TC7107 datasheet(18/26 Pages) MICROCHIP

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TC7106/A/TC7107/A

DS21455B-page 18

10.4

Ratiometric Resistance

Measurements

The true differential input and differential reference

make ratiometric reading possible. Typically in a ratio-

metric operation, an unknown resistance is measured,

with respect to a known standard resistance. No accu-

rately defined reference voltage is needed.

The unknown resistance is put in series with a known

standard and a current passed through the pair. The

voltage developed across the unknown is applied to the

input and the voltage across the known resistor is

applied to the reference input. If the unknown equals

the standard, the display will read 1000.

The displayed reading can be determined from the

following expression:

The display will over range for:

RUNKNOWN ≥ 2xRSTANDARD

FIGURE 10-2:

LOW PARTS COUNT

RATIOMETRIC

RESISTANCE

MEASUREMENT

FIGURE 10-3:

TEMPERATURE SENSOR

FIGURE 10-4:

POSITIVE TEMPERATURE

COEFFICIENT RESISTOR

TEMPERATURE SENSOR

FIGURE 10-5:

TC7106A, USING THE

INTERNAL REFERENCE:

200mV FULL SCALE, 3

READINGS-PER-SECOND

(RPS)

Displayed Reading

()

RUnknown

dard

tan

-------------------------------x1000

VREF+

VREF-

VIN+

VIN-

Analog

Common

TC7106A

LCD Display

RSTANDARD

RUNKNOWN

V+

V-

VIN-

VIN+

VREF+

VREF-

Common

50k

Ω

160k

Ω

300k

Ω

300k

Ω

50k

Ω

1N4148

Sensor

TC7106A

VFS = 2V

TC7106A

V+

V-

VIN-

VIN+

VREF+

VREF-

Common

5.6k

Ω

160k

Ω

20k

Ω

1N914

20k

Ω

0.7%/

°C

PTC

100k

Ω

100pF

0.47

µF

47k

Ω

0.22

µF

To Display

To Backplane

0.1

µF

Ω

22k

Ω

Set VREF = 100mV

TC7106A

0.01

µF

Ω

–

To Pin 1

–

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