TLC1542_13 Datasheet, PDF (19/33 Pages) Texas Instruments – 10-BIT ANALOG-TO-DIGITAL CONVERTERS WITH SERIAL CONTROL AND 11 ANALOG INPUTS

TLC1542I, TLC1542M, TLC1542Q
TLC1542C, TLC1543C, TLC1543I, TLC1543Q
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SLAS052G – MARCH 1992 – REVISED JANUARY 2006
APPLICATION INFORMATION (continued)
SIMPLIFIED ANALOG INPUT ANALYSIS
Using the equivalent circuit in Figure 17Figure 17, the time required to charge the analog input capacitance from
0 to V_S within 1/2 LSB can be derived as follows:
The capacitance charging voltage is given by
( ) V_C = V_S 1−e ^{−t c /RtCi}
where
R_t = R_s + r_i
(1)
The final voltage to 1/2 LSB is given by
V_C (1/2 LSB) = V_S − (V_S/2048)
(2)
Equating equation 1 to equation 2 and solving for time t_c gives
( ) VS −(VS/2048) = VS 1−e ^{−t c /RtCi}
and
t_c (1/2 LSB) = R_t × C_i × ln(2048)
(3)
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(2048)
(4)
This time must be less than the converter sample time shown in the timing diagrams.
Driving Source^†
TLC1542/3
R_s
V_I
r_i
V_S
V_C
1 kΩ MAX
C_i
50 pF MAX
V_I = Input Voltage at A0 −A10
V_S = External Driving Source Voltage
R_s = Source Resistance
r_i = Input Resistance
C_i = Equivalent Input Capacitance
† _{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 17. Equivalent Input Circuit Including the Driving Source
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