4-1/2 DIGIT ANALOG-TO-DIGITAL
CONVERTER WITH ON-CHIP LCD DRIVERS
TC7129
PIN DESCRIPTIONS
Pin No.
40-Pin
TC7129CPL
34
35
36
37
38
39
40
1
Pin No.
44-Pin
TC7129CKW
32
33
34
35
36
37
38
6,17, 28, 39
Pin No.
44-Pin
TC7129CLW
38
39
40
41
42
43
44
12, 23, 34,1
2
Symbol
REF HI
REF LO
DGND
RANGE
DP
2
DP
1
OSC
2
NC
Function
Positive reference voltage in
Negative reference voltage
Internal ground reference for digital section. See "±5V Power
Supply" paragraph.
3
µA
pull-down for 200 mV scale. Pulled HI externally for 2V
scale.
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.
No Connection
3
4
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 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."
5
6
7
Oscillator
(X
OSC
, C
O1
, C
O2
, R
O
)
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.
Integrating Resistor
(R
INT
)
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:
I
CHARGE
=
2V
13.3
µA
150 kΩ
Too high a value for R
INT
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.
8
TELCOM SEMICONDUCTOR, INC.
3-235
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
