TC7106(2002) View Datasheet(PDF) - Microchip Technology
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TC7106 Datasheet PDF : 26 Pages
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7.0 COMPONENT VALUE
SELECTION
7.1 Auto-Zero Capacitor (CAZ)
The CAZ capacitor size has some influence on system
noise. A 0.47µF capacitor is recommended for 200mV
full scale applications where 1LSB is 100µV. A 0.047µF
capacitor is adequate for 2.0V full scale applications. A
mylar type dielectric capacitor is adequate.
7.2 Reference Voltage Capacitor
(CREF)
The reference voltage used to ramp the integrator out-
put voltage back to zero during the reference integrate
cycle is stored on CREF. A 0.1µF capacitor is acceptable
when VIN- is tied to analog common. If a large Common
mode voltage exists (VREF- – analog common) and the
application requires 200mV full scale, increase CREF to
1.0µF. Rollover error will be held to less than 1/2 count.
A mylar dielectric capacitor is adequate.
7.3 Integrating Capacitor (CINT)
CINT should be selected to maximize the integrator out-
put voltage swing without causing output saturation.
Due to the TC7106A/7107A superior temperature coef-
ficient specification, analog common will normally sup-
ply the differential voltage reference. For this case, a
±2V full scale integrator output swing is satisfactory.
For 3 readings/second (FOSC = 48kHz), a 0.22µF value
is suggested. If a different oscillator frequency is used,
CINT must be changed in inverse proportion to maintain
the nominal ±2V integrator swing.
An exact expression for CINT is:
EQUATION 7-1:
CINT
=
(4000)
1
FOSC
VINT
VFS
RINT
Where:
FOSC = Clock Frequency at Pin 38
VFS = Full Scale Input Voltage
RINT = Integrating Resistor
VINT = Desired Full Scale Integrator Output Swing
CINT must have low dielectric absorption to minimize
rollover error. A polypropylene capacitor is recom-
mended.
© 2002 Microchip Technology Inc.
TC7106/A/TC7107/A
7.4 Integrating Resistor (RINT)
The input buffer amplifier and integrator are designed
with class A output stages. The output stage idling cur-
rent is 100µA. The integrator and buffer can supply
20µA drive currents with negligible linearity errors.
RINT is chosen to remain in the output stage linear drive
region, but not so large that printed circuit board leak-
age currents induce errors. For a 200mV full scale,
RINT is 47kΩ. 2.0V full scale requires 470kΩ.
Component
Value
Nominal Full Scale Voltage
200.0mV
2.000V
CAZ
RINT
CINT
0.47µF
47kΩ
0.22µF
0.047µF
470kΩ
0.22µF
Note: FOSC = 48kHz (3 readings per sec).
7.5 Oscillator Components
ROSC (Pin 40 to Pin 39) should be 100kΩ. COSC is
selected using the equation:
EQUATION 7-2:
FOSC =
0.45
RC
For FOSC of 48kHz, COSC is 100pF nominally.
Note that FOSC is divided by four to generate the
TC7106A internal control clock. The backplane drive
signal is derived by dividing FOSC by 800.
To achieve maximum rejection of 60Hz noise pickup,
the signal integrate period should be a multiple of
60Hz. Oscillator frequencies of 240kHz, 120kHz,
80kHz, 60kHz, 48kHz, 40kHz, etc. should be selected.
For 50Hz rejection, oscillator frequencies of 200kHz,
100kHz, 66-2/3kHz, 50kHz, 40kHz, etc. would be suit-
able. Note that 40kHz (2.5 readings/second) will reject
both 50Hz and 60Hz.
7.6 Reference Voltage Selection
A full scale reading (2000 counts) requires the input
signal be twice the reference voltage.
Required Full Scale Voltage*
200.0mV
2.000V
VREF
100.0mV
1.000V
* VFS = 2VREF.
In some applications, a scale factor other than unity
may exist between a transducer output voltage and the
required digital reading. Assume, for example, a pres-
sure transducer output is 400mV for 2000 lb/in2.
Rather than dividing the input voltage by two, the refer-
ence voltage should be set to 200mV. This permits the
transducer input to be used directly.
DS21455B-page 13
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