2002-2012 Microchip Technology Inc. DS21459E-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
elements: a voltage reference, a 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 3-1. See Section 4.0 “Typical Appli-
cations”, for variations. Typical values for each
component are shown. The sections below give
component selection criteria.
3.1 Oscillator (X
OSC
, C
O1
, C
O2
, R
O
)
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 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.
EQUATION 3-1:
This equation gives an oscillator frequency of 120 kHz.
A similar calculation 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 Section 4.5 “RC Oscillator”.
3.2 Integrating Resistor (R
INT
)
The integrating resistor sets the charging current for
the integrating capacitor. Choose a value that provides
a current between 5 A and 20 A at 2V, the maximum
full-scale input. The typical value chosen gives a
charging current of 13.3 A:
EQUATION 3-1:
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.
1/60 second = 16.7 msec =
1000 clock cycles *2 OSC cycles/clock cycle
OSC Frequency
I
CHARGE
=
2V
150 k
13.3 µA
