TLC548C, TLC548I, TLC549C, TLC549I
8-BIT ANALOG-TO-DIGITAL CONVERTERS
WITH SERIAL CONTROL
SLAS067C – NOVEMBER 1983 – REVISED SEPTEMBER 1996
APPLICATIONS INFORMATION
simplified analog input analysis
Using the equivalent circuit in Figure 2, 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
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)
Equating equation 1 to equation 2 and solving for time t
c
gives
V
S
– (V
S
/512) = V
S
1– e
and
t
c
(1/2 LSB) = R
t
×
C
i
×
ln(512)
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)
This time must be less than the converter sample time shown in the timing diagrams.
Driving Source†
Rs
VS
ri
VC
1 kΩ MAX
Ci
55 pF MAX
TLC548/9
(
– t
c
/R
t
C
i
)
(1)
(2)
(
– t
c
/R
t
C
i
)
(3)
(4)
(5)
VI
VI = Input Voltage at ANALOG IN
VS = External Driving Source Voltage
Rs = Source Resistance
ri = Input Resistance
Ci = 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 2. Equivalent Input Circuit Including the Driving Source
POST OFFICE BOX 655303
•
DALLAS, TEXAS 75265
7
TI [ TEXAS INSTRUMENTS ]
