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參數資料
| 型號： | AD7713 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Loop-Powered Signal Conditioning ADC(循環驅動LC2MOS信號調節A/D轉換器) |
| 中文描述： | 回路供電信號調理模數轉換器（循環驅動LC2MOS信號調節的A / D轉換器） |
| 文件頁數： | 16/28頁 |
| 文件大小： | 529K |
| 代理商： | AD7713 |
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REV. C
–16–
AD7713
System Synchronization
If multiple AD7713s are operated from a common master clock,
they can be synchronized to update their output registers simul-
taneously. A falling edge on the
SYNC
input resets the filter
and places the AD7713 into a consistent, known state. A com-
mon signal to the AD7713s’
SYNC
inputs will synchronize their
operation. This would normally be done after each AD7713 has
performed its own calibration or has had calibration coefficients
loaded to it.
The
SYNC
input can also be used to reset the digital filter in
systems where the turn-on time of the digital power supply
(DV
DD
) is very long. In such cases, the AD7713 will start oper-
ating internally before the DV
DD
line has reached its minimum
operating level, +4.75 V. With a low DV
DD
voltage, the
AD7713’s internal digital filter logic does not operate correctly.
Thus, the AD7713 may have clocked itself into an incorrect
operating condition by the time that DV
DD
has reached its cor-
rect level. The digital filter will be reset upon issue of a calibra-
tion command (whether it is self-calibration, system calibration
or background calibration) to the AD7713. This ensures correct
operation of the AD7713. In systems where the power-on de-
fault conditions of the AD7713 are acceptable, and no calibra-
tion is performed after power-on, issuing a
SYNC
pulse to the
AD7713 will reset the AD7713’s digital filter logic. An R, C on
the
SYNC
line, with R, C time constant longer than the DV
DD
power-on time, will perform the
SYNC
function.
ACCURACY
Sigma-delta ADCs, like VFCs and other integrating ADCs, do
not contain any source of nonmonotonicity and inherently offer
no missing codes performance. The AD7713 achieves excellent
linearity by the use of high quality, on-chip silicon dioxide ca-
pacitors, which have a very low capacitance/voltage coefficient.
The device also achieves low input drift through the use of chopper
stabilized techniques in its input stage. To ensure excellent perfor-
mance over time and temperature, the AD7713 uses digital calibra-
tion techniques that minimize offset and gain error.
AUTOCALIBRATION
Autocalibration on the AD7713 removes offset and gain errors
from the device. A calibration routine should be initiated on the
device whenever there is a change in the ambient operating tem-
perature or supply voltage. It should also be initiated if there is a
change in the selected gain, filter notch or bipolar/unipolar input
range. However, if the AD7713 is in its background calibration
mode, the above changes are all automatically taken care of
(after the settling time of the filter has been allowed for).
The AD7713 offers self-calibration, system calibration and
background calibration facilities. For calibration to occur on the
selected channel, the on-chip microcontroller must record the
modulator output for two different input conditions. These are
“zero-scale” and “full-scale” points. With these readings, the
microcontroller can calculate the gain slope for the input to out-
put transfer function of the converter. Internally, the part works
with a resolution of 33 bits to determine its conversion result of
either 16 bits or 24 bits.
voltage can go to +5 V with no degradation in performance
provided that the absolute value of REF IN(+) and REF IN(–)
does not exceed its AV
DD
and AGND limits. The part is also
functional with V
REF
voltages down to 1 V but with degraded
performance as the output noise will, in terms of LSB size, be
larger. REF IN(+) must always be greater than REF IN(–) for
correct operation of the AD7713.
Both reference inputs provide a high impedance, dynamic load
similar to the analog inputs. The maximum dc input leakage
current is 10 pA (
±
1 nA over temperature) and source resis-
tance may result in gain errors on the part. The reference inputs
look like the AIN1 analog input (see Figure 7). In this case,
R
INT
is 5 k
typ and C
INT
varies with gain. The input sample
rate is f
CLK IN
/256 and does not vary with gain. For gains of 1
to 8 C
INT
is 20 pF; for a gain of 16 it is 10 pF; for a gain of 32
it is 5 pF; for a gain of 64 it is 2.5 pF; and for a gain of 128 it is
1.25 pF.
The digital filter of the AD7713 removes noise from the refer-
ence input just as it does with the analog input, and the same
limitations apply regarding lack of noise rejection at integer mul-
tiples of the sampling frequency. The output noise performance
outlined in Tables I and II assumes a clean reference. If the ref-
erence noise in the bandwidth of interest is excessive, it can
degrade the performance of the AD7713. A recommended refer-
ence source for the AD7713 is the AD680, a 2.5 V reference.
USING THE AD7713
SYSTEM DESIGN CONSIDERATIONS
The AD7713 operates differently from successive approximation
ADCs or integrating ADCs. Since it samples the signal continu-
ously, like a tracking ADC, there is no need for a start convert
command. The output register is updated at a rate determined
by the first notch of the filter and the output can be read at any
time, either synchronously or asynchronously.
Clocking
The AD7713 requires a master clock input, which may be an
external TTL/CMOS compatible clock signal applied to the
MCLK IN pin with the MCLK OUT pin left unconnected.
Alternatively, a crystal of the correct frequency can be con-
nected between MCLK IN and MCLK OUT, in which case the
clock circuit will function as a crystal controlled oscillator. For
lower clock frequencies, a ceramic resonator may be used in-
stead of the crystal. For these lower frequency oscillators, exter-
nal capacitors may be required on either the ceramic resonator
or on the crystal.
The input sampling frequency, the modulator sampling fre-
quency, the –3 dB frequency, output update rate and calibration
time are all directly related to the master clock frequency,
f
CLK IN.
Reducing the master clock frequency by a factor of two
will halve the above frequencies and update rate and will double
the calibration time.
The current drawn from the DV
DD
power supply is also directly
related to f
CLK IN
. Reducing f
CLK IN
by a factor of two will halve
the DV
DD
current but will not affect the current drawn from the
AV
DD
power supply.
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