AD7858 Analog Devices, AD7858 Datasheet - Page 19

• Manufacturer Part Number: AD7858
• Description: 3 V to 5 V Single Supply, 200 kSPS, 8-Channel, 12-Bit, Serial Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7858

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 200kSPS
• Interface: Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: DIP,SOIC

PMGT1 PMGT0 SLEEP
Bit
0
0
0
1
1
A typical connection diagram for a low power application is
shown in Figure 21 (AD7858L is the low power version of the
AD7858).
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
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
ANALOG
SUPPLY
AUTO POWER
DOWN AFTER
POWER-UP
CONVERSION
AUTO CAL
ON
+3V
0V TO 2.5V
CURRENT,
I = 1.5mA
0.01 F
0.01 F
DD
0.1 F
TYP
INPUT
Bit
0
0
1
0
1
Table VI. Power Management Options
and DV
10 F
REF192
AIN(+)
AIN(–)
C
C
SLEEP
CAL
AGND
DGND
Pin
0
1
X
X
X
DD
REF1
REF2
0.1 F
REF
are powered up, the AD7858 should be
AV
OPTIONAL
EXTERNAL
REFERENCE
IN
AD7858L
AD7858/
DD
/REF
INTERNAL
REFERENCE
DV
CONVST
0.1 F
0.1 F
CLKIN
OUT
DD
DOUT
Comment
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
SCLK
SYNC
DIN
MASTER
SERIAL CLOCK
CLOCK
SERIAL DATA
SERIAL DATA
INPUT
CONVERSION
START INPUT
OSCILLATOR
OUTPUT
INPUT
INPUT
1.8MHz
GENERATOR
100kHz
PULSE
POWER
LOW
C/ P
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 µs) 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 µs 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 µs after this falling edge. The rising edge of CONVST
initiates a conversion so the CONVST pulse should be at least
5 µs wide. The part automatically powers down on completion
of the conversion.
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
and DV
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
pin. This time is given by the equation:
where R ≅ 150 kΩ and C = external capacitor.
CONVST
BUSY
DD
are first connected, it is recommended that the power-
START CONVERSION ON RISING EDGE
POWER-UP
POWER-UP ON FALLING EDGE
TIME
5 s
t
UP
OPERATION
= 9 × R × C
t
CONVERT
NORMAL
AD7858/AD7858L
POWER-DOWN
FULL
CONVST
POWER-UP
TIME
IN
/REF
OUT
DD