TC7129CLW datasheet(11/28 Pages) MICROCHIP

11 / 28 page

 2002-2012 Microchip Technology Inc.

DS21459E-page 11

TC7129

Figure 4-6:

Temperature Compensating Circuits.

4.5

RC Oscillator

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

V) resolution

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

mended value for the capacitor is 51 pF. 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 120 kHz frequency and C = 51 pF, the calculated

value of R is 75 k

. The RC oscillator and the crystal

oscillator circuits are shown in Figure 4-7.

Figure 4-7:

Oscillator Circuits.

4.6

Measuring Techniques

Two important techniques are used in the TC7129:

successive

integration

and

digital

auto-zeroing.

Successive integration is a refinement to the traditional

dual-slope conversion technique.

4.7

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, therefore,

proportional to the input voltage being measured. This

count can then be scaled and displayed as a measure-

ment 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

5 k

75 k

200 k

39 k

V–

V+

VDISP

DGND

TC7129

2N2222

39 k

V–

V+

VDISP

DGND

20 k

18 k

R =

0.45

Freq * C

TC7129

270 k

10 pF

V+

120 kHz

5 pF

V+

51 pF

75 k

De-integrate

Zero

Crossing

Time

Integrate

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