TC850CLW View Datasheet(PDF) - Microchip Technology
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TC850CLW Datasheet PDF : 26 Pages
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7.0 ANALOG SECTION TYPICAL
APPLICATIONS
7.1 Component Selection
7.1.1 REFERENCE VOLTAGE
The typical value for reference voltage VREF1 is
1.6384V. This value yields a full scale voltage of
3.2768V and resolution of 100 μV per step. The VREF2
value is derived by dividing VREF1 by 64. Thus, typical
VREF2 value is 1.6384V/64, or 25.6 mV. The VREF2
value should be adjusted within ±1% to maintain 15-bit
accuracy for the total conversion process;
EQUATION 7-1: :
VREF =
VREF1 ± 1%
64
The reference voltage is not limited to exactly 1.6384V,
however, because the TC850 performs a ratiometric
conversion. Therefore, the conversion result will be:
EQUATION 7-2:
Digital Counts =
VIN • 16384
VREF1
The full scale voltage can range from 3.2V to 3.5V. Full
scale voltages of less than 3.2V will result in increased
noise in the Least Significant bits, while a full scale
above 3.5V will exceed the input common-mode range.
7.1.2 INTEGRATION RESISTOR
The TC850 buffer supplies 25 μA of integrator charging
current with minimal linearity error. RINT is easily
calculated:
EQUATION 7-3:
VFULLSCALE
RINT =
25 μA
For a full scale voltage of 3.2768V, values of RINT
between 120 kΩ and 150 kΩ are acceptable.
TC850
7.1.3 INTEGRATION CAPACITOR
The integration capacitor should be selected to
produce an integrator swing of ≈ 4V at full scale. The
capacitor value is easily calculated:
EQUATION 7-4:
where:
VFS
4 • 256
C = RINT
•
4V FCLOCK
FCLOCK is the crystal or external oscillator
frequency and VFS is the maximum input voltage.
The integration capacitor should be selected for low
dielectric absorption to prevent rollover errors. A
polypropylene, polyester or polycarbonate dielectric
capacitor is recommended.
7.1.4 REFERENCE CAPACITORS
The reference capacitors require a low-leakage dielec-
tric, such as polypropylene, polyester or polycarbon-
ate. A value of 1 μF is recommended for operation over
the temperature range. If high-temperature operation is
not required, the CREF values can be reduced.
7.1.5 AUTO-ZERO CAPACITORS
Five capacitors are required to auto-zero the input
buffer, integrator amplifier and comparator. Recom-
mended capacitors are 0.1 μF film dielectric (such as
polyester or polypropylene). Ceramic capacitors are
not recommended.
© 2006 Microchip Technology Inc.
DS21479C-page 15
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