TLC2543-Q1 Datasheet, PDF (21/24 Pages) Texas Instruments – 12-BIT ANALOG-TO-DIGITAL CONVERTERS WITH SERIAL CONTROL AND 11 ANALOG INPUTS

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TLC2543ĆQ1
12ĆBIT ANALOGĆTOĆDIGITAL CONVERTERS
WITH SERIAL CONTROL AND 11 ANALOG INPUTS
SGLS218B − NOVEMBER 2003 − REVISED NOVEMBER 2004
APPLICATION INFORMATION
simplified analog input analysis
Using the equivalent circuit in Figure 18, the time required to charge the analog input capacitance from 0 V to
V_S within 1/2 LSB can be derived as follows:
The capacitance charging voltage is given by:
ǒ Ǔ VC + VS 1–e–tc^ńRtCi
(1)
Where:
R_t = R_s + r_i
The final voltage to 1/2 LSB is given by:
V_C (1/2 LSB) = V_S − (V_S /8192)
(2)
Equating equation 1 to equation 2 and solving for time t_c gives:
ǒ Ǔ ǒ Ǔ VS * VSń8192 + VS 1–e–tc^ńRtCi
(3)
and
t_c (1/2 LSB) = R_t × C_i × ln(8192)
(4)
Therefore, with the values given, the time for the analog input signal to settle is:
t_c (1/2 LSB) = (R_s + 1 kΩ) × 60 pF × ln(8192)
(5)
This time must be less than the converter sample time shown in the timing diagrams.
Driving Source†
TLC2543
Rs
VI
ri
VS
VC
1 kΩ Max
Ci
60 pF Max
VI = Input Voltage at AIN
VS = External Driving Source Voltage
Rs = Source Resistance
ri = Input Resistance
Ci = Input Capacitance
VC = Capacitance Charging Voltage
† _{Driving source requirements:}
• Noise and distortion for the source must be equivalent to the
resolution of the converter.
• Rs must be real at the input frequency.
Figure 18. Equivalent Input Circuit Including the Driving Source
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