CS5460 datasheet(29/34 Pages) CIRRUS

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CS5460

DS279PP5

5.5.2 Calibration Tips

To minimize digital noise near the device, the user

should wait for each calibration step to be complet-

ed before reading or writing to the serial port.

After a calibration is performed, the offset and gain

cro-controller and recorded in memory. The same

calibration words can be uploaded into the offset

and gain registers of the converters when power is

first applied to the system, or when the gain range

on the current channel is changed.

An offset calibration must be performed before a

gain calibration. Each gain calibration depends on

the zero calibration point obtained from the offset

calibration.

The offset and gain calibration steps each take one

conversion cycle to complete. At the end of the cal-

ibration step, DRDY is set to indicate that the cali-

bration is complete.

5.6 Input Current Protection

In Figure 3 and Figure 4, note the series resistor RPI

which is connected to the IIN+ input pin. This re-

sistor is used to provide current-limit protection for

the current-channel input pin in the event of a pow-

er surge or lightening surge.

The voltage/cur-

rent-channel inputs have surge-current limits of

100mA.

This applies to brief voltage/current

spikes (<500 msec). The limit is 10mA for DC in-

put overload situations.

The VIN+ pin does not need a protection resistor

for the configurations shown in Figure 3 and Figure

4. This is because a resistive voltage-divider is

used as the sensor, and so series resistance is al-

ready provided to the VIN+ input pin. (If it was in-

stalled, it would be called RPV). If the negative

sides of the CS5460 input channels are not ground-

ed (i.e., if VIN- and IIN- are connected in a differ-

ential configuration) then it is appropriate to put

protection resistors on these inputs as well.

Capacitors CPV and CPI should be included to pro-

vide for attenuation of high-frequency noise that

may be coupled into the input lines. In differential

input configurations, such a capacitor should be

added to the VIN- and IIN- pins in addition to the

VIN+ and IIN+ pins.

Values for RPV/I and CPV/I must be chosen with the

approximate input lowpass cutoff frequency in

mind. In general, the cutoff frequency should not

be less than 10 times the roll-off frequencies of the

internal voltage/current channel filters (see Figure

6 and Figure 7). From these figures we see that the

internal voltage channel roll-off is at ~1400Hz

while the current channel roll-off is at ~1600Hz. If

the cutoff frequency of the external protection is

much less than 10x these values (14000Hz and

16000Hz), then some of the harmonic content in

the power signal may start to get attenuated by this

input filtering, which is undesirable.

The exact values of RPV/I and CPV/I must be calcu-

lated for each particular application. The primary

goal is to make sure that the input pins never re-

ceive transient input currents greater than 100mA.

Also, they should never be exposed to DC currents

greater than 10mA. The user-supplied protection

resistors RPV and RPI will limit the current that

comes into the pins in over-voltage--where the in-

ternal protection diodes turn on inside the CS5460.

For example, suppose that the value for RPI (on the

current channel input) was chosen to be 500 Ohms.

Then we know that the current channel can with-

stand brief voltage spikes of up to ~50V (refer-

enced to GND) without damage to the part. This is

because 50V / 500Ohm = 100mA. We can also say

that the pin can withstand a common mode DC

voltage of up to 5V.

When computing appropriate values for RPV/I, the

differential input impedance of the CS5460’s volt-

age channel and current channel should also be

considered. This is especially true for the current

channel, which has a lower differential input im-

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