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參數(shù)資料
| 型號(hào)： | AD7813 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 8-/10-Bit Sampling ADC(400kSPS,8位/10位采樣A/D轉(zhuǎn)換器) |
| 中文描述： | 8-/10-Bit采樣ADC（以其400 kSPS，8位/ 10位采樣的A / D轉(zhuǎn)換器） |
| 文件頁(yè)數(shù)： | 7/12頁(yè) |
| 文件大?。?/td> | 232K |
| 代理商： | AD7813 |
AD7813
–7–
REV. 0
During the acquisition phase the sampling capacitor must be
charged to within a 1/2 LSB of its final value. The time it takes
to charge the sampling capacitor (T
CHARGE
) is given by the
following formula:
T
CHARGE
= 7.6
×
(
R
2 + 125
)
×
3.5
pF
For small values of source impedance, the settling time associ-
ated with the sampling circuit (100 ns) is, in effect, the acquisi-
tion time of the ADC. For example, with a source impedance
(R2) of 10
the charge time for the sampling capacitor is
approximately 4 ns. The charge time becomes significant for
source impedances of 2 k
and greater.
AC Acquisition Time
In ac applications it is recommended to always buffer analog
input signals. The source impedance of the drive circuitry must
be kept as low as possible to minimize the acquisition time of
the ADC. Large values of source impedance will cause the
THD to degrade at high throughput rates.
ADC TRANSFER FUNCTION
The output coding of the AD7813 is straight binary. The de-
signed code transitions occur at successive integer LSB values
(i.e., 1 LSB, 2 LSBs, etc.). The LSB size is = V
REF
/1024. The
ideal transfer characteristic for the AD7813 is shown in Figure 7.
000...000
0V
ANALOG INPUT
111...111
111...110
000...001
000...010
111...000
011...111
1LSB
+V
REF
–1LSB
1LSB = V
REF
/1024
A
Figure 7. Transfer Characteristic
POWER-UP TIMES
The AD7813 has a 1
μ
s power-up time. When V
DD
is first con-
nected, the AD7813 is in a low current mode of operation. In
order to carry out a conversion the AD7813 must first be pow-
ered up. The ADC is powered up by a rising edge on an inter-
nally generated
CONVST
signal, which occurs as a result of a
rising edge on the external
CONVST
pin. The rising edge of the
external
CONVST
signal initiates a 1
μ
s pulse on the internal
CONVST
signal. This pulse is present to ensure the part has
enough time to power up before a conversion is initiated, as a
conversion is initiated on the falling edge of gated
CONVST
.
See Timing and Control section. Care must be taken to ensure
that the
CONVST
pin of the AD7813 is logic low when V
DD
is
first applied.
When operating in Mode 2, the ADC is powered down at the
end of each conversion and powered up again before the next
conversion is initiated. (See Figure 8.)
t
POWER-UP
t
POWER-UP
t
POWER-UP
MODE 1
MODE 2
V
DD
EXT CONVST
INT CONVST
V
DD
EXT CONVST
INT CONVST
Figure 8. Power-Up Times
POWER VS. THROUGHPUT RATE
By operating the AD7813 in Mode 2, the average power con-
sumption of the AD7813 decreases at lower throughput rates.
Figure 9 shows how the Automatic Power-Down is implemented
using the external
CONVST
signal to achieve the optimum
power performance for the AD7813. The AD7813 is operated
in Mode 2, and the duration of the external
CONVST
pulse is
set to be equal to or less than the power-up time of the device.
As the throughput rate is reduced, the device remains in its power-
down state longer and the average power consumption over time
drops accordingly.
EXT CONVST
INT CONVST
POWER-DOWN
t
POWER-UP
t
CONVERT
t
100 s @ 10kSPS
Figure 9. Automatic Power-Down
For example, if the AD7813 is operated in a continuous sam-
pling mode, with a throughput rate of 10 kSPS, the power con-
sumption is calculated as follows. The power dissipation during
normal operation is 10.5 mW, V
DD
= 3 V. If the power-up time
is 1
μ
s and the conversion time is 2.3
μ
s, the AD7813 can then
be said to dissipate 10.5 mW for 3.3
μ
s (worst case) during each
conversion cycle. If the throughput rate is 10 kSPS, the cycle
time is 100
μ
s and the average power dissipated during each
cycle is (3.3/100)
×
(10.5 mW) = 346.5
μ
W.
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