TC7129(2002) View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

TC7129
(Rev.:2002)

4-1/2 Digit Analog-to-Digital Converters with On-Chip LCD Drivers

Microchip Technology 

TC7129

FIGURE 4-6:

TEMPERATURE COMPENSATING CIRCUITS

V+

V+

1N4148

5kΩ

75kΩ

39kΩ

200kΩ

–

+

24

TC7129

19 VDISP

36 DGND

23

39kΩ

24

20kΩ

2N2222 TC7129

18kΩ

19

VDISP

36

DGND

23

V-

4.8 RC Oscillator

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

R = 0.45

Freq * C

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

value of R is 75kΩ. The RC oscillator and the crystal

oscillator circuits are shown in Figure 4-7.

FIGURE 4-7:

OSCILLATOR CIRCUITS

1

5pF

V+

120kHz

40

270kΩ

10pF

TC7129

2

V+

V-

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:

Integrate

DUAL SLOPE

CONVERSION

De-integrate

TC7129

1

40

2

75kΩ

51pF

© 2002 Microchip Technology Inc.

Zero

Crossing

Time

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.

DS21459B-page 11

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