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參數資料
| 型號： | AD7878SE |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | LC2MOS Complete 12-Bit 100 kHz Sampling ADC with DSP Interface |
| 中文描述： | 12-BIT SUCCESSIVE APPROXIMATION ADC, PARALLEL ACCESS, CQCC28 |
| 封裝： | CERAMIC, LCC-28 |
| 文件頁數： | 11/16頁 |
| 文件大小： | 417K |
| 代理商： | AD7878SE |
AD7878
–11–
REV. A
Positive Full-Scale Adjust
Apply a voltage of 2.9978 V (FS/2 – 3/2 LSBs) at V
1
. Adjust R2
until the ADC output code flickers between 0111 1111 1110
and 0111 1111 1111.
Negative Full-Scale Adjust
Apply a voltage of –2.9993 V (–FS/2 + 1/2 LSB) at V
l
and ad-
just R2 until the ADC output code flickers between 1000 0000
0000 and 1000 0000 0001.
Figure 17. AD7878 Full-Scale Adjust Circuit
MICROPROCESSOR INTERFACING
The AD7878 high speed bus timing allows direct interfacing to
DSP processors. Due to the complexity of the AD7878 internal
logic, only synchronous interfacing is allowed. This means that
the ADC clock must be the same as, or a derivative of, the pro-
cessor clock. Suitable processor interfaces are shown in Figures
18 to 21.
AD7878–ADSP-2100/TMS32010/TMS32020
All three interfaces use an external timer for conversion control,
allowing the ADC to sample the analog input asynchronously to
the microprocessor. The AD7878
ALFL
output interrupts the
processor when the FIFO preprogrammed word count is
reached. The processor then reads the conversion results from
the AD7878 internal FIFO memory.
Figure 18. AD7878–ADSP-2700 Interface
Figure 19. AD7878–TMS32020 Interface
The interfaces to the ADSP-2100 and the TMS32020 gate the
AD7878
CS
and the
BUSY
to provide a signal which drives the
processor into a wait state if a read/write operation to the ADC
is attempted while the ADC track/hold amplifier is going from
the track to the hold mode. This avoids digital feedthrough to
the analog circuitry. The TMS32020 does not have separate
RD
and
WR
outputs to drive the AD7878
DMWR
and
DMRD
inputs. These are generated from the processor
STRB
and R/
W
outputs with the addition of some logic gates.
Figure 20. AD7878–TMS32020 Interface
AD7878–M CC8000
This interface also uses an external timer for conversion control
as described for the previous three interfaces. It is discussed
separately because it needs extra logic due to the nature of its
interrupts. The MC68000 has eight levels of external interrupt.
When interrupting this processor one of these levels (0 to 7)
has to be encoded onto the
IPL2
–
IPL0
inputs. This is achieved
with a 74148 encoder in Figure 21, (interrupt Level 1 is taken
for example purposes only). The MC68000 places this inter-
rupt level on address bits A3 to A1 at the start of the interrupt
service routine. Additional logic is used to decode this interrupt
level on the address bus and the FC2–FC0 outputs to generate
a
VPA
signal for the MC68000. This results in an autovectored
interrupt, the start address for the service routine must be
loaded into the appropriate auto vector location during initial-
ization. For further information on the 68000 interrupts con-
sult the 68000
User’s Manual
.
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