TC850
7.1.3
INTEGRATION CAPACITOR
7.0
ANALOG SECTION TYPICAL
APPLICATIONS
The integration capacitor should be selected to pro-
duce an integrator swing of ≈ 4V at full scale. The
capacitor value is easily calculated:
7.1
Component Selection
7.1.1
REFERENCE VOLTAGE
EQUATION 7-4:
V
The typical value for reference voltage V
1.6384V. This value yields a full scale voltage of
is
FS
4 • 256
4V F
REF1
•
C =
R
INT
CLOCK
3.2768V and resolution of 100µV per step. The V
REF2
where:
value is derived by dividing V
by 64. Thus, typical
REF1
F
is the crystal or external oscillator
CLOCK
V
value is 1.6384V/64, or 25.6mV. The V
REF2
REF2
frequency and V is the maximum input voltage.
FS
value should be adjusted within ±1% to maintain 15-bit
accuracy for the total conversion process;
The integration capacitor should be selected for low
dielectric absorption to prevent rollover errors. A
polypropylene, polyester or polycarbonate dielectric
capacitor is recommended.
EQUATION 7-1:
:
V
± 1%
REF1
V
=
REF
64
7.1.4
REFERENCE CAPACITORS
The reference voltage is not limited to exactly 1.6384V,
however, because the TC850 performs a ratiometric
conversion. Therefore, the conversion result will be:
The reference capacitors require a low-leakage dielec-
tric, such as polypropylene, polyester or polycarbon-
ate. A value of 1µF is recommended for operation over
the temperature range. If high-temperature operation is
not required, the C
values can be reduced.
EQUATION 7-2:
REF
V
IN
Digital Counts =
• 16384
7.1.5
AUTO ZERO CAPACITORS
V
REF1
Five capacitors are required to auto zero the input
buffer, integrator amplifier and comparator. Recom-
mended capacitors are 0.1µF film dielectric (such as
polyester or polypropylene). Ceramic capacitors are
not recommended.
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 25µA of integrator charging
current with minimal linearity error. R
lated:
is easily calcu-
INT
EQUATION 7-3:
V
FULLSCALE
R
=
INT
25µA
For a full scale voltage of 3.2768V, values of R
INT
between 120kΩ and 150kΩ are acceptable.
2002 Microchip Technology Inc.
DS21479B-page 15
MICROCHIP [ MICROCHIP ]
