TLC2543-EP Datasheet, PDF (21/25 Pages) Texas Instruments – 12-BIT ANALOG-TO-DIGITAL CONVERTER WITH SERIAL CONTROL AND 11 ANALOG INPUTS

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TLC2543-EP
12-BIT ANALOG-TO-DIGITAL CONVERTER
WITH SERIAL CONTROL AND 11 ANALOG INPUTS
SGLS125A – JULY 2002 – REVISED NOVEMBER 2006
APPLICATION INFORMATION (continued)
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 one-half LSB can be derived as follows:
The capacitance charging voltage is given by:
ǒ Ǔ _{VC + VS 1 * e}^*tcńRtCi
(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}^*tcńRtCi
(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^(A)
TLC2543
R_s
V_I
r_i
V_S
V_C
1 kΩ Max
C_i
60 pF Max
V_I = Input Voltage at AIN
V_S = External Driving Source Voltage
R_s = Source Resistance
r_i = Input Resistance
C_i = Input Capacitance
V_C = Capacitance Charging Voltage
A. Driving source requirements:
• Noise and distortion for the source must be equivalent to the resolution of the converter.
• R_s must be real at the input frequency.
Figure 18. Equivalent Input Circuit Including the Driving Source
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