CS5463 datasheet(14/46 Pages) CIRRUS

14 / 46 page

CS5463

DS678F2

4. THEORY OF OPERATION

The CS5463 is a dual-channel analog-to-digital convert-

er (ADC) followed by a computation engine that per-

forms

power

calculations

and

energy-to-pulse

conversion. The data flow for the voltage and current

channel measurement and the power calculation algo-

rithms are depicted in Figure 3 and 4, respectively.

The analog inputs are structured with two dedicated

channels, Voltage and Current, then optimized to simpli-

fy interfacing to various sensing elements.

The voltage-sensing element introduces a voltage

waveform on the voltage channel input VIN± and is sub-

ject to a gain of 10x. A second-order delta-sigma modu-

lator samples the amplified signal for digitization.

Simultaneously, the current-sensing element introduces

a voltage waveform on the current channel input IIN±

and is subject to two selectable gains of the program-

mable gain amplifier (PGA). The amplified signal is

sampled by a fourth-order delta-sigma modulator for

digitization. Both converters sample at a rate of

MCLK/8, the over-sampling provides a wide dynamic

range and simplified anti-alias filter design.

4.1 Digital Filters

The decimating digital filters on both channels are Sinc3

filters followed by 4th-order IIR filters. The single-bit

data is passed to the low-pass decimation filter and out-

put at a fixed word rate. The output word is passed to an

optional IIR filter to compensate for the magnitude roll

off of the low-pass filtering operation.

An optional digital high-pass filter (HPF in Figure 3) re-

moves any DC component from the selected signal

path. By removing the DC component from the voltage

and/or the current channel, any DC content will also be

removed from the calculated active power as well. With

both HPFs enabled the DC component will be removed

from the calculated VRMS and IRMS as well as the appar-

ent power.

When the optional HPF in either channel is disabled, an

all-pass filter (APF) is implemented. The APF has an

amplitude response that is flat within the channel band-

width and is used for matching phase in systems where

only one HPF is engaged.

4.2 Voltage and Current Measurements

The digital filter output word is then subject to a DC off-

set adjustment and a gain calibration (See Section 7.

System Calibration

on page 37). The calibrated mea-

surement is available by reading the instantaneous volt-

age and current registers.

The Root Mean Square (RMS in Figure 4) calculations

are performed on N instantaneous voltage and current

samples, Vn and In, respectively (where N is the cycle

count), using the formula:

and likewise for VRMS, using Vn. IRMS and VRMS are ac-

cessible by register reads, which are updated once ev-

ery cycle count (referred to as a computational cycle).

4.3 Power Measurements

The instantaneous voltage and current samples are

multiplied to obtain the instantaneous power (see Fig-

ure 3). The product is then averaged over N conver-

sions to compute active power and is used to drive

energy pulse output E1. Energy output E2 is selectable,

providing an energy sign or a pulse output that is pro-

portional to the apparent power. Energy output E3

VOLTAGE

SINC3

CURRENT

SINC3

DELAY

REG

DELAY

REG

DCoff*

PGA

Configuration Register *

Digital Filter

HPF

2nd Order

∆Σ

Modulator

4th Order

∆Σ

Modulator

x10

SYS

Gain *

PC6 PC5 PC4 PC3 PC2 PC1 PC0

*DENOTES REGISTER NAME.

DELAY

REG

DELAY

REG

HPF

XVDEL XIDEL

2322

...

Operational Modes Register *

∫

V *

I *

VHPF

IHPF

ε*

APF

HPF

APF

IIR

Figure 3. Data Measurement Flow Diagram.

RMS

–

∑

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