TLC540I Datasheet, PDF (8/11 Pages) Texas Instruments – 8-BIT ANALOG-TO-DIGITAL CONVERTERS WITH SERIAL CONTROL AND 11 INPUTS

TLC540I, TLC541I
8-BIT ANALOG-TO-DIGITAL CONVERTERS
WITH SERIAL CONTROL AND 11 INPUTS
SLAS065A – OCTOBER 1983 – REVISED MARCH 1995
APPLICATION INFORMATION
simplified analog input analysis
Using the equivalent circuit in Figure 1, the time required to charge the analog input capacitance from 0 to V_S
within 1/2 LSB can be derived as follows:
+ ǒ ń Ǔ The capacitance charging voltage is given by
VC VS 1–e–tc ^RtCi
(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 /512)
(2)
* ǒ ń Ǔ + ǒ ń Ǔ Equating equation 1 to equation 2 and solving for time tc gives
VS VS 512 VS 1–e–tc ^RtCi
(3)
and
t_c (1/2 LSB) = R_t × C_i × ln(512)
(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(512)
(5)
This time must be less than the converter sample time shown in the timing diagrams.
Driving Source†
TLC540/1
Rs
VI
ri
VS
VC
1 kΩ MAX
Ci
50 pF MAX
VI = Input Voltage at INPUT A0 – A10
VS = External Driving Source Voltage
Rs = Source Resistance
ri = Input Resistance
Ci = Equivalent Input Capacitance
† _{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 1. Equivalent Input Circuit Including the Driving Source
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