AN929 datasheet(3/22 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS00929A-page 3

AN929

Ramp Rate Method

The resistor-capacitor (RC) ramp rate method shown in

Figure 5 provides a simple solution for resistive

sensors, such as thermistors. This method provides a

low-cost solution with an accuracy of approximately

±1%. When voltage is applied to a RC combination, the

capacitor’s voltage will increase exponentially and the

ramping time can be measured with a comparator. The

accuracy of the RC timing method can be improved by

comparing the ramp rates of a sensor resistor (RSensor)

and a known resistance (RREF) to reduce the effects of

temperature and component tolerance.

The RC timing method can be implemented using the

circuitry inside a microcontroller. The timing measure-

ment begins by configuring the GP1 pin as a logic ‘1’

output and GP2 as an input. This connects RSENSOR to

a logic-high voltage (VOH ≅ VDD), while RREF is discon-

nected from the circuit via the high impedance of an

input pin. The I/O pin AN0 is configured as an input and

connects the capacitor to a comparator inside the

microcontroller. The time required for the capacitor

voltage to ramp-up to the reference voltage of the

comparator (VTH ≅ VREF) can be determined through

either a hardware or software timer. Once the measure-

ment is completed, the GP1 pin is toggled to a logic ‘0’

output to discharge the capacitor. The configuration

procedure of GP1 and GP2 is then reversed in order to

measure RREF.

FIGURE 5:

Ramp Rate Method.

Duty Cycle Output

Silicon IC sensors that provide an output proportional

to the duty cycle of a digital signal are available, as

shown in Figure 6. These sensors typically have a

constant “on” time (t1) and a variable “off” time (t2).

However, it is often necessary to calculate the t1-to-t2

ratio in order to achieve the specified sensor accuracy.

It is possible to have a duty cycle output with a thermo-

couple, RTD or thermistor. However, the circuit is

relatively complex when compared to other signal

conditioning options.

FIGURE 6:

Duty Cycle Output.

VREF

Comparator

Input

Output

Input

Output

RREF

RSensor

AN0

GP1

GP2

PIC16C5X

t = 0

t = t1

t = t2

Voltage (V)

Time (t)

VTH

RREF

RSensor

c t

()

DD 1e

–

--------

–







tRC In 1

-----------

–





–

Sensor

-----





REF

Temp.

∝ t

1 / t2

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