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參數(shù)資料
| 型號(hào)： | AD7478A |
| 英文描述： | AD7476A/AD7477A/AD7478A: 2.35 V to 5.25 V. 1 MSPS. 12-/10-/8-Bit ADCs in 6-Lead SC70 Data Sheet (Rev. A. 2/03) |
| 中文描述： | AD7476A/AD7477A/AD7478A：2.35 V至5.25五，1 MSPS的。 12/10/8位ADC的6引腳SC70數(shù)據(jù)手冊(cè)（版本A 2月3日） |
| 文件頁數(shù)： | 14/20頁 |
| 文件大?。?/td> | 729K |
| 代理商： | AD7478A |
REV. A
–14–
AD7476A/AD7477A/AD7478A
Table II provides some typical performance data with various
op amps used as the input buffer for a 100 kHz input tone at
room temperature under the same setup conditions.
Table II. AD7476A Typical Performance with Various
Input Buffers, V
DD
= 3 V
Op Amp in the
Input Buffer
AD7476A SNR Performance
(dB)
AD711
AD797
AD845
72.3
72.5
71.4
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance will depend on the amount of total harmonic
distortion (THD) that can be tolerated. The THD will increase
as the source impedance increases and the performance will
degrade. See TPC 7.
Digital Inputs
The digital inputs applied to the AD7476A/AD7477A/AD7478A
are not limited by the maximum ratings that limit the analog
input. Instead, the digital inputs applied can go to 7 V and are
not restricted by the V
DD
+ 0.3 V limit as on the analog input.
For example, if the AD7476A/AD7477A/AD7478A were oper-
ated with a V
DD
of 3 V, then 5 V logic levels could be used on
the digital inputs. However, it is important to note that the data
output on SDATA will still have 3 V logic levels when V
DD
= 3 V.
Another advantage of SCLK and
CS
not being restricted by the
V
DD
+ 0.3 V limit is the fact that power supply sequencing
issues are avoided. If
CS
or SCLK is applied before V
DD
, there
is no risk of latch-up as there would be on the analog input if a
signal greater than 0.3 V was applied prior to V
DD
.
MODES OF OPERATION
The mode of operation of the AD7476A/AD7477A/AD7478A is
selected by controlling the (logic) state of the
CS
signal during a
conversion. There are two possible modes of operation: normal
and power-down. The point at which
CS
is pulled high after the
conversion has been initiated will determine whether the
AD7476A/AD7477A/AD7478A will enter power-down mode or
not. Similarly, if already in power-down,
CS
can control
whether the device will return to normal operation or remain in
power-down. These modes of operation are designed to provide
flexible power management options. These options can be
chosen to optimize the power dissipation/throughput rate ratio for
different application requirements.
Normal Mode
This mode is intended for the fastest throughput rate perfor-
mance, as the user does not have to worry about any power-up
times with the AD7476A/AD7477A/AD7478A remaining fully
powered all the time. Figure 9 shows the general diagram of the
operation of the AD7476A/AD7477A/AD7478A in this mode.
The conversion is initiated on the falling edge of
CS
as described
in the Serial Interface section. To ensure that the part remains
fully powered up at all times,
CS
must remain low until at least
10 SCLK falling edges have elapsed after the falling edge of
CS
.
If
CS
is brought high any time after the 10th SCLK falling
edge, but before the end of the t
CONVERT
, the part will remain
powered up, but the conversion
will be terminated and SDATA
will go back into three-state.
For the AD7476A, 16 serial clock cycles are required to com-
plete the conversion and access the complete conversion results.
For the AD7477A and AD7478A, a minimum of 14 and 12
serial clock cycles are required to complete the conversion and
access the complete conversion results, respectively.
CS
may idle high until the next conversion or may idle low until
CS
returns high sometime prior to the next conversion (effec-
tively idling
CS
low).
Once a data transfer is complete (SDATA has returned to
three-state), another conversion can be initiated after the quiet
time, t
QUIET
, has elapsed by bringing
CS
low again.
Power-Down Mode
This mode is intended for use in applications where slower
throughput rates are required; either the ADC is powered down
between each conversion, or a series of conversions is performed
at a high throughput rate and the ADC is then powered down
for a relatively long duration between these bursts of several
conversions. When the AD7476A/AD7477A/AD7478A is in
power-down, all analog circuitry is powered down.
To enter power-down, the conversion process must be inter-
rupted by bringing
CS
high anywhere after the second falling
edge of SCLK and before the 10th falling edge of SCLK as
shown in Figure 10. Once
CS
has been brought high in this
window of SCLKs, the part will enter power-down, the con-
version that was initiated by the falling edge of
CS
will be
terminated, and SDATA will go back into three-state. If
CS
is
brought high before the second SCLK falling edge, then the
part will remain in normal mode and will not power down. This
will avoid accidental power-down due to glitches on the
CS
line.
In order to exit this mode of operation and power the AD7476A/
AD7477A/AD7478A up again, a dummy conversion is performed.
On the falling edge of
CS
, the device will begin to power up and
will continue to power up as long as
CS
is held low until after the
falling edge of the 10th SCLK. The device will be fully powered
up once 16 SCLKs have elapsed and valid data will result from
the next conversion as shown in Figure 11. If
CS
is brought high
before the 10th falling edge of SCLK, then the AD7476A/
AD7477A/AD7478A will go back into power-down. This avoids
accidental power-up due to glitches on the
CS
line or an inad-
vertent burst of eight SCLK cycles while
CS
is low. So although
the device may begin to power up on the falling edge of
CS
, it
will power down again on the rising edge of
CS
as long as it occurs
before the 10th SCLK falling edge.
Power-Up Time
The power-up time of the AD7476A/AD7477A/AD7478A is
1
μ
s, which means that with any frequency of SCLK up to 20 MHz,
one dummy cycle will always be sufficient to allow the device to
power up. Once the dummy cycle is complete, the ADC will be
fully powered up and the input signal will be acquired properly.
The quiet time, t
QUIET
, must still be allowed from the point
where the bus goes back into three-state after the dummy con-
version to the next falling edge of
CS
. When running at a 1 MSPS
throughput rate, the AD7476A/AD7477A/AD7478A will power
up and acquire a signal within
±
0.5 LSB in one dummy
cycle, i.e., 1
μ
s.
When powering up from the power-down mode with a dummy
cycle, as in Figure 11, the track-and-hold that was in hold mode
while the part was powered down returns to track mode after
the first SCLK edge the part receives after the falling edge of
CS
. This is shown as Point A in Figure 11. Although at any
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