Datasheet
11
Signal Chain Design Guide
Temperature Sensing Solutions
Thermistor Solution
Thermistors are non-linear and require a look up table for
compensation. The solution is to use Microchip’s Linear
Active Thermistors, the MCP9700 and the 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 at the non-linear region and noise
immunity circuit development overhead can be avoided by
using the low-cost Linear Active Thermistors. The voltage
output pin (V
out) can be directly connected to the ADC
input of a microcontroller. The MCP9700/9700A and
MCP9701/9701A temperature coefficients are scaled
to provide a 1°C/bit resolution for an 8-bit ADC with a
reference voltage of 2.5V and 5V, respectively.The MCP9700
and MCP9701 sensors output can be compensated for
improved sensor accuracy as shown below, refer to the
AN1001 application note.
MCP9700 and MCP9701 Key Features
■ 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.)
Applications
■ Refrigeration equipment
■ Power supply over temperature protection
■ General purpose temperature monitoring
Typical Sensor Accuracy Before and After
Compensation
Resistive Temperature Detector (RTD)
Solutions
RTD Solution with Precision Delta-Sigma ADC
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.
Microchip’s RTD solution uses a high performance Delta-
Sigma Analog to Digital converter, two external resistors, and a
reference voltage to measure RTD resistance or temperature
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, R
A
, is needed to set the
measurement resolution ratio (shown in equation below).
RTD Resistance
For instance, a 2V ADC reference voltage (Vref) results in
a 1μV/LSb (Least Significant Bit) resolution. Setting R
A
=
RB=6.8 kΩ provides 111.6 μV/°C temperature coefficient
(PT100 RTD with 0.385Ω/°C temperature coefficient). 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 figure 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.
This solution can be evaluated using Microchip’s RTD
Reference Design Board (TMPSNSRD-RTD2).
Code
R
RTD
= R
A
(
2
n − 1
−
Code
)
Where:
Code = ADC output code
R
A
= Biasing resistor
n = ADC number of bits
(22 bits with sign, MCP3551)
RTD Instrumentation Circuit Block Diagram and Output Performance (see Application Note AN1154)
Measured Accuracy (°C)
-200 0
Temperature (°C)
600400200
0.1
0.05
0
-0.05
-0.1
800
V
REF
V
DD
1 µF
MCP3551
–
+
RTD
R
B
5%
R
A
1%
V
DD
V
REF
SPI
LDO
V
LDO
C* C*
3
*See LDO Data Sheet at: www.microchip.com/LDO
PIC
®
MCU