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AD974BRS Datasheet PDF : 20 Pages
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AD974
CONVERSION CONTROL
The AD974 is controlled by two signals: R/C and CS. When
R/C is brought low, with CS low, for a minimum of 50 ns, the
input signal will be held on the internal capacitor array and a
conversion “n” will begin. Once the conversion process does
begin, the BUSY signal will go low until the conversion is com-
plete. Internally, the signals R/C and CS are ORed together and
there is no requirement on which signal is taken low first when
initiating a conversion. The only requirement is that there be at
least 10 ns of delay between the two signals being taken low.
After the conversion is complete, the BUSY signal will return
high and the AD974 will again resume tracking the input signal.
Under certain conditions the CS pin can be tied Low and R/C
will be used to determine whether you are initiating a conver-
sion or reading data. On the first conversion, after the AD974 is
powered up, the DATA output will be indeterminate.
Conversion results can be clocked serially, using either an
internal clock generated by the AD974 or an external clock.
The AD974 is configured for the internal data clock mode by
pulling the EXT/INT pin low. It is configured for the external
clock mode by pulling the EXT/INT pin high.
INTERNAL DATA CLOCK MODE
The AD974 is configured to generate and provide the data clock
when the EXT/INT pin is held low. Typically CS will be tied
low and R/C will be used to initiate a conversion “n.” During
the conversion the AD974 will output 16 bits of data, MSB first,
from conversion “n-1” on the DATA pin. This data will be
synchronized with 16 clock pulses provided on the DATACLK
pin. The output data will be valid on both the rising and falling
edge of the data clock as shown in Figure 3. After the LSB has
been presented, the DATACLK pin will stay low until another
conversion is initiated.
In this mode, the digital input/output pins’ transitions are suit-
ably positioned to minimize degradation on the conversion
result, mainly during the second half of the conversion process.
EXTERNAL DATA CLOCK MODE
The AD974 is configured to accept an externally supplied data
clock when the EXT/INT pin is held high. This mode of opera-
tion provides several methods by which conversion results can
be read. The output data from conversion “n-1” can be read
during conversion “n,” or the output data from conversion “n”
t1
CS, R/C
A0, A1
WR1, WR2
BUSY
MODE
t23 t25
t24
t3
t2
t5
t4
ACQUIRE
CONVERT
ACQUIRE
t6
t7
Figure 2. Basic Conversion Timing
CONVERT
R/C
DATACLK
DATA
BUSY
t8
t1
t2
t9
1
2
3
t10
t11
MSB
VALID
BIT 14
VALID
BIT 13
VALID
t6
15
16
BIT 1
VALID
LSB
VALID
Figure 3. Serial Data Timing for Reading Previous Conversion Results with Internal Clock
(CS and EXT/ INT Set to Logic Low)
REV. A
–7–
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