CS5460A datasheet(36/54 Pages) CIRRUS

36 / 54 page

CS5460A

DS487F4

b) The common-mode rejection performance of the

CS5460A is sufficient within the frequency range

over which the CS5460A performs A/D conver-

sions. Addition of such common-mode caps can

actually often degrade the common-mode rejec-

tion performance of the entire voltage/current input

networks. Therefore, choosing relatively small val-

ues for (CV+, CV-, CI+, CI-) will provide necessary

common-mode rejection at the much higher fre-

quencies, and will allow the CS5460A to realize its

CMRR performance in the frequency-range of in-

terest.

[Note that this discussion does not include correc-

tion of phase-shifts caused by the voltage-sense

transformer and current-sense transformer, al-

though these phase-shifts should definitely be con-

sidered in a real-life practical meter design.] On the

current channel, using commonly available values

for the components, RI+ and RI- can be set to

470

Ω. Then a value of CIdiff = 18 nF and a value of

0.22 nF for CI- and CI- will yield a -3 dB cutoff fre-

quency of 15341 Hz for the current channel. For

the voltage channel, if RI+ and RI- are also set to

470

Ω, CVdiff = 18 nF, and CV- =CV- =0.22nF

(same as current channel), the -3 dB cutoff fre-

quency of the voltage channel’s input filter will be

14870 Hz. The difference in the two cutoff frequen-

cies is due to the difference in the input impedance

between the voltage/current channels.

If there is concern about the effect that the differ-

ence in these two cutoff frequencies (and therefore

the mis-match between the time-constants of the

overall voltage/current input networks) would have

on the accuracy of the power/energy registration, a

non-standard resistor value for RV+ =RV- of (for ex-

ample) 455

Ω can be used. This would shift the

(differential) -3 dB cutoff frequency of the voltage

channel’s input filter (at the voltage channel inputs)

to ~15370 Hz, which would cause the first-order

time-constants of the voltage/current channel input

filters to be closer in value.

Agreement between the voltage/current channel

time-constants can also be obtained by adjusting

the phase compensation bits, instead of using less

commonly-available

resistor/capacitor

values

(such as RI+ =RI- = 455

Ω). If the values of RI+ and

RI- are again 470

Ω, the first-order time-constants

of the two R-C filters are estimated by taking the re-

ciprocal of the -3 dB cutoff frequencies (when ex-

pressed

rads/s).

Subtracting

these

two

time-constants shows that after the voltage/current

signals pass through their respective anti-aliasing

filters, the sensed voltage signal will be delayed

~0.329 µs more than the current signal. If metering

a 60 Hz power system, this implies that the input

voltage-sense

signal

will

delayed

~0.007 degrees more than the delay imposed on

the input current-sense signal. Note that when the

PC[6:0] bits are set to their default setting of

“0000000”, the internal filtering stages of the

CS5460A will impose an additional delay on the

fundamental frequency component of the 60Hz

voltage signal of 0.0215 degrees, with respect to

the current signal. The total difference between the

delay on the voltage-sense fundamental and the

current-sense

fundamental

will

therefore

~0.286 degrees. But if the phase compensation

bits are set to 1111111, the CS5460A will delay the

voltage channel signal by an additional -0.04 de-

grees, which is equivalent to shifting the voltage

signal forward by 0.04 degrees. The total phase

shift on the voltage-sense signal (with respect to

the

fundamental

frequency)

would

then

~0.011 degrees ahead of the current-sense signal,

which

would

therefore

provide

close-

ly-matched

delay

values

between

the

volt-

age-sense and current-sense signals. Adjustment

of the PC[6:0] bits therefore can provide an effec-

tive way to more closely match the delays of the

voltage/current sensor signals, allowing for more

commonly available R and C component values to

be used in both of these filters.

As a final note, tolerances of the R and C compo-

nents that are used to build the two R-C filters

should also be taken into consideration. A com-

mon tolerance of ±0.1% can vary the delay by as

much as much as ~±2.07 µs, which means that the

difference between the delays of the voltage-sense

and current-sense signals that is caused by these

filters could vary by as much as ~±4.1 µs, which is

equivalent to a phase shift of ~±0.089 degrees (at

60 Hz). This in turn implies that our decision to ad-

just the PC[6:0] bits (to shift the voltage signal for-

ward by 0.04 degrees) could actually cause the

voltage signal to be shifted by as much as

~0.100 degrees ahead of the current signal.

Thus, adjustment of the PC[6:0] bits to more close-

ly match the two time-constants/delays may only

be useful if a precise calibration operation can be

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