EVAL-AD7623CBZ Analog Devices Inc, EVAL-AD7623CBZ Datasheet - Page 18

• Manufacturer Part Number: EVAL-AD7623CBZ
• Description: BOARD EVALUATION FOR AD7623
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7623CBZ

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 1.33M
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 50mW @ 1.33MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7623
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7623
•
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 V
±V
and C = 1 nF provides a corner frequency of 16 MHz.
If the application can tolerate more noise, the AD8139 differen-
tial driver can be used.
REF
one-pole, low-pass filter made by R
external filter, if one is used. The SNR degradation due to
the amplifier is
where:
f
cutoff frequency of the input filter (16 MHz), if one is used.
N is the noise factor of the amplifier (+1 in buffer
configuration).
e
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 using it.
The driver needs to have a THD performance suitable to
that of the AD7623. Figure 13 gives the THD vs. frequency
that the driver should exceed.
–3dB
N
with midscale at V
is the equivalent input voltage noise density of the op
is the input bandwidth of the AD7623 (50 MHz) or the
SNR
LOSS
=
20
REF
log
/2. The one-pole filter using R = 10 Ω
⎛
⎜
⎜ ⎜
⎝
2809
REF
+
, produces a differential
π
53
f
−
IN
3
dB
and C
(
Ne
N
IN
)
2
or by the
⎞
⎟
⎟ ⎟
⎠
Rev. 0 | Page 18 of 28
(UNIPOLAR 0V TO 2.048V)
VOLTAGE REFERENCE INPUT
The AD7623 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 AD7623 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 AD7623 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.
ANALOG INPUT
1kΩ
1kΩ
Figure 27. Single-Ended-to-Differential Driver Circuit
100nF
(Internal Reference Buffer Used)
U1
U2
590Ω
590Ω
10pF
10pF
AD8021
AD8021
10Ω
10Ω
1nF
1nF
IN+
IN–
AD7623
10μF
REF