TC7135 datasheet(12/22 Pages) MICROCHIP

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TC7135

DS21460B-page 12

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 20µAdrive

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 buildup will not saturate the inte-

grator swing (approximately 0.3V from either supply).

For ±5V supplies and ANALOG COMMON tied to sup-

ply ground, a ±3.5V to ±4V full scale integrator swing is

adequate. A 0.10

µFto0.47µF is recommended. In

general, the value of CINT is given by:

EQUATION 6-2:

A very important characteristic of the integrating capac-

itor CINT is that it has low dielectric absorption to pre-

vent 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. A large capacitor reduces

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 out-

put is VIN =2VREF.

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 60Hz line

frequency will maximize 60Hz "line noise" rejection. A

100kHz clock frequency will reject 50Hz, 60Hz and

400Hz noise. This corresponds to five readings per

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

TABLE 6-1:

CONVERSION RATE VS.

CLOCK FREQUENCY

RINT =

Full scale voltage

µA

CINT =

[10,000 x clock period] x IINT

Integrator output voltage swing

(10,000) (clock period) (20

µA)

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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