TC7136ACPI datasheet(12/22 Pages) MICROCHIP

12 / 22 page

TC7136/TC7136A

DS21461B-page 12

An exact expression for CINT is:

EQUATION 6-1:

CINT must have low dielectric absorption to minimize

rollover

error.

polypropylene

capacitor

recommended.

6.4

Integrating Resistor (RINT)

The input buffer amplifier and integrator are designed

with Class A output stages. The output stage idling cur-

rent is 6

µA. The integrator and buffer can supply 1µA

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 200mV full scale, RINT is 180kΩ.A

2V full scale requires 1.8M

Ω (see Table 6-1).

TABLE 6-1:

Note:

FOSC = 48kHz (3 reading per sec).

ROSC =180kΩ, COSC =50pF.

6.5

Oscillator Components

COSC should be 50pF. ROSC is selected from the

equation:

EQUATION 6-2:

Note that FOSC is ÷ 4 to generate the TC7136A's inter-

nal 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

50Hz rejection, oscillator frequencies of 200kHz,

100kHz, 66-2/3kHz, 50kHz, 40kHz, etc. would be suit-

able. Note that 40kHz (2.5 readings per second) will

reject both 50Hz and 60Hz.

6.6

Reference Voltage Selection

A full scale reading (2000 counts) requires the input

signal be twice the reference voltage.

Note:

*VREF =2VREF.

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 pres-

sure transducer output for 2000 lb/in2 is 400mV. Rather

than dividing the input voltage by two, the reference

voltage should be set to 200mV. This permits the trans-

ducer input to be used directly. The differential refer-

ence 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 compen-

sating offset voltage can be applied between analog

common and VIN-. The transducer output is connected

between VIN+ and analog common.

Component

Value

Nominal Full Scale Voltage

200mV

CAZ

0.47

µF0.1µF

RINT

180k

Ω

1.8M

Ω

CINT

0.047

µF

0.047

µF

(4000)

FOSC









CINT =

VFS

RINT









VINT

Where:

FOSC = Clock frequency at Pin 38

VFS = Full scale input voltage

RINT = Integrating resistor

VINT = Desired full scale integrator output swing

Required Full Scale Voltage*

VREF

200mV

100mV

FOSC =

0.45

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