TC7135CPL datasheet(12/24 Pages) MICROCHIP

12 / 24 page

TC7135

DS21460C-page 12

 2004 Microchip Technology Inc.

6.0

TYPICAL APPLICATIONS

6.1

Component Value Selection

6.1.1

INTEGRATING RESISTOR

The integrating resistor RINT is determined by the full-

scale input voltage and the output current of the buffer

used to charge the integrator capacitor (CINT). Both the

buffer amplifier and the integrator have a class A output

stage, with 100 µA of quiescent current. A 20 µA drive

current gives negligible linearity errors. Values of 5 µA

to 40 µA give good results. The exact value of an

integrating resistor for a 20 µA current is easily

calculated.

EQUATION 6-1:

6.1.2

INTEGRATING CAPACITOR (CINT)

The product of integrating resistor and capacitor should

be selected to give the maximum voltage swing that

ensures the tolerance build-up will not saturate the

integrator swing (approximately 0.3V from either

supply). For ±5V supplies and ANALOG COMMON tied

to supply ground, a ±3.5V to ±4V full scale integrator

swing

adequate.

0.10 µF

0.47 µF

recommended. In general, the value of CINT is given

by:

EQUATION 6-2:

A very important characteristic of the integrating

capacitor CINT is that it has low dielectric absorption to

prevent rollover or ratiometric errors. A good test for

dielectric absorption is to use the capacitor with the

input tied to the reference. This ratiometric condition

should read half scale 0.9999, with any deviation

probably due to dielectric absorption. Polypropylene

capacitors give undetectable errors at reasonable cost.

Polystyrene and polycarbonate capacitors may also be

used in less critical applications.

6.1.3

AUTO-ZERO AND REFERENCE

CAPACITORS

The size of the auto-zero capacitor has some influence

on the noise of the system, with a larger capacitor

reducing the noise. The reference capacitor should be

large enough such that stray capacitance to ground

from its nodes is negligible.

The dielectric absorption of the reference and auto-

zero capacitors are only important at power-on or when

the circuit is recovering from an overload. Smaller or

cheaper capacitors can be used if accurate readings

are not required for the first few seconds of recovery.

6.1.4

REFERENCE VOLTAGE

The analog input required to generate a full-scale

output is VIN = 2 VREF.

The stability of the reference voltage is a major factor in

the overall absolute accuracy of the converter. For this

reason, it is recommended that a high-quality reference

be used where high-accuracy absolute measurements

are being made.

6.2

Conversion Timing

6.2.1

LINE FREQUENCY REJECTION

A signal integration period at a multiple of the 60 Hz

line frequency will maximize 60 Hz “line noise”

rejection. A 100 kHz clock frequency will reject 50 Hz,

60 Hz and 400 Hz noise. This corresponds to five

readings per second (see Table 6-1 and Table 6-2).

TABLE 6-1:

CONVERSION RATE VS.

CLOCK FREQUENCY

INT

Full Scale Voltage

µA

--------------------------------------------

IN T

10 000

clock period

[] I

IN T

integrator output voltage swing

---------------------------------------------------------------------------

10 000

() clock period

() 20µA

integrator output voltage swing

------------------------------------------------------------------------------

Oscillator Frequency

(kHz)

Conversion Rate

(Conv./Sec.)

100

2.5

120

200

300

7.5

400

800

1200

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