4-1/2 DIGIT ANALOG-TO-DIGITAL
CONVERTER WITH ON-CHIP LCD DRIVERS
1
2
3
4
5
6
7
8
TC7129
PIN DESCRIPTIONS
Pin No.
40-Pin
TC7129CPL
Pin No.
44-Pin
TC7129CKW
Pin No.
44-Pin
TC7129CLW
Symbol
Function
34
35
36
32
33
34
38
39
40
REF HI
REF LO
DGND
Positive reference voltage in
Negative reference voltage
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
39
40
36
37
38
42
43
44
DP2
DP1
Internal 3 µA pull-down. When HI, decimal point 2 will be on.
Internal 3 µA pull-down. When HI, decimal point 1 will be on.
Output of first clock inverter. Input of second clock inverter.
OSC2
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
componentvaluesarenotcritical;substitutescanbechosen
based on the information given below.
The basic circuit for a digital multimeter application is
shown in Figure 1. See "Special Applications" for variations.
Typicalvaluesforeachcomponentareshown. Thesections
below give component selection criteria.
The resistor and capacitor values are not critical; those
shown work for most applications. In some situations, the
capacitorvaluesmayhavetobeadjustedtocompensatefor
parasitic capacitance in the circuit. The capacitors can be
low-cost ceramic devices.
SomeapplicationscanuseasimpleRCnetworkinstead
of a crystal oscillator. The RC oscillator has more potential
for jitter, especially in the least significant digit. See "RC
Oscillator."
Integrating Resistor (RINT
)
Oscillator (XOSC, CO1, CO2, RO)
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:
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
cycleisequaltotwooscillatorcycles.For60Hzrejection,the
oscillator frequency should be chosen so that the period of
one line cycle equals the integration time for the 2V range:
2V
ICHARGE
=
13.3 µA
150 kΩ
Too high a value for RINT increases the sensitivity to
noise pickup and increases errors due to leakage current.
Too low a value degrades the linearity of the integration,
leading to inaccurate readings.
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.
TELCOM SEMICONDUCTOR, INC.
3-235
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
