AN929 datasheet(13/22 Pages) MICROCHIP

13 / 22 page

 2004 Microchip Technology Inc.

DS00929A-page 13

AN929

SINGLE POWER SUPPLY CIRCUIT

Figure 19 provides a low-cost, single power supply

thermocouple amplifier circuit using a quad op amp.

The buffered input differential amplifier topology is

similar to an instrumentation amplifier and offers the

feature of equal and high input impedance at the ampli-

fier inputs. An instrumentation amplifier with integrated

gain resistors can also be used to implement this cir-

cuit. The gain of the amplifier was selected to be 249,

providing a temperature coefficient of 10 mV/

°C. The

thermocouple inputs are biased to VDD/2 through

10 M

Ω resistors, providing the ability to detect a failed

open-circuit thermocouple.

FIGURE 19:

Schematic of Single Supply Voltage Thermocouple Amplifier.

RTD Oscillator Circuits

Oscillator circuits can be used to provide an accurate

temperature measurement with an RTD sensor. The

state variable oscillator provides an output frequency

that is proportional to the square root of the product of

two temperature-sensing resistors and is a good circuit

for precision applications. The astable multi-vibrator or

relaxation oscillator provides a square wave output

with a single amplifier and is a good alternative for

cost-sensitive applications.

The components must be chosen carefully so that the

change in the oscillation frequency results primarily

from the RTD and not from variation due to the compo-

nent tolerance, temperature coefficient and drift rate.

Metal film resistors, metal foil resistors and NPO

porcelain capacitors are recommended to minimize the

component error. Capacitors are relatively poor in

performance when compared to resistors. Typically, the

capacitor limits the accuracy of the oscillator. Further-

more, precision capacitors are only available in

relatively small values. The state variable oscillator

requires two 100 nF capacitors, while the relaxation

oscillator uses a 0.68 µF capacitor to produce a nomi-

nal oscillation of 1 kHz. The state variable and relax-

ation circuits have an uncalibrated measurement

accuracy of approximately 1

°C and 3°C, respectively.

The difference is primarily due to the capacitor error.

An application that requires an accuracy of better than

±1

°C may require a temperature calibration and burn-

in procedure. A temperature compensation algorithm

can easily be implemented using the E2 non-volatile

memory of a microcontroller to store temperature

correction data in a look-up table. The temperature

coefficients are obtained by calibrating the circuit over

temperature and comparing the measured temperature

against the actual temperature. A burn-in or tempera-

ture-cycling procedure can significantly reduce the drift

of the resistors and capacitors. Burn-in procedures are

useful because the majority of the change in magnitude

of resistors and capacitors occurs within the 500 hours

of a life test.

EMI Filter

Connector

Shielded

Cable

TC1047A

ADC

IN_1

IN_2

+5V

R1 = R2 = 1 MΩ

MCP619

Cold Junction Compensation

C1 = C2 = 1 nF

R7 = R8 = 249 kΩ

R5 = R6 = 1 kΩ

R3 = R4 = 10 MΩ

C3 = 0.1 µF

+5V

Type K

Thermocouple

VIN1

VIN2

U1A

U1B

OUT

IN 2

IN1

–

()

-------





REF

∆

() 249k

------------





Temp. Coef.

249

µV °C

⁄

10mV

°C

⁄

≅

U1C

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