Datasheet

AD977/AD977A
–13–REV. D
EXTERNAL CONTINUOUS CLOCK DATA READ DURING
CONVERSION WITH SYNC OUTPUT GENERATED
Figure 9 illustrates the method by which data from conversion
“n-1” can be read during conversion “n” while using a continu-
ous external clock with the generation of a SYNC output. What
permits the generation of a SYNC output is a transition of
DATACLK while either CS is high or while both CS and R/C
are low.
With a continuous clock the CS pin cannot be tied low as it
could be with a discontinuous clock. Use of a continuous clock
while a conversion is occurring can increase the DNL and
Transition Noise of the AD977/AD977A.
In Figure 9 a conversion is initiated by taking R/C low with CS
held low. While this condition exists a transition of DATACLK,
clock pulse #0, will enable the generation of a SYNC pulse.
Less then 83 ns after R/C is taken low the BUSY output will go
low to indicate that the conversion process has began. Figure 9
shows R/C then going high and after a delay of greater than
15 ns (t
15
), clock pulse #1 can be taken high to request the
SYNC output. The SYNC output will appear approximately
50 ns after this rising edge and will be valid on the falling edge
of clock pulse #1 and the rising edge of clock pulse #2. The
MSB will be valid approximately 40 ns after the rising edge of
clock pulse #2 and can be latched off either the falling edge of
clock pulse #2 or the rising edge of clock pulse #3. The LSB
will be valid on the falling edge of clock pulse #17 and the rising
edge of clock pulse #18. Approximately 40 ns after the rising
edge of clock pulse #18, the DATA output pin will reflect the
state of the TAG input pin during the rising edge of clock
pulse #2.
For both the AD977 and the AD977A the data should be
clocked out during the 1st half of BUSY so as not to degrade
conversion performance. For the AD977 this requires use of a
4.8 MHz DATACLK or greater with data being read out as
soon as the conversion process begins. For the AD977A it
requires use of a 10 MHz DATACLK or greater.
CS
BUSY
R/C
EXT
DATACLK
BIT 15
(MSB)
1
DATA
SYNC
03
BIT 0
(LSB)
TAG 0 TAG 1
TAG 0
TAG
2
t
13
t
14
t
12
t
19
18
t
15
t
16
t
1
t
20
t
2
t
17
t
12
t
18
t
23
t
24
t
18
TAG 2
TAG 1 TAG 16 TAG 17 TAG 18 TAG 19
Figure 9. Conversion and Read Timing for Reading Previous Conversion Results During a Conversion Using An External
Continuous Data Clock (EXT/
INT Set to Logic High)