Datasheet

Data Sheet ADAS3023
suitable decoupling on the REFIN output and the RCAP
internally regulated supply.
Figure 42. 4.096V Internal Reference Connection
External Reference and Internal Buffer
The external reference and internal buffer are useful where a
common system reference is used or when improved drift
performance is required.
Setting Bit REFEN to 0 disables the internal band gap reference,
allowing the user to provide an external voltage reference (2.5 V
typical) to the REFIN pin. The internal buffer remains enabled,
thus reducing the need for an external buffer amplifier to generate
the main system reference. Where REFIN = 2.5 V and REF1 and
REF2 output 4.096 V, this serves as the main system reference.
For this configuration, connect the external source, as shown in
Figure 43. Any type of 2.5 V reference can be used in this config-
uration (low power, low drift, small package, and so forth) because
the internal buffer handles the dynamics of the ADAS3023
reference requirements.
Figure 43. External Reference Using Internal Buffer
External Reference
For applications that require a precise, low drift, 4.096 V
reference, an external reference can be used. Note that in this
mode, disabling the internal buffer requires setting REFEN to 0,
and driving or connecting REFIN to AGND; thus, both hardware
and software control are necessary. Attempting to drive the REF1
and REF2 pins alone prior to disabling the internal buffer can
cause source/sink contention in the outputs of the driving
amplifiers.
Connect the precision 4.096 V reference directly to REF1 and
REF2, which are the main system reference (see Figure 44); two
recommended references are the ADR434 or ADR444.
If an op amp is used as an external reference source, take note of
the concerns regarding driving capacitive loads. Capacitive
loading for op amps usually refers to the ability of the amplifier
to remain marginally stable in ac applications but can also play
a role in dc applications, such as a reference source.
Keep in mind that the reference source sees the dynamics of the
bit decision process on the reference pins and further analysis
beyond the scope of this data sheet may be required.
Figure 44. External Reference
Reference Decoupling
With any of the reference topologies described in the Voltage
Reference Input/Output section, the REF1 and REF2 reference
pins of the ADAS3023 have dynamic impedances and require
sufficient decoupling, regardless of whether the pins are used as
inputs or outputs. This decoupling usually consists of a low ESR
capacitor connected to each REF1 and REF2 pin and to the accom-
panying REFN return paths. Ceramic chip capacitors (X5R, 1206
size) are recommended for decoupling in all of the reference
topologies described in the Voltage Reference Input/Output
section.
The placement of the reference decoupling capacitors plays an
important role in system performance. Using thick printed circuit
board (PCB) traces, mount the decoupling capacitors on the same
side as the ADAS3023, close to the REF1 and REF2 pins. Route the
return paths to the REFN inputs that, in turn, connect to the analog
ground plane of the system. When it is necessary to connect to an
internal PCB, minimize the resistance of the return path to ground
by using as many through vias as possible.
Using the shortest distance and several vias, connect the REFN
and RGND inputs to the analog ground plane of the system,
preferably adjacent to the solder pads. One common mistake is
to route these traces to an individual trace that connects to the
ground of the system. This can introduce noise, which may
adversely affect the LSB sensitivity. To prevent such noise, use
PCBs with multiple layers, including ground planes, rather than
using single or double sided boards.
Smaller reference decoupling capacitor values (as low as 2.2 µF)
can be used with little impact, mainly on DNL and THD. Further-
more, there is no need for an additional lower value ceramic
decoupling capacitor (for example, 100 nF) that is common in
decoupling schemes for high frequency noise rejection.
For applications that use multiple ADAS3023 devices or other
PulSAR ADCs, using the internal reference buffer is most effective
ADAS3023
REFN REF2
10µF
0.1µF
REFN REF1
10µF
0.1µF
REFN
REFIN
10µF
0.1µF
BAND
GAP
RCAP
1µF
RGND
10942-013
ADAS3023
REFN REF2
10µF
0.1µF
REFN REF1
10µF
0.1µF
REFN REFIN
10µF
0.1µF
BAND
GAP
RCAP
1µF
RGND
REFERENCE
SOURCE = 2.5V
10942-014
ADAS3023
REFN REF2
10µF
0.1µF
REFN REF1
10µF
0.1µF
REFIN
BAND
GAP
RCAP
1µF
RGND
REFERENCE
SOURCE = 4.096V
10942-015
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