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Signal Chain Design Guide

Temperature Sensing Solutions

Resistive Temperature Detector (RTD)

Solutions

RTD Solution with RC Oscillators

RC oscillators offer several advantages in precision

sensing applications. They do not require an Analog-to-

Digital Converter (ADC), and oscillator can be directly

connected to an Input/Output pin of a microcontroller

to measure change in frequency proportional to sensor

output. The accuracy of the frequency measurement is

directly related to the quality of the microcontroller’s clock

signal, and high-frequency oscillators for the controller are

available with accuracies of better than 10 ppm.

The oscillator circuits shown in the Oscillator Circuits

For Sensors section can be used for this method. The

variable resistor of the circuits (Figure: Oscillator Circuits

for Resistive Sensors) are replaced with the RTD sensor.

There is an example of a state variable RC oscillator, which

provides an output frequency that is proportional to the

square root of the product of the two RTD resistances

(α 1/(R1 × R2)

1/2

). A second example shows the

relaxation oscillator (or astable multi-vibrator), which

provides a square wave output with a single comparator.

The state variable RC oscillator is good for precision

applications, while the relaxation oscillator is an

alternative for cost-sensitive applications.

RTD Solution with Instrumentation Ampliﬁer

This Wheatstone bridge reference design board

demonstrates the performance of Microchip’s MCP6N11

instrumentation amplifier (INA) and a traditional three op amp

INA using Microchip’s MCP6V26 and MCP6V27 auto-zeroed

op amps. The input signal comes from an RTD temperature

sensor in a Wheatstone bridge. Real world interference is

added to the bridge’s output, to provide realistic performance

comparisons. Data is gathered and displayed on a PC, for

ease of use. The USB PIC® microcontroller and included

Graphical User Interface (GUI) provides the means to

configure the board and collect sample data.

MCP6N11 and MCP6V2X Wheatstone Bridge

Reference Design (ARD00354)

–

RTD

Using USB

PWM

Coupling

(Thermal Management Software)

VREF

12-bit ADC Module

PIC18F2550 (USB) Microcontroller

PWM

Input

Filter

Output

Filter

INA

Thermocouple Sensor Solutions

Thermocouple Solution with Precision

Delta-Sigma ADC

Delta-Sigma ADCs can be used to directly measure

thermocouple voltage. Microchip’s MCP3421 ADC can

be used to accurately measure temperature using a

Thermocouple. The device provides a plug and play solution

for various types of thermocouples, greatly simplifying the

circuit design. In this case, the Thermocouple linearization

routine is implemented in firmware or software. Cold

Junction Compensation is implemented using Microchip’s

stand alone digital temperature sensors, such as the

±0.5C accurate MCP9808.

This solution can be evaluated using Microchip’s

Thermocouple Reference Design Board (TMPSNSRD-TCPL1).

Thermocouple Solution with Auto-Zero’ed Op Amp

Microchip’s auto-zeroed op amp can be used to accurately

measure thermocouple voltage. The MCP6V01 op amp

ultra low offset voltage and high common mode rejection

makes it ideal for low cost thermocouple applications.

The MCP6V01 Thermocouple Auto-Zeroed Reference

design demonstrates how to accurately measure

temperature (MCP6V01RD-TCPL).

Wireless Temperature Monitoring Solution

MCP9804

Temp. Sensor

PIC18F2550

USB PIC

Microcontroller

MCP3421

18-bit ADC

Thermocouple

Thermal Pad

MCP3421

–

(Thermocouple)

Heat

PIC

MCU

18-bit ∆

∑

ADC

2.4 GHz

MRF24J40

MCP9804

Temp Sensor

±1°C