3-252

TELCOM SEMICONDUCTOR, INC.

TC7136

TC7136A

LOW POWER, 3-1/2 DIGIT

ANALOG-TO-DIGITAL CONVERTERS

Figure 2. Normal-Mode Rejection of Dual-Slope Converter

The dual-slope converter accuracy is unrelated to the

integrating resistor and capacitor values, as long as they are

stable during a measurement cycle. Noise immunity is an

inherent benefit. Noise spikes are integrated, or averaged,

to zero during integration periods. Integrating ADCs are

immune to the large conversion errors that plague succes-

sive approximation converters in high-noise environments.

Interfering signals with frequency components at multiples

of the averaging period will be attenuated. Integrating ADCs

commonly operate with the signal integration period set to a

multiple of the 50 Hz/60 Hz power line period.

ANALOG SECTION

In addition to the basic integrate and deintegrate dual-

slope cycles discussed above, the TC7136/A designs incor-

porate an "integrator output-zero cycle" and an "auto-zero

cycle." These additional cycles ensure the integrator starts

at 0V (even after a severe overrange conversion) and that all

offset voltage errors (buffer amplifier, integrator and com-

parator) are removed from the conversion. A true digital zero

reading is assured without any external adjustments.

A complete conversion consists of four distinct phases:

(1) Integrator output-zero phase

(2) Auto-zero phase

(3) Signal integrate phase

(4) Reference deintegrate phase

Integrator Output-Zero Phase

This phase guarantees the integrator output is at 0V

before the system-zero phase is entered. This ensures that

true system offset voltages will be compensated for even

after an overrange conversion. The count for this phase is a

function of the number of counts required by the deintegrate

phase.

The count lasts from 11 to 140 counts for non-overrange

conversions and from 31 to 640 counts for overrange

conversions.

Auto-Zero Phase

During the auto-zero phase, 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 volt-

age error compensation. The voltage level established on

phase residual is typically 10

µV to 15 µV.

The auto-zero duration is from 910 to 2900 counts for

non-overrange conversions and from 300 to 910 counts for

overrange conversions.

Signal Integration Phase

The auto-zero loop is entered and the internal differen-

+

and V

–

is integrated for a fixed time period. The TC7136/A 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:

1000,

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, V

–

mon.

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.

Reference Integrate Phase

–

+

is con-

nected 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 internal clock periods. The digital

reading displayed is:

1000

30

20

10

0

0.1/t

1/t

10/t

INPUT FREQUENCY

t = MEASUREMENT PERIOD

4