TC7129CLW datasheet(11/24 Pages) MICROCHIP

11 / 24 page

DS21459B-page 11

TC7129

FIGURE 4-6:

TEMPERATURE COMPENSATING CIRCUITS

4.8

RC Oscillator

For applications in which 3-1/2 digit (100

µV) resolution

is sufficient, an RC oscillator is adequate. A recom-

mended value for the capacitor is 51pF. Other values

can be used as long as they are sufficiently larger than

the circuit parasitic capacitance. The resistor value is

calculated as:

EQUATION 4-1:

For 120kHz frequency and C = 51pF, the calculated

value of R is 75k

Ω. The RC oscillator and the crystal

oscillator circuits are shown in Figure 4-7.

FIGURE 4-7:

OSCILLATOR CIRCUITS

4.9

Measuring Techniques

Two important techniques are used in the TC7129: suc-

cessive integration and digital auto-zeroing. Succes-

sive integration is a refinement to the traditional dual

slope conversion technique.

4.10

Dual Slope Conversion

A dual slope conversion has two basic phases: inte-

grate and de-integrate. During the integrate phase, the

input signal is integrated for a fixed period of time; the

integrated voltage level is thus proportional to the input

voltage. During the de-integrate phase, the integrated

voltage is ramped down at a fixed slope, and a counter

counts the clock cycles until the integrator voltage

crosses zero. The count is a measurement of the time

to ramp the integrated voltage to zero, and is, there-

fore, proportional to the input voltage being measured.

This count can then be scaled and displayed as a mea-

surement of the input voltage. Figure 4-8 shows the

phases of the dual slope conversion.

FIGURE 4-8:

DUAL SLOPE

CONVERSION

The dual slope method has a fundamental limitation.

The count can only stop on a clock cycle, so that mea-

surement accuracy is limited to the clock frequency. In

addition, a delay in the zero crossing comparator can

add to the inaccuracy. Figure 4-9 shows these errors in

an actual measurement.

TC7129

1N4148

Ω

75k

Ω

200k

Ω

39k

Ω

V-

V+

VDISP

DGND

TC7129

2N2222

39k

Ω

V-

V+

VDISP

DGND

20k

Ω

18k

Ω

0.45

Freq * C

TC7129

270k

Ω

10pF

V+

120kHz

5pF

V+

51pF

75k

Ω

De-integrate

Zero

Crossing

Time

Integrate

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