CS5460 datasheet(30/34 Pages) CIRRUS

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CS5460

DS279PP5

pedance than the voltage channel. These imped-

ance specs are given at the beginning of this data

sheet (see the specification titled “Effective Input

Impedance” for the voltage and current channels).

For example, the differential input impedance in

the current channel is spec’d to be 30 kOhm. As

the user increases the value of RPI to provide for

more and more common-mode surge protection,

the voltage drop across the external protection re-

sistor increases, and it divides the input signal

down more and more. This in turn reduces the dy-

namic range of the signals that are ultimately pre-

sented to the CS5460’s inputs. As an example,

suppose that the user creates a current-sensor con-

figuration that provides a differential voltage of

150mV (RMS) across shunt resistor RS at maxi-

mum line-current level. However, the user has set

RPI to 500 Ohms. This means that when there is

150mV across the shunt resistor (RS), the voltage

across the IIN+ and IIN- inputs is actually 150mV

* [ 30K / (500 + 30K)] = ~148mV (RMS). We see

that this has decreased the maximum signal input

level.

To avoid this voltage division, the user

should first consider the input protection that is go-

ing to be necessary, and then calculate the sensor

gains such that the drop across the protection resis-

tors is taken into account.

Typical values for these components are RPI = 500

Ohm, CPI = 0.02uF, CPV = 0.002uF and if neces-

sary, RPV = 5 KOhm.

5.7 PCB Layout

The CS5460 should be placed entirely over an ana-

log ground plane with both the VA- and DGND

pins of the device connected to the analog plane.

Place the analog-digital plane split immediately ad-

jacent to the digital portion of the chip.

Note:

See the CDB5460 data sheet for suggested

layout details and Applications Note 18 for

more detailed layout guidelines. Before

layout, please call for our Free Schematic

Review Service.

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