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參數資料
| 型號： | AD774BBR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | Complete 12-Bit A/D Converters |
| 中文描述： | 1-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, PARALLEL ACCESS, PDSO28 |
| 封裝： | PLASTIC, SOIC-28 |
| 文件頁數： | 9/12頁 |
| 文件大小： | 247K |
| 代理商： | AD774BBR |
REV. C
–9–
TO
OUTPUT
BUFFERS
CE
CS
R/
C
A
0
12/
8
NYBBLE A
ENABLE
NYBBLE B
ENABLE
NYBBLE C
ENABLE
NYBBLE = 0
ENABLE
STATUS
CLK EN
HIGH IF CONVERSION
IN PROGRESS
SAR
RESET
EOC 12
EOC 8
START CONVERT
S
R
Q
QB
READ
VALUE OF A
0
AT LAST CONVERT COMMAND
D
EN
D
EN
Q
R
S
Q
Figure 9. Equivalent Internal Logic Circuitry
CONTROL LOGIC
The AD674B and AD774B contain on-chip logic to provide
conversion initiation and data read operations from signals
commonly available in microprocessor systems; this internal
logic circuitry is shown in Figure 9.
The control signals CE,
CS
, and R/
C
control the operation of
the converter. The state of R/
C
when CE and
CS
are both
asserted determines whether a data read (R/
C
= 1) or a convert
(R/
C
= 0) is in progress. The register control inputs, A
0
and
12/
8
, control conversion length and data format. If a conversion
is started with A
0
low, a full 12-bit conversion cycle is initiated.
If A
0
is high during a convert start, a shorter 8-bit conversion
cycle results. During data read operations, A
0
determines
whether the three-state buffers containing the 8 MSBs of the
conversion result (A
0
= 0) or the 4 LSBs (A
0
= 1) are enabled.
The 12/
8
pin determines whether the output data is to be orga-
nized as two 8-bit words (12/
8
tied to DIGITAL COMMON)
or a single 12-bit word (12/
8
tied to V
LOGIC
). In the 8-bit mode,
the byte addressed when A
0
is high contains the 4 LSBs from
the conversion followed by four trailing zeroes. This organiza-
tion allows the data lines to be overlapped for direct interface to
8-bit buses without the need for external three-state buffers.
An output signal, STS, indicates the status of the converter.
STS goes high at the beginning of a conversion and returns low
when the conversion cycle is complete.
Table I. Truth Table
CE
CS
R/
C
12/
8
A
0
Operation
0
X
1
1
1
1
1
X
1
0
0
0
0
0
X
X
0
0
1
1
1
X
X
X
X
1
0
0
X
X
0
1
X
0
1
None
None
Initiate 12-Bit Conversion
Initiate 8-Bit Conversion
Enable 12-Bit Parallel Output
Enable 8 Most Significant Bits
Enable 4 LSBs + 4 Trailing Zeroes
The ADC may be operated in one of two modes, the full-control
mode and the standalone mode. The full-control mode uses all
the control signals and is useful in systems that address decode
multiple devices on a single data bus. The standalone mode is
useful in systems with dedicated input ports available. In gen-
eral, the standalone mode is capable of issuing start-convert
commands on a more precise basis and therefore produces
higher accuracy results. The following sections describe these
two modes in more detail.
FULL-CONTROL MODE
Chip Enable (CE), Chip Select (
CS
), and Read/Convert (R/
C
)
are used to control Convert or Read modes of operation. Either
CE or
CS
may be used to initiate a conversion. The state of R/
C
when CE and
CS
are both asserted determines whether a data
Read (R/
C
= 1) or a Convert (R/
C
= 0) is in progress. R/
C
should be LOW before both CE and
CS
are asserted; if R/
C
is
HIGH, a Read operation will momentarily occur, possibly
resulting in system bus contention.
AD674B/AD774B
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