ADAS3023 datasheet(23/32 Pages) AD

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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

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

ADAS3023

REFN

REF2

10µF

0.1µF

REFN

REF1

10µF

0.1µF

REFIN

BAND

GAP

RCAP

1µF

RGND

REFERENCE

SOURCE = 4.096V

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