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CS5461A Datasheet PDF : 44 Pages
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CS5461A
VOLTAGE x10
Digital Filter
2nd Order
∆Σ
DELAY
REG
SINC 3
X
IIR
Modulator
VDCoff* Vgn*
+
HPF
Option
+Σ
X
V*
ΣN
X
÷N
VACoff*
+
√ +Σ
V RMS*
APF
6
Option
Poff * PulseRateE1,2 * X
Energy-to-pulse
E3
PC6 PC5 PC4 PC3 PC2 PC1 PC0
Configuration Register *
SYSGain *
+
X +Σ
ΣN
÷N
PActive*
X
S*
APF
Option
P* PulseRateE3 * X
Energy-to-pulse
E1
E2
CURRENT PGA
4th Order
∆Σ
Modulator
SINC 3
DELAY
REG
X
IIR
Digital Filter
*DENOTES REGISTER NAME.
HPF
Option
+Σ
X
+
IDCoff* I gn*
ΣN
X
÷N
I*
+
√
Σ
+
IACoff*
I RMS *
4. THEORY OF OPERATION
Figure 2. Data Flow.
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 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
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 VRMS and IRMS 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, Vn and In respectively (where N is the cycle count),
using the formula:
IRMS =
N–1
∑ In
n=0
-------------------
N
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 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.
DS661F2
13
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