CS5466 datasheet(10/16 Pages) CIRRUS

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CS5466

THEORY OF OPERATION

The CS5466 is a dual channel Analog-to-Digital Con-

verter (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 the four 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 output 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 cur-

rent channel, any dc content will also be removed from

the calculated average real power as well.

4.2

Average Real Power Computation

The instantaneous voltage and current data samples

are multiplied together to obtain the instantaneous pow-

er. The product is then averaged over 400 conversions

to compute the average real 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 stepper motor or a electromechanical counter or

interface to a microcontroller or infrared LED. The alter-

nating output pulses of E1 and E2 allows for use with

low cost electromechanical counters.

Output FOUT provides a uniform pulse stream that is

proportional to the average real power and is designed

for system calibration. The FREQ2:0 inputs set the out-

put pulse rate of E1, E2 and FOUT. See Section 5.3 for

more details.

2nd Order

∆Σ

Modulator

4th Order

∆Σ

Modulator

VIN±

IIN±

Energy-to-

Pulse Rate

Converter

FOUT

Sinc3

PGA

10x

HPF

NEG

IGAIN[1:0]

HPF

FREQ[2:0]

IIR

Sinc3

N = 400

Current Channel

Voltage Channel

Digital Filter

HPF

Figure 2. Data Flow

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