Datasheet

ADAS3022 Data Sheet
Rev. B | Page 28 of 40
VOLTAGE REFERENCE OUTPUT/INPUT
The ADAS3022 allows the choice of an internal reference or an
external reference using the on-chip buffer/amplifier, or an
external reference.
The internal reference of the ADAS3022 provides excellent
performance and can be used in almost all applications. To set
the reference selection mode, use the internal reference enable bit
(REFEN) and the REFIN pin as described in this section. REF1
and REF2 must be tied together externally.
Internal Reference
The precision internal reference is factory trimmed and is
suitable for most applications.
Setting the REFEN bit in the CFG register to 1 (default) enables
the internal reference and produces 4.096 V on the REF1 and
REF2 pins; this 4.096 V output serves as the main system
reference. The unbuffered 2.5 V (typical) band gap voltage is
output on the REFIN pin, which requires an external parallel
decoupling using 10 µF and 0.1 µF capacitors to reduce the
noise on the output. Because the current output of REFIN is
limited, it can be used as a source if followed by a suitable buffer,
such as the AD8031. Note that excessive loading of the REFIN
output will also lower the 4.096 V system reference because the
internal amplifier uses a fixed gain.
The internal reference output is trimmed to the targeted value
of 4.096 V with an initial accuracy of ±8 mV. The reference is
also temperature compensated to provide a typical drift of
±5 ppm/°C.
When the internal reference is used, the ADAS3022 should be
decoupled as shown in Figure 62. Note that both REF1 and
REF2 connections are required, along with suitable decoupling
on the REFIN output and the RCAP internally regulated supply.
REF2 REF1
RCA
P
REFN
RGND
BAND
GAP
ADAS3022
0.1
µ F
REFIN
REFN
10
µ F
10
µ F
0.1
µ F
10
µ F
0.1
µ
F
1
µ
F
REFN
10516-016
Figure 62. 4.096 V Internal Reference Connection
External Reference and Internal Buffer
The external reference and internal buffer are useful when a com-
mon system reference is used or if improved drift performance
is required.
Setting 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. With REFIN = 2.5 V, REF1 and REF2
output 4.096 V, which serves as the main system reference.
For this configuration, connect the external source as shown
in Figure 63. Any type of 2.5 V reference, including those with
low power, low drift, and a small package, can be used in this
configuration because the internal buffer handles the dynamics
of the ADAS3022 reference.
REF2 REF1
RCAP
REFN
RGND
BAND
GAP
ADAS3022
0.1
µ F
REFIN
REFN
10 µ F
10
µ
F
0.1
µ F
10
µ F
0.1
µ F
1
µ
F
REFN
REFERENCE
SOURCE = 2.5V
10516-017
Figure 63. External Reference Using Internal Buffer
External Reference
For applications that require a precise, low drift 4.096 V reference,
an external reference can also be used.
This option requires disabling the internal buffer by setting
REFEN to 0 and driving or connecting REFIN to AGND; therefore,
both hardware and software control are necessary. Attempting
to drive the REF1 and REF2 pins prior to disabling the internal
buffer can cause source/sink contention in the driving amplifiers.
Connect the precision 4.096 V reference, which serves as the
main system reference, through a low impedance buffer (such as
the AD8031 or the AD8605) to REF1 and REF2 as shown in
Figure 64. Recommended references include the ADR434,
ADR444, and ADR4540.
REF2 REF1
RCAP
REFN
RGND
BAND
GAP
ADAS3022
0.1 µ
F
REFIN
REFN
1
0 µ F
10 µ F
0.1 µ
F
1µF
REFERENCE
SOURCE = 4.096V
10516-018
Figure 64. External Reference
If an op amp is used as the external reference source, take note
of any 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.