TC7129 datasheet(7/24 Pages) MICROCHIP

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DS21459B-page 7

TC7129

3.0

DETAILED DESCRIPTION

(All Pin Designations Refer to 40-Pin PDIP.)

The TC7129 is designed to be the heart of a high

resolution analog measurement instrument. The only

additional components required are a few passive ele-

ments: a voltage reference, an LCD, and a power

source. Most component values are not critical; substi-

tutes can be chosen based on the information given

below.

The basic circuit for a digital multimeter application is

shown in Figure 3-1. See Section 4.0, Typical Applica-

tions for variations. Typical values for each component

are shown. The sections below give component selec-

tion criteria.

3.1

Oscillator (XOSC,CO1,CO2,RO)

The primary criterion for selecting the crystal oscillator

is to choose a frequency that achieves maximum rejec-

tion of line frequency noise. To do this, the integration

phase should last an integral number of line cycles.

The integration phase of the TC7129 is 10,000 clock

cycles on the 200mV range and 1000 clock cycles on

the 2V range. One clock cycle is equal to two oscillator

cycles. For 60Hz rejection, the oscillator frequency

should be chosen so that the period of one line cycle

equals the integration time for the 2V range:

EQUATION 3-1:

This equation gives an oscillator frequency of 120kHz.

A similar calculation gives an optimum frequency of

100kHz for 50Hz rejection.

The resistor and capacitor values are not critical; those

shown work for most applications. In some situations,

the capacitor values may have to be adjusted to com-

pensate for parasitic capacitance in the circuit. The

capacitors can be low cost ceramic devices.

Some applications can use a simple RC network

instead of a crystal oscillator. The RC oscillator has

more potential for jitter, especially in the least

significant digit. See Section 4.8, RC Oscillator.

3.2

Integrating Resistor (RINT)

The integrating resistor sets the charging current for

the integrating capacitor. Choose a value that provides

a current between 5

µA and 20µA at 2V, the maximum

full scale input. The typical value chosen gives a

charging current of 13.3

µA:

EQUATION 3-2:

Toohighavalue for RINT increases the sensitivity to

noise pickup and increases errors due to leakage cur-

rent. Too low a value degrades the linearity of the

integration, leading to inaccurate readings.

1/60 second = 16.7msec =

1000 clock cycles *2 OSC cycles/clock cycle

OSC Frequency

ICHARGE =

150k

Ω

13.3

µA

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