TC850_13 datasheet(8/30 Pages) MICROCHIP

Nome de Peças	TC850

Descrição Electrónicos	15-Bit, Fast Integrating CMOS A/D Converter

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Fabricante Electrônico	MICROCHIP [Microchip Technology]

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TC850

DS21479D-page 8

 2001-2012 Microchip Technology Inc.

3.0

DETAILED DESCRIPTION

The TC850 is a multiple-slope, integrating A/D con-

verter (ADC). The multiple-slope conversion process,

combined with chopper-stabilized amplifiers, results in

a significant increase in ADC speed, while maintaining

very high resolution and accuracy.

3.1

Dual-Slope Conversion Principles

The conventional dual-slope converter measurement

cycle (shown in Figure 3-1) has two distinct phases:

1.

Input signal integration

2.

Reference voltage integration (de-integration).

FIGURE 3-1:

Dual-Slope ADC Cycle

The input signal being converted is integrated for a

fixed time period, measured by counting clock pulses.

An opposite polarity constant reference voltage is then

de-integrated until the integrator output voltage returns

to zero. The reference integration time is directly

proportional to the input signal.

In a simple dual-slope converter, complete conversion

requires the integrator output to “ramp-up” and “ramp-

down.” Most dual-slope converters add a third phase,

auto-zero. During auto-zero, offset voltages of the input

buffer, integrator and comparator are nulled, thereby

eliminating the need for zero offset adjustments.

Dual-slope converter accuracy is unrelated to the inte-

grating resistor and capacitor values, as long as they

are stable during a measurement cycle. By converting

the unknown analog input voltage into an easily mea-

sured function of time, the dual-slope converter

reduces the need for expensive, precision passive

components.

Noise immunity is an inherent benefit of the integrating

conversion method. Noise spikes are integrated, or

averaged, to zero during the integration period. Inte-

grating ADCs are immune to the large conversion

errors

that

plague

successive

approximation

converters in high-noise environments.

A simple mathematical equation relates the input

signal, reference voltage and integration time:

EQUATION 3-1:

3.2

Multiple-Slope Conversion

Principles

One limitation of the dual-slope measurement tech-

nique is conversion speed. In a typical dual-slope

method, the auto-zero and integrate times are each

one-half of the de-integrate time. For a 15-bit conver-

sion, 214 +214 +215 (65,536) clock pulses are required

for auto-zero, integrate and de-integrate phases,

respectively. The large number of clock cycles

effectively limits the conversion rate to about 2.5

conversions per second, when a typical analog CMOS

fabrication process is used.

The TC850 uses a multiple-slope conversion technique

to increase conversion speed (Figure 3-2). This tech-

nique makes use of a two-slope de-integration phase

and permits 15-bit resolution up to 40 conversions per

second.

During the TC850’s de-integration phase, the integra-

tion capacitor is rapidly discharged to yield a

resolution of 9 bits. At this point, some charge will

remain on the capacitor. This remaining charge is then

slowly de-integrated, producing an additional 6 bits of

resolution. The result is 15 bits of resolution achieved

with only 29 +26 (512 + 64, or 576) clock pulses for

de-integration. A complete conversion cycle occupies

only 1280 clock pulses.

In order to generate “fast-slow” de-integration phases,

two voltage references are required. The primary refer-

ence (VREF1) is set to one-half of the full scale voltage

(typically VREF1 = 1.6384V, and VFS = 3.2768V). The

secondary voltage reference (VREF2) is set to VREF1/64

(typically 25.6 mV). To maintain 15-bit linearity, a

tolerance of 0.5% for VREF2 is recommended.

FIGURE 3-2:

“Fast Slow” Reference De-

Integration Cycle

Auto

Zero

0V

Integrator

Output

End of Conversion

Reference

De-integrate

Signal De-integrate

Time

1

RINTCINT

TINT

0

VIN(T)DT =

VREF TDEINT

RINTCINT

where:

VREF

= Reference voltage

TINT

= Signal integration time (fixed)

TDEINT = Reference voltage integration time

(variable).

Auto

Zero

0V

Integrator

Output

End of Conversion

"Slow" Reference De-integrate

(6-Bit Resolution)

"Fast" Reference

De-integrate

(9-Bit Resolution)

Signal Integrate

Time

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