Datasheet

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SBAS255C − JUNE 2004 − REVISED APRIL 2009
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25
RETRIEVAL BEFORE AND AFTER CONV
TOGGLES (CONTINUOUS MODE)
For the absolute maximum time for data retrieval, data can
be retrieved before and after CONV toggles. Nearly all of
T
INT
is available for data retrieval. Figure 25 illustrates how
this is done by combining the two previous methods. You
must pause the retrieval during CONV toggling to prevent
digital noise, as discussed previously, and finish before the
next data is ready. The maximum number of DDC114s that
can be daisy-chained together (FORMAT = high) is:
T
INT
* 20ms * 1.75ms
80t
DCLK
NOTE: 64τ
DCLK
is used for FORMAT = low.
For T
INT
= 400µs and DCLK = 10MHz, the maximum
number of DDC114s is 47 (or 59 for FORMAT = low).
RETRIEVAL: NONCONTINUOUS MODE
Retrieving in noncontinuous mode is slightly different than
compared with the continuous mode. As illustrated in
Figure 26, DVALID
goes low in time t
30
after the first
integration completes. If T
INT
is shorter than this time, all
of t
31
is available to retrieve data before the other side data
is ready. For T
INT
> t
30
, the first integration data is ready
before the second integration completes. Data retrieval
must be delayed until the second integration completes,
leaving less time available for retrieval. The time available
is t
31
– (T
INT
– t
30
). The second integration data must be
retrieved before the next round of integration begins. This
time is highly dependent on the pattern used to generate
CONV. As with the continuous mode, data retrieval must
halt before and after CONV toggles (t
28
, t
29
) and be
completed before new data is ready (t
26
).
SYMBOL
DESCRIPTION
CLK = 4MHZ, CLK_4X = 0 CLK = 4.8MHZ, CLK_4X = 0
UNITS
SYMBOL
DESCRIPTION
MIN TYP MAX MIN TYP MAX
UNITS
t
26
Hold Time that DOUT is Valid Before Falling Edge of DVALID 1.75 1.458 µs
t
28
Data Retrieval Shutdown Before Edge of CONV 10 10 µs
t
29
Data Retrieval Start-Up After Edge of CONV 10 10 µs
Figure 25. Readback Before and After CONV Toggles
SYMBOL
DESCRIPTION
CLK = 4MHz, CLK_4X = 0 CLK = 4.8MHz, CLK_4X = 0
UNITS
SYMBOL
DESCRIPTION
MIN TYP MAX MIN TYP MAX
UNITS
t
30
1st ncont Mode Data Ready 344.75 ± 0.25 287.292 ± 0.208 µs
t
31
2nd ncont Mode Data Ready 362.500 302.083 µs
Figure 26. Readback in Non-Continuous Mode
………… … …
………
… … …
DCLK
DVALID
CONV
DOUT
Side B
Data
Side A
Data
T
INT
t
29
t
28
t
26
T
INT
T
INT
……
……
T
IN T
T
INT
t
30
T
INT
T
INT
t
31
Side A
Data
Side B
Data
CONV
DVALID
DCLK
DOUT