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| 型号： | TC7117CPI |
| 元件分类： | ADC |
| 英文描述： | 1-CH DUAL-SLOPE ADC, PDIP40 |
| 封装： | PLASTIC, DIP-40 |
| 文件页数： | 5/32页 |
| 文件大小： | 515K |
| 代理商： | TC7117CPI |
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2006 Microchip Technology Inc.
DS21455C-page 13
TC7106/A/TC7107/A
7.0
COMPONENT VALUE
SELECTION
7.1
Auto-Zero Capacitor (CAZ)
The CAZ capacitor size has some influence on system
noise. A 0.47
μF capacitor is recommended for 200mV
full scale applications where 1LSB is 100
μV. 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
1.0
μF. 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
coefficient specification, analog common will normally
supply 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 must have low dielectric absorption to minimize
rollover
error.
A
polypropylene
capacitor
is
recommended.
7.4
Integrating Resistor (RINT)
The input buffer amplifier and integrator are designed
with class A output stages. The output stage idling
current is 100
μA. The integrator and buffer can supply
20
μA 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
leakage currents induce errors. For a 200mV full scale,
RINT is 47kΩ. 2.0V full scale requires 470kΩ.
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:
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
suitable. 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.
*VFS = 2VREF.
CINT =
(4000)
VINT
1
FOSC
VFS
RINT
Where:
FOSC = Clock Frequency at Pin 38
VFS = Full Scale Input Voltage
RINT = Integrating Resistor
VINT = Desired Full Scale Integrator Output Swing
Component
Value
Nominal Full Scale Voltage
200.0mV
2.000V
CAZ
0.47
μF0.047μF
RINT
47k
Ω
470k
Ω
CINT
0.22
μF0.22μF
Required Full Scale Voltage*
VREF
200.0mV
100.0mV
2.000V
1.000V
FOSC =
0.45
RC
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