TC820CPL Просмотр технического описания (PDF) - TelCom Semiconductor Inc => Microchip

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TC820CPL

3-3/4 DIGIT A/D CONVERTER WITH FREQUENCY COUNTER AND LOGIC PROBE

TelCom Semiconductor Inc => Microchip 

3-3/4 A/D CONVERTER WITH

FREQUENCY COUNTER

1

AND LOGIC PROBE

TC820

∫ 1

tINT

RINT CINT 0

VIN (t) dt =

VREF tDEINT

RINT CINT

where: VREF = Reference voltage

tINT = Integration time

tDEINT = Deintegration time

For a constant t INT:

VIN

=

VREF

ϫ

t DEINT

t INT

Accuracy in a dual-slope converter is unrelated to the

integrating resistor and capacitor values as long as they are

stable during a measurement cycle. An inherent benefit of

the dual-slope technique is noise immunity. Noise spikes

are integrated or averaged to zero during the integration

periods, making integrating ADCs immune to the large

conversion errors that plague successive approximation

converters in high-noise environments. Interfering signals,

with frequency components at multiples of the averaging

(integrating) period, will be attenuated (Figure 3). Integrating

ADCs commonly operate with the signal integration period

set to a multiple of the 50/60Hz power line period.

30

T = MEASUREMENT

PERIOD

20

10

0

0.1/T

1/T

10/T

INPUT FREQUENCY

Figure 3. Normal-Mode Rejection of Dual-Slope Converter

Analog Section

In addition to the basic integrate and deintegrate dual-

slope phases discussed above, the TC820 design incorpo-

rates a "zero integrator output" phase and an "auto-zero"

phase. These additional phases ensure that the integrator

starts at 0V (even after a severe overrange conversion), and

that all offset voltage errors (buffer amplifier, integrator and

TELCOM SEMICONDUCTOR, INC.

comparator) are removed from the conversion. A true digital

2 zero reading is assured without any external adjustments.

A complete conversion consists of four distinct phases:

(1) Zero Integrator Output

(2) Auto-Zero

(3) Signal Integrate

(4) Reference Deintegrate

Zero Integrator Output Phase

3 This phase guarantees that the integrator output is at 0V

before the system zero phase is entered, ensuring that the

true system offset voltages will be compensated for even

after an overrange conversion. The duration of this phase is

500 counts plus the unused deintegrate counts.

Auto-Zero Phase

During the auto-zero phase, the differential input signal

4 is disconnected from the measurement circuit by opening

internal analog switches, and the internal nodes are shorted

to Analog Common (0VREF) to establish a zero input condi-

tion. Additional analog switches close a feedback loop

around the integrator and comparator to permit comparator

offset voltage error compensation. A voltage established on

CAZ then compensates for internal device offset voltages

during the measurement cycle. The auto-zero phase re-

5 sidual is typically 10µV to 15µV. The auto-zero duration is

1500 counts.

Signal Integration Phase

Upon completion of the auto-zero phase, the auto-zero

loop is opened and the internal differential inputs connect to

VIN+ and VIN–. The differential input signal is then integrated

for a fixed time period, which is 2000 counts (4000 clock

6 periods). The externally-set clock frequency is divided by

two before clocking the internal counters. The integration

time period is:

tINT =

4000

fOSC

The differential input voltage must be within the device's

7 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, as in battery-powered applications,

VIN– should be tied to analog common.

Polarity is determined at the end of signal integration

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

signals less than 1 LSB are correctly determined. This

8 allows precision null detection that is limited only by device

noise and auto-zero residual offsets.

3-157

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