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參數(shù)資料
| 型號： | AD7703 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | LC2MOS 20-Bit A/D Converter |
| 中文描述： | LC2MOS 20位A / D轉(zhuǎn)換器 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大小： | 251K |
| 代理商： | AD7703 |
AD7703
–12–
REV. D
INPUT SIGNAL CONDIT IONING
Reference voltages from +1 V to +3 V may be used with the
AD7703, with little degradation in performance. Input ranges
that cannot be accommodated by this range of reference voltages
may be achieved by input signal conditioning. T his may take the
form of gain to accommodate a smaller signal range, or passive
attenuation to reduce a larger input voltage range.
Source Resistance
If passive attenuators are used in front of the AD7703, care
must be taken to ensure that the source impedance is suffi-
ciently low. T he dc input resistance for the AD7703 is over
1 G
. In parallel with this there as a small dynamic load which
varies with the clock frequency (see Figure 14). Each time the
A
IN
R1
R2
C
EXT
AGND
AD7703
V
OS
≤
100mV
C
10pF
V
IN
1G
Figure 14. Equivalent Input Circuit and Input Attenuator
analog input is sampled, a 10 pF capacitor draws a charge
packet of maximum 1 pC (10 pF
3
100 mV) from the analog
source with a frequency f
CLK IN
/256. For a 4.096 MHz CLK IN,
this yields an average current draw of 16 nA. After each sample
the AD7703 allows 62 clock periods for the input voltage to
settle. T he equation which defines settling time is:
V
O
=
V
IN
[1±
e
±
t
/
RC
]
where
V
O
, is the final settled value,
V
IN
, is the value of the input
signal,
R
is the value of the input source resistance,
C
is the
10 pF sample capacitor. T he value of
t
is
equal to 62/f
CLK IN
.
T he following equation can be developed which gives the max-
imum allowable source resistance, R
S(MAX )
for an error of V
E
.
R
S
(
MAX
)
=
62
f
CLKIN
·(10
pF
)·
ln
(100
mV
/
V
E
)
Provided the source resistance is less than this value, the analog
input will settle within the desired error band in the requisite 62
clock periods. Insufficient settling leads to offset errors. T hese
can be calibrated in system calibration schemes.
If a limit of 600 nV (0.25 LSB at 20 bits) is set for the maxi-
mum offset voltage, then the maximum allowable source resis-
tance is 125 k
from the above equation, assuming that there is
no external stray capacitance.
An RC filter may be added in front of the AD7703 to reduce
high frequency noise. With an external capacitor added from
A
IN
to AGND, the following equation will specify the maximum
allowable source resistance:
R
S
(
MAX
)
=
62
f
CLKIN
·(
C
IN
+
C
EX T
) ·
ln
100
mV
·
C
IN
(
C
IN
+
C
EX T
)
V
E
T he practical limit to the maximum value of source resistance is
thermal (Johnson) noise. A practical resistor may be modeled as
an ideal (noiseless) resistor in series with a noise voltage source
or in parallel with a noise current source.
V
n
=
4
kTRf Volts
i
n
=
4
kTf
/
R Amperes
where k is Boltzmann’s constant (1.38
3
10
–23
J/K ), and T is
temperature in degrees K elvin (
°
C + 273).
Active signal conditioning circuits such as op amps generally do
not suffer from problems of high source impedance. T heir open
loop output resistance is normally only tens of ohms and, in any
case, most modern general purpose op amps have sufficiently
fast closed loop settling time for this not to be a problem. Offset
voltage in op amps can be eliminated in a system calibration
routine.
Antialias Considerations
T he digital filter of the AD7703 does not provide any rejection
at integer multiples of the sampling frequency (nf
CLK IN
/256,
where
n
= 1, 2, 3 . . . ).
With a 4.096 MHz master clock there are narrow (
±
10 Hz)
bands at 16 kHz, 32 kHz, 48 kHz, etc., where noise passes
unattenuated to the output.
However, due to the AD7703’s high oversampling ratio of 800
(16 kHz to 20 Hz) these bands occupy only a small fraction of
the spectrum, and most broadband noise is filtered.
T he reduction in broadband noise is given by:
e
out
=
e
in
2
f
C
/
f
S
=
0.035
e
in
where
e
in
and
e
out
are rms noise terms referred to the input and
f
C
is the filter –3 dB corner frequency (f
CLK IN
/409600) and
f
S
is the
sampling frequency (f
CLK IN
/256).
Since the ratio of f
S
to f
CLK IN
is fixed, the digital filter reduces
broadband white noise by 96.5% independent of the master
clock frequency.
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