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參數資料
| 型號： | AD7858AN |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | 3 V to 5 V Single Supply, 200 kSPS 8-Channel, 12-Bit Sampling ADC |
| 中文描述： | 8-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, SERIAL ACCESS, PDIP24 |
| 封裝： | 0.300 INCH, PLASTIC, DIP-24 |
| 文件頁數： | 19/32頁 |
| 文件大小： | 315K |
| 代理商： | AD7858AN |
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AD7858/AD7858L
REV. B
–19–
Table VI. Power Management Options
PMGT1 PMGT0
SLEEP
Bit
Bit
Pin
Comment
0
0
0
Full Power-Down
if Not
Calibrating or Converting
(Default Condition
After Power-On)
Normal Operation
Normal Operation
(Independent of the
SLEEP
Pin)
Full Power-Down
Partial Power-Down
if Not
Converting
0
0
0
1
1
X
1
1
0
1
X
X
A typical connection diagram for a low power application is
shown in Figure 21 (AD7858L is the low power version of the
AD7858).
AUTO POWER
DOWN AFTER
CONVERSION
AUTO CAL
ON
POWER-UP
OPTIONAL
EXTERNAL
REFERENCE
1.8MHz
OSCILLATOR
MASTER
CLOCK
INPUT
0.1 F
0.1 F
10 F
ANALOG
SUPPLY
+3V
0V TO 2.5V
INPUT
0.1 F
0.01 F
0.01 F
0.1 F
INTERNAL
REFERENCE
100kHz
PULSE
GENERATOR
CONVERSION
START INPUT
SERIAL CLOCK
INPUT
SERIAL DATA
OUTPUT
CLKIN
SCLK
CONVST
SYNC
DIN
DOUT
DGND
AGND
CAL
SLEEP
C
REF2
C
REF1
AIN(
–
)
AIN(+)
AV
DD
DV
DD
AD7858/
AD7858L
LOW
POWER
C/ P
CURRENT,
I = 1.5mA
TYP
SERIAL DATA
INPUT
REF
IN
/REF
OUT
REF192
Figure 21. Typical Low Power Circuit
POWER-UP TIMES
Using an External Reference
When the AD7858 is powered up, the part is powered up from
one of two conditions. First, when the power supplies are ini-
tially powered up and, secondly, when the part is powered up
from either a hardware or software power-down (see last section).
When AV
DD
and DV
DD
are powered up, the AD7858 should be
left idle for approximately 32 ms (4 MHz CLK) to allow for the
autocalibration if a 10 nF cap is placed on the
CAL
pin, (see
Calibration section). During power-up the functionality of the
SLEEP
pin is disabled, i.e., the part will not power down until
the end of the calibration if
SLEEP
is tied logic low. The auto-
calibration on power-up can be disabled if the
CAL
pin is tied to
a logic high. If the autocalibration is disabled, then the user must
take into account the time required by the AD7858 to power-up
before a self-calibration is carried out. This power-up time is the
time taken for the AD7858 to power up when power is first
applied (300
μ
s) typ) or the time it takes the external reference
to settle to the 12-bit level
–
whichever is the longer.
The AD7858 powers up from a full hardware or software
power-down in 5
μ
s typ. This limits the throughput which the
part is capable of to 104 kSPS for the AD7858 operating with a
4 MHz CLK and 66 kSPS for the AD7858L with a 1.8 MHz
CLK when powering down between conversions. Figure 22
shows how power-down between conversions is implemented
using the
CONVST
pin. The user first selects the power-down
between conversions option by using the
SLEEP
pin and the
power management bits, PMGT1 and PMGT0, in the control
register, (see last section). In this mode the AD7858 automati-
cally enters a full power-down at the end of a conversion, i.e.,
when BUSY goes low. The falling edge of the next
CONVST
pulse causes the part to power up. Assuming the external refer-
ence is left powered up, the AD7858 should be ready for normal
operation 5
μ
s after this falling edge. The rising edge of
CONVST
initiates a conversion so the
CONVST
pulse should be at least
5
μ
s wide. The part automatically powers down on completion
of the conversion.
5 s
t
CONVERT
POWER-UP
TIME
NORMAL
OPERATION
FULL
POWER-DOWN
POWER-UP
TIME
START CONVERSION ON RISING EDGE
POWER-UP ON FALLING EDGE
CONVST
BUSY
Figure 22. Power-Up Timing When Using
CONVST
Pin
NOTE: Where the software CONVST is used or automatic full
power-down, the part must be powered up in software with an
extra write setting PMGT1 = 0 and PMGT0 = 1 before a con-
version is initiated in the next write. Automatic partial power-
down after a calibration is not possible; the part must be
powered down manually. If software calibrations are to be used
when operating in the partial power-down mode, then three
separate writes are required. The first initiates the type of cali-
bration required, the second write powers the part down into
partial power-down mode, while the third write powers the part
up again before the next calibration command is issued.
Using the Internal (On-Chip) Reference
As in the case of an external reference, the AD7858 can power-
up from one of two conditions, power-up after the supplies are
connected or power-up from hardware/software power-down.
When using the on-chip reference and powering up when AV
DD
and DV
DD
are first connected, it is recommended that the power-
up calibration mode be disabled as explained above. When using
the on-chip reference, the power-up time is effectively the time
it takes to charge up the external capacitor on the REF
IN
/REF
OUT
pin. This time is given by the equation:
t
UP
= 9
×
R
×
C
where
R
150 k
and
C
= external capacitor.
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