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參數資料
| 型號： | AD7890SQ |
| 英文描述： | 450mA, Low Noise Current Mode Step-Down DC/DC Converter; Package: SSOP; No of Pins: 20; Temperature Range: -40°C to +85°C |
| 中文描述： | 立法會2馬鞍山8通道12位串行數據采集系統(tǒng)（191.20十一） |
| 文件頁數： | 12/20頁 |
| 文件大小： | 191K |
| 代理商： | AD7890SQ |
REV. B
AD7890
–12–
In the Self-Clocking Mode, the AD7890 indicates when conver-
sion is complete by bringing the
RFS
line low and initiating a
serial data transfer. In the external clocking mode, there is no
indication of when conversion is complete. In many applica-
tions, this will not be a problem as the data can be read from the
part during conversion or after conversion. However, applications
that seek to achieve optimum performance from the AD7890
will have to ensure that the data read does not occur during
conversion or during 500 ns prior to the rising edge of
CONVST
.
This can be achieved in either of two ways. The first is to ensure
in software that the read operation is not initiated until 5.9
μ
s
after the rising edge of
CONVST
. This will only be possible if
the software knows when the
CONVST
command is issued. The
second scheme would be to use the
CONVST
signal as both the
conversion start signal and an interrupt signal. The simplest way
to do this would be to generate a square wave signal for
CONVST
with high and low times of 5.9
μ
s (see Figure 6). Conversion is
initiated on the rising edge of
CONVST
. The falling edge of
CONVST
occurs 5.9
μ
s later and can be used as either an active
low or falling edge-triggered interrupt signal to tell the processor
to read the data from the AD7890. Provided the read operation
is completed 500 ns before the rising edge of
CONVST
, the
AD7890 will operate to specification.
This scheme limits the throughput rate to 11.8
μ
s minimum. How-
ever, depending upon the response time of the microprocessor
to the interrupt signal and the time taken by the processor to
read the data, this may the fastest which the system could have
operated. In any case, the
CONVST
signal does not have to
have a 50:50 duty cycle. This can be tailored to optimize the
throughput rate of the part for a given system.
Alternatively, the
CONVST
signal can be used as a normal nar-
row pulsewidth. The rising edge of
CONVST
can be used as
an active high or rising edge-triggered interrupt. A software
delay of 5.9
μ
s can then be implemented before data is read
from the part.
C
EXT
FUNCTIONING
The C
EXT
input on the AD7890 provides a means of determining
how long after a new channel address is written to the part that a
conversion can take place. The reason behind this is two-fold.
Firstly, when the input channel to the AD7890 is changed, the
input voltage on this new channel is likely to be very different
from the previous channel voltage. Therefore, the part’s track/
hold has to acquire the new voltage before an accurate con-
version can take place. An internal pulse delays any conversion
start command (as well as the signal to send the track/hold
into hold) until after this pulse has timed out. The second
reason is to allow the user to connect external antialiasing or
signal conditioning circuitry between MUX OUT and SHA IN.
This external circuitry will introduce extra settling time into
the system. The C
EXT
pin provides a means for the user to
extend the internal pulse to take this extra settling time into
account. Basically, varying the value of the capacitor on the
C
EXT
pin varies the duration of the internal pulse. Figure 7
shows the relationship between the value of the C
EXT
capaci-
tor and the internal delay.
C
EXT
CAPACITANCE – pF
0
I
64
56
48
40
32
24
16
8
0
250
500
750
1000
1250
1500
1750
2000
T
A
= –40 C
T
A
= +85 C
T
A
= +25 C
Figure 7. Internal Pulsewidth vs. C
EXT
RFS
TFS
t
CONVERT
500ns MIN
CONVST
SCLK
CONVERSION IS
INITIATED AND
TRACK/HOLD GOES
INTO HOLD
CONVERSION
ENDS 5.9 s
LATER
SERIAL READ
AND WRITE
OPERATIONS
READ AND WRITE
OPERATIONS SHOULD
END 500ns PRIOR
TO NEXT RISING EDGE
OF
CONVST
NEXT
CONVST
RISING EDGE
P INT
SERVICE OR
POLLING
ROUTINE
Figure 6.
CONVST
Used as Status Signal in External Clocking Mode
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