CS5374 datasheet(33/44 Pages) CIRRUS

33 / 44 page

CS5374

DS862F1

regulator output, with additional power supply

bulk capacitance placed among the analog compo-

nent route if space permits.

8.4 PCB Layers and Routing

The CS5374 is a high-performance device, and

special care must be taken to ensure power and

ground routing is correct. Power can be supplied ei-

ther through dedicated power planes or routed trac-

es. When routing power traces, it is recommended

to use a “star” routing scheme with the star point ei-

ther at the voltage regulator output or at a local

power supply bulk capacitor.

It is also recommended to dedicate a full PCB layer

to a solid ground plane, without splits or routing.

All bypass capacitors should connect between the

power supply circuit and the solid ground plane as

near as possible to the device power supply pins.

The CS5374 analog signals are differentially rout-

ed and do not normally require connection to a sep-

arate analog ground. However, if a separate analog

ground is required, it should be routed using a

“star” routing scheme on a separate layer from the

solid ground plane and connected to the ground

plane only at a single point. Be sure all active de-

vices and passive components connected to the

separate analog ground are included in the “star”

route to ensure sensitive analog currents do not re-

turn through the ground plane.

8.5 Power Supply Rejection

Power supply rejection of the CS5374 is frequency

dependent. The CS5376A digital filter fully rejects

power supply noise for frequencies above the se-

lected digital filter corner frequency. Power supply

noise frequencies between DC and the digital filter

corner frequency are rejected as specified in the

“Power Supply Characteristics” on page 13.

8.6 SCR Latch-up Considerations

It is recommended to connect the VA– power sup-

ply to system ground (GND) through a reverse-bi-

ased Schottky diode. At power up, if the VA+

power supply ramps up before the VA– supply is

established, the VA- pin voltage could be pulled

above ground potential through the CS5374 de-

vice. If the VA– supply is pulled 0.7 V or more

above GND, SCR latch-up can occur. A reverse-bi-

ased Schottky diode will clamp the VA– voltage a

maximum of 0.3 V above ground to ensure SCR

latch-up does not occur at power up.

For similar reasons, care should be taken to connect

the CS5374 thermal pad on the bottom of the pack-

age to VA–, not system ground (GND), since it in-

ternally connects to VA– and is expected to be the

most negative applied voltage.

8.7 DC-DC Converters

Many low-frequency measurement systems are

battery powered and utilize DC-DC converters to

efficiently generate power supply voltages. To

minimize interference effects, operate the DC-DC

converter at a frequency which is rejected by the

digital filter, or operate it synchronous to the

MCLK rate.

A synchronous DC-DC converter whose operating

frequency is derived from MCLK will theoretically

minimize the potential for “beat frequencies” to ap-

pear in the measurement bandwidth. However this

requires the source clock to remain jitter free with-

in the DC-DC converter circuitry. If clock jitter can

occur within the DC-DC converter (as in a PLL-

based architecture), it’s better to use a non-syn-

chronous DC-DC converter whose switching fre-

quency is rejected by the digital filter.

During PCB layout, do not place high-current DC-

DC converters near sensitive analog components.

Carefully routing a separate DC-DC “star” ground

will help isolate noisy switching currents away

from the sensitive analog components.

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