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EVAL-CONTROLBRD33 Datasheet(Fiches technique) 18 Page - Analog Devices

Numéro de pièce EVAL-CONTROLBRD33
Description  16-Bit, 2 LSB INL, 3 MSPS PulSAR® ADC
Télécharger  32 Pages
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
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AD7621
Rev. 0 | Page 18 of 32
coming from the driver is filtered by the AD7621 analog
input circuit one-pole, low-pass filter made by RIN and CIN
or by the external filter, if one is used. The SNR
degradation due to the amplifier is
() ⎟⎟
⎜⎜
+
=
2
3
2809
53
log
20
N
dB
LOSS
Ne
f
SNR
π
where:
f–3dB is the input bandwidth of the AD7621 (50 MHz) or
the cutoff frequency of the input filter (16 MHz), if
one is used.
N
is the noise factor of the amplifier (+1 in buffer
configuration).
eN
is the equivalent input voltage noise density of the op
amp, in nV/√Hz.
For instance, a driver with an equivalent input noise
density of 2.1 nV/√Hz, like the AD8021 with a noise gain
of +1 when configured as a buffer, degrades the SNR by
only 0.33 dB when using the RC filter in Figure 23, and by
1 dB without.
The driver needs to have a THD performance suitable to
that of the AD7621. Figure 13 gives the THD vs. frequency
that the driver should exceed.
The AD8021 meets these requirements and is appropriate for
almost all applications. The AD8021 needs a 10 pF external
compensation capacitor that should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
+1 gain arrangement is recommended and helps to obtain the
best signal-to-noise ratio.
The AD8022 can also be used when a dual version is needed
and a gain of 1 is present. The AD829 is an alternative in
applications where high frequency (above 100 kHz) performance
is not required. In applications with a gain of 1, an 82 pF
compensation capacitor is required. The AD8610 is an option
when low bias current is needed in low frequency applications.
Single-to-Differential Driver
For applications using unipolar analog signals, a single-ended-
to-differential driver, as shown in Figure 27, allows for a
differential input into the part. This configuration, when
provided an input signal of 0 to VREF, will produce a differential
±VREF with midscale at VREF/2. The one-pole filter using R = 10 Ω
and C = 1 nF provides a corner frequency of 16 MHz.
If the application can tolerate more noise, the AD8139
differential driver can be used.
AD8021
ANALOG INPUT
(UNIPOLAR 0V TO 2.048V)
AD8021
IN+
IN–
AD7621
REF
10
μF
10
Ω
10
Ω
100nF
1nF
1nF
U2
U1
10pF
10pF
1k
Ω
1k
Ω
590
Ω
590
Ω
Figure 27. Single-Ended-to-Differential Driver Circuit
(Internal Reference Buffer Used)
VOLTAGE REFERENCE INPUT
The AD7621 allows the choice of either a very low temperature
drift internal voltage reference or an external reference.
Unlike many ADCs with internal references, the internal
reference of the AD7621 provides excellent performance and
can be used in almost all applications.
Internal Reference
(PDBUF = Low, PDREF = Low)
To use the internal reference, the PDREF and PDBUF inputs
must be low. This produces a 1.2 V band gap output on
REFBUFIN which, amplified by the internal buffer, results in a
2.048 V reference on the REF pin.
The internal reference is temperature-compensated to 2.048 V ±
10 mV. The reference is trimmed to provide a typical drift of
7 ppm/°C. This typical drift characteristic is shown in Figure 7.
The output resistance of the REFBUFIN is 6.33 kΩ (minimum)
when the internal reference is enabled. It is necessary to
decouple this with a ceramic capacitor greater than 100 nF.
Thus, the capacitor provides an RC filter for noise reduction.
Since the output impedance of REFBUFIN is typically 6.33 kΩ,
relative humidity (among other industrial contaminates) can
directly affect the drift characteristics of the reference. Typically,
a guard ring is used to reduce the effects of drift under such
circumstances. However, since the AD7621 has a fine lead pitch,
guarding this node is not practical. Therefore, in these
industrial and other types of applications, it is recommended to
use a conformal coating such as Dow Corning 1-2577 or
Humiseal 1B73.
External 1.2 V Reference and Internal Buffer
(PDREF = High, PBBUF = Low)
To use an external reference with the internal buffer, PDREF
should be high and PDBUF should be low. This powers down
the internal reference and allows the 1.2 V reference to be
applied to REFBUFIN.




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