ADSP-21MOD870 데이터 시트보기 (PDF) - Analog Devices

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ADSP-21MOD870

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Analog Devices 

ADSP-21mod870

CAPACITIVE LOADING

Figures 16 and 17 show the capacitive loading characteristics of

the ADSP-21mod870.

18

T = +85 C

16 VDD = 30V

14

12

10

8

6

4

2

0

0

50

100

150

200

250

CL – pF

Figure 16. Typical Output Rise Time vs. Load Capacitance,

CL (at Maximum Ambient Operating Temperature)

10

9

8

7

6

5

4

3

2

1

NOMINAL

–1

–2

–3

–4

0

20 40 60 80 100 120 140 160 180 20.0

CL – pF

Figure 17. Typical Output Valid Delay or Hold vs. Load

Capacitance, CL (at Maximum Ambient Operating

Temperature)

INPUT

OR

OUTPUT

1.5V

1.5V

Figure 18. Voltage Reference Levels for AC Measure-

ments (Except Output Enable/Disable)

TEST CONDITIONS

Output Disable Time

Output pins are considered to be disabled when they have

stopped driving and started a transition from the measured

output high or low voltage to a high impedance state. The out-

put disable time (tDIS) is the difference of tMEASURED and tDECAY.

The time is the interval from when a reference signal reaches

a high or low voltage level to when the output voltages have

changed by 0.5 V from the measured output high or low voltage,

see Figure 19.

The decay time, tDECAY, is dependent on the capacitive load, CL,

and the current load, iL, on the output pin. It can be approxi-

mated by the following equation:

tDECAY

= CL

× 0.5V

iL

from which

tDIS = tMEASURED – tDECAY

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.

Output Enable Time

Output pins are considered to be enabled when they have made

a transition from a high-impedance state to when they start

driving. The output enable time (tENA) is the interval from when

a reference signal reaches a high or low voltage level to when the

output has reached a specified high or low trip point, see Figure

19. If multiple pins (such as the data bus) are enabled, the mea-

surement value is that of the first pin to start driving. Figure 20

shows the equivalent device loading for ac measurements.

REFERENCE

SIGNAL

tMEASURED

VOH tDIS

(MEASURED)

OUTPUT

VOL

(MEASURED)

VOH (MEASURED) – 0.5V

VOL (MEASURED) +0.5V

tDECAY

tENA

2.0V

1.0V

VOH

(MEASURED)

VOL

(MEASURED)

OUTPUT STOPS

DRIVING

OUTPUT STARTS

DRIVING

HIGH-IMPEDANCE STATE. TEST CONDITIONS CAUSE

THIS VOLTAGE LEVEL TO BE APPROXIMATELY 1.5V.

Figure 19. Output Enable/Disable

IOL

TO

OUTPUT

PIN

50pF

+1.5V

IOH

Figure 20. Equivalent Device Loading for AC Measure-

ments (Including All Fixtures)

–18–

REV. 0

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