TC7136ACPL datasheet(9/12 Pages) TELCOM

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TELCOM SEMICONDUCTOR, INC.

Component

Value

Nominal Full-Scale Voltage

200mV

CAZ

0.47

µF

0.1

µF

RINT

180 k

Ω

1.8 M

Ω

CINT

0.047

µF

0.047

µF

NOTE:fOSC = 48 kHz (3 readings per sec). ROSC = 180kΩ, COSC = 50

output voltage back to zero during the reference integrate

phase, is stored on CREF. A 0.1 µF capacitor is acceptable

when V

–

REF is tied to analog common. If a large common-

mode voltage exists (V

–

REF ≠ analog common) and the

application requires a 200 mV full scale, increase CREF to

µF. Roll-over error will be held to less than 0.5 count. A

Mylar-type dielectric capacitor is adequate.

Integrating Capacitor (CINT)

CINT should be selected to maximize integrator output

voltage swing without causing output saturation. Analog

common will normally supply the differential voltage refer-

ence this case, a

±2V full-scale integrator output swing is

satisfactory. For 3 readings per second (fOSC = 48 kHz) a

0.047

µF value is suggested. For one reading per second,

0.15

µF is recommended. If a different oscillator frequency

is used, CINT must be changed in inverse proportion to

maintain the nominal

±2V integrator swing.

An exact expression for CINT is:

Oscillator Components

COSC should be 50 pF. ROSC is selected from the

equation:

fOSC =

Note that fOSC is 4 to generate the TC7136A's internal

clock. The backplane drive signal is derived by dividing fOSC

by 800.

To achieve maximum rejection of 60Hz noise pickup,

the signal integrate period should be a multiple of 60Hz.

Oscillator frequencies of 240kHz, 120kHz, 80kHz, 60kHz,

40kHz, etc. should be selected. For 50 Hz rejection, oscil-

lator frequencies of 200kHz, 100kHz, 66-2/3 kHz, 50kHz,

40kHz, etc. would be suitable. Note that 40kHz (2.5 read-

ings per second) will reject both 50Hz and 60Hz.

Reference Voltage Selection

A full-scale reading (2000 counts) requires the input

signal be twice the reference voltage.

0.45

In some applications, a scale factor other than unity may

exist between a transducer output voltage and the required

digital reading. Assume, for example, a pressure transducer

output for 2000 lb/in.2 is 400 mV. Rather than dividing the

input voltage by two, the reference voltage should be set to

200 mV. This permits the transducer input to be used

directly.

The differential reference can also be used when a

digital zero reading is required when VIN is not equal to zero.

This is common in temperature measuring instrumentation.

A compensating offset voltage can be applied between

analog common and V

–

IN The transducer output is connected

between V

IN and analog common.

DEVICE PIN FUNCTIONAL DESCRIPTION

Differential Signal Inputs

IN (Pin 31), V

–

IN (Pin 30)

The TC7136/A is designed with true differential inputs

and accepts input signals within the input stage common-

mode voltage range (VCM). The typical range is V+ –1V to V–

+1V. Common-mode voltages are removed from the system

when the TC7136A operates from a battery or floating power

source (isolated from measured system), and VIN

– is con-

nected to analog common (VCOM). (See Figure 7.)

Required Full-Scale Voltage*

VREF

200 mV

100 mV

*VFS = 2 VREF.

where: fOSC = Clock frequency at pin 38

VFS = Full-scale input voltage

RINT = Integrating resistor

VINT = Desired full-scale integrator output swing.

CINT must have low dielectric absorption to minimize

roll-over error. A polypropylene capacitor is recommended.

Integrating Resistor (RINT)

The input buffer amplifier and integrator are designed

with Class A output stages. The output stage idling current

is 6

µA. The integrator and buffer can supply 1 µA drive

currents with negligible linearity errors. RINT is chosen to

remain in the output stage linear drive region, but not so

large that PC board leakage currents induce errors. For a

200 mV full scale, RINT is 180 kΩ. A 2V full scale requires

1.8 M

Ω.

CINT =,

fOSC

(

VFS

RINT

(

)

(4000)

VINT

LOW POWER, 3-1/2 DIGIT

ANALOG-TO-DIGITAL CONVERTERS

TC7136

TC7136A

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