100Ω resistor provides
±0.1°C accuracy as shown in the fi gure below.
This approach provides a plug-and-play solution with
minimum adjustment. However, the system accuracy
depends on several factors such as the RTD type, biasing
circuit tolerance and stability, error due to power dissipation
or self-heat, and RTD non-linear characteristics.
R RTD = R A
Where:
Code = ADC output code
-0.05
0.05
-0.1
R A = Biasing resistor
0.1
n = ADC number of bits
0
-200
(22 bits with sign, MCP3551)
(
2 n – 1 – Code
0
Code
Temperature (°C)
200
)
400
600
REF
) results in
800