TC850CPL datasheet(10/26 Pages) MICROCHIP

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TC850

DS21479C-page 10

4.2

Zero Integrator Phase

During the zero integrator phase, the differential input

signal is disconnected from the circuit by opening inter-

nal analog gates. The internal nodes are shorted to

analog common (ground) to establish a zero input con-

dition. At the same time, a feedback loop is closed

around the input buffer, integrator and comparator. The

feedback loop ensures the integrator output is near 0V

before the signal integrate phase begins.

During this phase, a chopper-stabilization technique is

used to cancel offset errors in the input buffer, integra-

tor and comparator. Error voltages are stored on the

CBUFF, CINT and COMP capacitors. The zero integrate

phase requires 246 clock cycles.

4.3

Signal Integrate Phase

The zero integrator loop is opened and the internal dif-

ferential inputs are connected to IN+ and IN-. The differ-

ential input signal is integrated for a fixed time period.

The TC850 signal integrate period is 256 clock periods,

or counts. The crystal oscillator frequency is

÷4 before

clocking the internal counters.

The integration time period is:

EQUATION 4-1:

4.4

Reference Integrate Phase

During reference integrate phase, the charge stored on

the integrator capacitor is discharged. The time

required to discharge the capacitor is proportional to

the analog input voltage.

The reference integrate phase is divided into three

subphases:

Fast

Slow

Overrange de-integrate

During fast de-integrate, VIN- is internally connected to

analog common and VIN+ is connected across the pre-

viously-charged reference capacitor (CREF1). The inte-

grator capacitor is rapidly discharged for a maximum of

512 internal clock pulses, yielding 9 bits of resolution.

During the slow de-integrate phase, the internal VIN+

node is now connected to the CREF2 capacitor and the

residual charge on the integrator capacitor is further

discharged a maximum of 64 clock pulses. At this point,

the analog input voltage has been converted with 15

bits of resolution.

If the analog input is greater than full scale, the TC850

performs up to three overrange de-integrate sub-

phases. Each subphase occupies a maximum of 64

clock pulses. The overrange feature permits analog

inputs up to 192 LSBs greater than full scale to be

correctly converted. This feature permits the user to

digitally null up to 192 counts of input offset, while

retaining full 15-bit resolution.

In addition to 512 counts of fast, 64 counts of slow and

192 counts of overrange de-integrate, the reference

integrate phase uses 10 clock pulses to permit internal

nodes to settle. Therefore, the reference integrate

cycle occupies 778 clock pulses.

TINT =

4 x 256

FOSC

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