Cirrus Logic CS5461A - Manual (Page 13)

CS5461A
DS661F3 13
4. THEORY OF OPERATION
The CS5461A is a dual-channel analog-to-digital con-
verter (ADC) followed by a computation engine that per-
forms power calculations and energy-to-pulse
conversion. The flow diagram for the two data paths is
depicted in
Figure 2 . The analog inputs are structured
with two dedicated channels, voltage and current, then
optimized to simplify interfacing to 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 mod-
ulator 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 the two selectable gains of the pro-
grammable 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 Sinc
3
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
the IIR filter to compensate for the magnitude roll-off of
the low-pass filtering operation.
An optional digital High-pass Filter (HPF in Figure 2 ) 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 V
RMS
and I
RMS
as well as the appar-
ent power.
When the HPF option is used in only one channel, the
APF (all pass filter) option can be applied to the other
channel to preserve the phase match between the two
channels.
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 35). The calibrated mea-
surement is available to the user by reading the instan-
taneous voltage and current registers.
The Root Mean Square (RMS) calculations are per-
formed on N instantaneous voltage and current sam-
ples, V
n and I n respectively (where N is the cycle count),
using the formula:
and likewise for V
RMS
, using V n . I
RMS
and V
RMS
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 2 ). The product is then averaged over N conver-
sions to compute active power and used to drive energy
pulse outputs E1, E2 and E3. Output E3 provides a uni-
form pulse stream that is proportional to the active pow-
er and is designed for system calibration.
VOLTAGE SINC 3
+
X
V *
gn
X
V *
CURRENT SINC 3
+
X
I * gn
DELAY
REG
DELAY
REG
HPF
Option
X I * RMS
V * RMS
E1
IIR
I *
I
DCoff
*
V
DCoff
*
PGA
IIR
X
+
+


Energy-to-pulse X E3
+

X
+
Configuration Register *
Digital Filter
Digital Filter
HPF
Option
X S *
2nd Order

Modulator
4th Order

Modulator
x10
+
I
ACoff
*

+
+
V
ACoff
*
 +
E2

N ÷
N 
N ÷
N 
P *
Active

N ÷
N 
Poff *
P*
X
X
SYS Gain * PC6 PC5 PC4 PC3 PC2 PC1 PC0
6
PulseRateE 1,2
*
PulseRateE 3 * Energy-to-pulse
*DENOTES REGISTER NAME.
APF
Option
APF
Option
Figure 2. Data Flow.
I
RMS
I
n
n 0 =
N 1 –

N
--------------------
=