Cirrus Logic CS5466 - Manual (Page 10)

CS5466
10 DS659F2
4. THEORY OF OPERATION
The CS5466 is a dual-channel analog-to-digital convert-
er (ADC) followed by a computation engine that per-
forms an energy-to-pulse conversion. The flow diagram
for the two data paths is depicted in Figure 2. The ana-
log inputs are structured with two dedicated channels,
voltage and current, then optimized to simplify interfac-
ing to sensing elements.
The voltage-sensing element introduces a voltage
waveform on the voltage channel input VIN
 and is sub-
ject to a fixed 10x gain amplifier. A second-order delta-
sigma modulator samples the amplified signal for digiti-
zation.
Simultaneously, the current sensing element introduces
a voltage waveform on the current channel input IIN

and is subject to four programmable gains. The ampli-
fied 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 fourth-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
or current channel, any DC content will also be removed
from the calculated average active (real) power as well.
4.2 Active Power Computation
The instantaneous voltage and current data samples
are multiplied to obtain the instantaneous power. The
product is then averaged over 400 conversions to com-
pute the active power value used to drive pulse outputs
E1, E2, and FOUT. Output pulse rate of E1 and E2 can
be set to one of four frequencies to directly drive a step-
per motor or a electromechanical counter or interface to
a microcontroller or infrared LED. The alternating output
pulses of E1 and E2 allows for use with low-cost elec-
tromechanical counters.
Output FOUT provides a uniform pulse stream that is
proportional to the active power and is designed for sys-
tem calibration. The FREQ[2:0] inputs set the output
pulse rate of E1, E2, and FOUT. See
”Energy Pulse
Outputs” on page 11. for more details.
2nd Order

Modulator
4th Order

Modulator
x
VIN±
IIN±
Energy-to-
Pulse Rate
Converter
E1
E2
FOUT
Sinc 3
PGA
10x
HPF
NEG
IGAIN[1:0]
FREQ[2:0]
IIR
IIR
Sinc 3
Current Channel
Voltage Channel
Digital Filter
HPF
N=400 
N
HPF
Config
Digital Filter
Figure 2. Data Flow