TC7129 datasheet(13/28 Pages) MICROCHIP

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DS21459D-page 13

TC7129

4.8

Successive Integration

The successive integration technique picks up where

dual-slope conversion ends. The overshoot voltage

shown in Figure 4-9 (called the “integrator residue

voltage”) is measured to obtain a correction to the initial

count. Figure 4-10 shows the cycles in a successive

integration measurement.

The waveform shown is for a negative input signal. The

sequence of events during the measurement cycle is

shown in Table 4-1.

TABLE 4-1:

MEASUREMENT CYCLE

SEQUENCE

4.9

Digital Auto-Zeroing

To eliminate the effect of amplifier offset errors, the

TC7129 uses a digital auto-zeroing technique. After the

input voltage is measured as described above, the

measurement is repeated with the inputs shorted

internally. The reading with inputs shorted is a

measurement of the internal errors and is subtracted

from the previous reading to obtain a corrected

measurement. Digital auto-zeroing eliminates the need

for an external auto-zeroing capacitor used in other

ADCs.

4.10

Inside the TC7129

Figure 4-11 shows a simplified block diagram of the

TC7129.

Phase

Description

INT1

Input signal is integrated for fixed time (1000 clock

cycles on 2V scale, 10,000 on 200 mV).

DE1

Integrator voltage is ramped to zero. Counter

counts up until zero-crossing to produce reading

accurate to 3-1/2 digits. Residue represents an

overshoot of the actual input voltage.

REST

Rest; circuit settles.

X10

Residue voltage is amplified 10 times and

inverted.

DE2

Integrator voltage is ramped to zero. Counter

counts down until zero-crossing to correct reading

to 4-1/2 digits. Residue represents an undershoot

of the actual input voltage.

REST

Rest; circuit settles.

X10

Residue voltage is amplified 10 times and

inverted.

DE3

Integrator voltage is ramped to zero. Counter

counts up until zero-crossing to correct reading to

5-1/2 digits. Residue is discarded.

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