TC7106 datasheet(9/34 Pages) MICROCHIP

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DS21455D-page 9

TC7106/A/TC7107/A

4.0

ANALOG SECTION

In addition to the basic signal integrate and de-

integrate cycles discussed, the circuit incorporates an

auto-zero cycle. This cycle removes buffer amplifier,

integrator, and comparator offset voltage error terms

from the conversion. A true digital zero reading results

without adjusting external potentiometers. A complete

conversion consists of three cycles: an auto-zero,

signal integrate, and reference integrate cycle.

4.1

Auto-Zero Cycle

During the auto-zero cycle, the differential input signal

is disconnected from the circuit by opening internal

analog gates. The internal nodes are shorted to analog

common (ground) to establish a zero input condition.

Additional analog gates close a feedback loop around

the integrator and comparator. This loop permits

comparator offset voltage error compensation. The

voltage level established on CAZ compensates for

device offset voltages. The offset error referred to the

input is less than 10 µV.

The auto-zero cycle length is 1000 to 3000 counts.

4.2

Signal Integrate Cycle

The auto-zero loop is entered and the internal

differential inputs connect to VIN+ and VIN-. The

differential input signal is integrated for a fixed time

period. The TC7106/TC7106A signal integration period

is 1000 clock periods or counts. The externally set

clock frequency is divided by four before clocking the

internal counters.

The integration time period is:

EQUATION 4-1:

The differential input voltage must be within the device

Common mode range when the converter and

measured system share the same power supply

common (ground). If the converter and measured

system do not share the same power supply common,

VIN- should be tied to analog common.

Polarity is determined at the end of signal integrate

phase. The sign bit is a true polarity indication, in that

signals less than 1 LSB are correctly determined. This

allows precision null detection limited only by device

noise and auto-zero residual offsets.

4.3

Reference Integrate Phase

The third phase is reference integrate or de-integrate.

VIN- is internally connected to analog common and

VIN+ is connected across the previously charged

reference capacitor. Circuitry within the chip ensures

that the capacitor will be connected with the correct

polarity to cause the integrator output to return to zero.

The time required for the output to return to zero is

proportional to the input signal and is between 0 and

2000 counts.

The digital reading displayed is:

EQUATION 4-2:

Where:

FOSC

Externally set clock frequency

OSC

-------------

1000

REF

-------------

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