Analog Devices ADSP-2181 Manual page 19

ADSP-2183
CAPACITIVE LOADING
Figures 17 and 18 show the capacitive loading characteristics of
the ADSP-2183.
30
T = +85 C
V = 3.0V
DD
25
20
15
10
5
0
0 25 50 75
Figure 17. Typical Output Rise Time vs. Load Capacitance,
C (at Maximum Ambient Operating Temperature)
L
18
16
14
12
10
8
6
4
2
NOMINAL
–2
–4
–6
0 25 50
Figure 18. Typical Output Valid Delay or Hold vs. Load
Capacitance, C (at Maximum Ambient Operating
L
Temperature)
TEST CONDITIONS
Output Disable Time
Output pins are considered to be disabled when they have
stopped driving and started a transition from the measured out-
put high or low voltage to a high impedance state. The output
disable time (t ) is the difference of t
DIS
shown in the Output Enable/Disable diagram. The time is the
interval from when a reference signal reaches a high or low volt-
age level to when the output voltages have changed by 0.5 V
from the measured output high or low voltage. The decay time,
t , is dependent on the capacitive load, C
DECAY
load, i , on the output pin. It can be approximated by the fol-
L
lowing equation:
t
DECAY
REV. 0
100 125 150 175 200
C – pF
L
75 100 125 150 175 200
C – pF
L
and t
MEASURED DECAY
, and the current
L
C • 0.5V
L
i
L
from which
is calculated. If multiple pins (such as the data bus) are dis-
abled, the measurement value is that of the last pin to stop
driving.
INPUT
OUTPUT
Figure 19. Voltage Reference Levels for AC Measure-
ments (Except Output Enable/Disable)
Output Enable Time
Output pins are considered to be enabled when that have made
a transition from a high-impedance state to when they start
driving. The output enable time (t
a reference signal reaches a high or low voltage level to when
the output has reached a specified high or low trip point, as
shown in the Output Enable/Disable diagram. If multiple pins
(such as the data bus) are enabled, the measurement value is
that of the first pin to start driving.
REFERENCE
SIGNAL
t
MEASURED
t
V
DIS
OH
(MEASURED)
OUTPUT
V
OL
(MEASURED)
OUTPUT STOPS
DRIVING
Figure 20. Output Enable/Disable
TO
OUTPUT
PIN
, as
Figure 21. Equivalent Device Loading for AC Measure-
ments (Including All Fixtures)
–19–
ADSP-2181/ADSP-2183
t t – t
DIS MEASURED DECAY
3.0V
1.5V
0.0V
2.0V
1.5V
0.3V
) is the interval from when
ENA
t
ENA
V (MEASURED) – 0.5V
OH 2.0V
1.0V
V (MEASURED) +0.5V
OL
t
DECAY
OUTPUT STARTS
DRIVING
HIGH-IMPEDANCE STATE. TEST CONDITIONS CAUSE
THIS VOLTAGE LEVEL TO BE APPROXIMATELY 1.5V.
I
OL
50pF
I
OH
V
OH
(MEASURED)
V
OL
(MEASURED)
+1.5V