Datasheet

TLC548C, TLC548I, TLC549C, TLC549I
8-BIT ANALOG-TO-DIGITAL CONVERTERS
WITH SERIAL CONTROL
SLAS067C – NOVEMBER 1983 – REVISED SEPTEMBER 1996
7
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
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
–t
c
/R
t
C
i
( )
(1)
where
R
t
= R
s
+ r
i
The final voltage to 1/2 LSB is given by
(2)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 (3)
–t
c
/R
t
C
i
( )
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.
R
s
r
i
V
S
V
C
1 kΩ MAX
Driving Source
†
TLC548/9
V
I
V
I
= Input Voltage at ANALOG IN
V
S
= External Driving Source Voltage
R
s
= Source Resistance
r
i
= Input Resistance
C
i
= 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.
C
i
55 pF MAX
Figure 2. Equivalent Input Circuit Including the Driving Source