3-235

TELCOM SEMICONDUCTOR, INC.

7

6

5

4

3

1

2

8

PIN DESCRIPTIONS

Pin No.

Pin No.

Pin No.

40-Pin

44-Pin

44-Pin

TC7129CPL

TC7129CKW

TC7129CLW

Symbol

Function

34

32

38

REF HI

Positive reference voltage in

35

33

39

REF LO

Negative reference voltage

36

34

40

DGND

Internal ground reference for digital section. See "

±5V Power

Supply" paragraph.

37

35

41

RANGE

3

µA pull-down for 200 mV scale. Pulled HI externally for 2V

scale.

38

36

42

Internal 3

µA pull-down. When HI, decimal point 2 will be on.

39

37

43

Internal 3

µA pull-down. When HI, decimal point 1 will be on.

40

38

44

Output of first clock inverter. Input of second clock inverter.

6,17, 28, 39

12, 23, 34,1

NC

No Connection

COMPONENT SELECTION

(All pin designations refer to 40-Pin Dip)

The TC7129 is designed to be the heart of a high-

resolution analog measurement instrument. The only addi-

tional components required are a few passive elements, a

voltage reference, an LCD, and a power source. Most

component values are not critical; substitutes can be chosen

based on the information given below.

The basic circuit for a digital multimeter application is

shown in Figure 1. See "Special Applications" for variations.

Typical values for each component are shown. The sections

below give component selection criteria.

The primary criterion for selecting the crystal oscillator

is to chose a frequency that achieves maximum rejection 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 200 mV

range and 1000 clock cycles on the 2V range. One clock

cycle is equal to two oscillator cycles. For 60 Hz rejection, the

oscillator frequency should be chosen so that the period of

one line cycle equals the integration time for the 2V range:

1/60 second = 16.7 msec =

1000 clock cycles

2 osc cycles/clock cycle

oscillator frequency

giving an oscillator frequency of 120 kHz. A similar calcula-

tion gives an optimum frequency of 100 kHz for 50 Hz

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 compensate 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 "RC

Oscillator."

The integrating resistor sets the charging current for

the integrating capacitor. Choose a value that provides a

current between 5

µA and 20 µA at 2V, the maximum full-

scale input. The typical value chosen gives a charging

current of 13.3

µA:

13.3

µA

noise pickup and increases errors due to leakage current.

Too low a value degrades the linearity of the integration,

leading to inaccurate readings.

2V

150 k

Ω

TC7129

4-1/2 DIGIT ANALOG-TO-DIGITAL

CONVERTER WITH ON-CHIP LCD DRIVERS