Cirrus-logic CS5460A User Manual download pdf (Page 34)

CS5460A

34 DS487F5

3.13 Input Filtering

Figure 6 shows how the analog inputs can be con-

nected for a single-ended input configuration. Note

here that the Vin- and Iin- input pins are held at a

constant DC common-mode level, and the varia-

tion of the differential input signal occurs only on

the Vin+ and Vin- pins. The common-mode level

on the Vin-/Iin- pins is often set at (or very near) the

CS5460A’s common-mode ground reference po-

tential. (The common-mode ground reference po-

tential is defined by the voltage at the VA- pin.)

But this is not required--the DC reference level of

the Vin-/Iin- pins can be set to any potential be-

tween [VA-] and [(VA+) - 250 mV]. In Figure 6, ob-

serve the circuitry which has been placed in front

of the current channel input pins, as one example.

The anti-aliasing filter can be constructed by calcu-

lating appropriate values for R

I+ =RI-, CIdiff, and

I+ = CI-. The sensor voltage that is created by the

voltage drop across R

SHUNT is fed into the Iin+ pin,

while the voltage at the Iin- pin is held constant.

Figure 7 shows a differential bipolar input configu-

ration. Note in Figure 7 that the “+” and “-” input

pins for the voltage/current channels are equally

referenced above and below the CS5460A’s

ground reference voltage. Such a differential bipo-

lar input configuration can be used because the

CS5460A voltage/current channel inputs are able

to accept input voltage levels as low as -250 mV

(common-mode) below the VA- pin ground refer-

ence, which is defined by the voltage at the VA-

pin. (In fact, if desired, the center-tapped reference

of these differential input pairs could be connected

to a DC voltage of, for example, +2 V, because

+2 V is within the available common-mode range

of [VA-] and [VA+ - 250mV]. But this configuration

may not be practical for most metering applica-

tions.) In the differential bipolar input configuration,

the voltage signals at the Vin- and Iin- pins will fluc-

tuate in similar fashion to the Vin+/Iin+ pins, except

the voltages at the “-” pins will be 180 degrees out

of phase with respect to the voltage signals at the

“+” pins. Therefore the signal paths to the “+” and

“-” pins play an equal role in defining the differential

voltage input signal. Because of this, the protection

resistors placed on Vin-/Iin- pins will play an equal-

ly important role as the resistors on the Vin+/Iin+

pins, in defining the differential responses of the

voltage/current channel input anti-aliasing filters.

These resistors also serve as the current-limit pro-

tection resistors (mentioned earlier).

Before determining a typical set of values for R

V+,

V-, CV+, CV-, CVdiff, RI+, RI-, CI+, CI-, and CIdiff in

Figure 7, several other factors should be consid-

ered:

1. Values for the above resistors/capacitors should

be chosen with the desired differential-mode (and

common-mode) lowpass cutoff frequencies in

mind. In general, the differential cutoff frequencies

should not be less than 10 times the cut-off fre-

quencies of the internal voltage/current channel fil-

ters, which can be estimated by studying Figure 4

and Figure 5. In these figures, the internal voltage

channel cutoff frequency is ~1400 Hz while the

current channel cutoff frequency is ~1600 Hz. If

the cutoff frequency of the external anti-aliasing fil-

ter is much less than 10x these values (14000 Hz

and 16000 Hz), then some of the harmonic content

that may be present in the voltage/current signals

will be attenuated by the voltage/current channel

input anti-aliasing filters, because such R-C filters

will begin to roll off at a frequency of 1/10th of the

filter’s -3 dB cutoff frequency. If the designer is not

interested in metering energy that may be present

in the higher harmonics (with respect to the funda-

mental power line frequency) then the differen-

tial-mode cutoff frequencies on the voltage/current

input networks can be reduced. However, relaxing

the metering bandwidth is usually unacceptable,

as most modern power meters are required to reg-

ister energy out to the 11th harmonic (at a mini-

mum).

2. The first-order time-constants of the overall volt-

age and current channel sensor networks should

be set such that they are equal (within reason), or

at least close in magnitude. If the time-constants of

the voltage/current sensor networks are not

well-matched, then the phase relationship be-

tween the voltage-sense and current-sense sig-

nals will suffer an undesirable shift. In this

situation, the real (true) power/energy measure-

ments reported by the CS5460A can contain signif-

icant error, because the power factor of the sensed

voltage and current signals will be significantly dif-

ferent than the actual power factor of the power

line voltage/current waveforms.

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