MCP3906AT-ISLASHSS datasheet(18/32 Pages) MICROCHIP

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MCP3905A/05L/06A

DS22011B-page 18

The multiplier output gives the product of the two high-

pass filtered channels, corresponding to instantaneous

real power. Multiplying two sine wave signals by the

same

ω frequency gives a DC component and a 2ω

component. The instantaneous power signal contains

the real power of its DC component, while also contain-

ing 2

ω components coming from the line frequency

multiplication. These 2

ω components come for the line

frequency (and its harmonics) and must be removed in

order to extract the real-power information. This is

accomplished using the low-pass filter and DTF

converter.

4.6

Low-Pass Filter and DTF

Converter

The MCP3905A/05L/06A low-pass filter is a first-order

IIR filter that extracts the active real-power information

(DC component) from the instantaneous power signal.

The magnitude response of this filter is detailed in

Figure 4-5. Due to the fact that the instantaneous power

signal has harmonic content (coming from the 2

components of the inputs), and since the filter is not

ideal, there will be some ripple at the output of the low-

pass filter at the harmonics of the line frequency.

The cut-off frequency of the filter (8.9 Hz) has been

chosen to have sufficient rejection for commonly-used

line frequencies (50 Hz and 60 Hz). With a standard

input clock (MCLK = 3.58 MHz) and a 50 Hz line

frequency, the rejection of the 2

ω component (100 Hz)

will be more than 20 dB. This equates to a 2

component containing 10 times less power than the

main DC component (i.e., the average active real

power).

FIGURE 4-5:

LPF Magnitude Response

(MCLK = 3.58 MHz).

The output of the low-pass filter is accumulated in the

digital-to-frequency converter. This accumulation is

compared to a different digital threshold for FOUT0/1

and HFOUT, representing a quantity of real energy mea-

sured by the part. Every time the digital threshold on

FOUT0/1 or HFOUT is crossed, the part will output a

pulse (See Section 4.7 “FOUT0/1 and HFOUT Output

Frequencies”).

The equivalent quantity of real energy required to

output a pulse is much larger for the FOUT0/1 outputs

than the HFOUT. This is such that the integration period

for the FOUT0/1 outputs is much larger. This larger

integration period acts as another low-pass filter so that

the output ripple due to the 2

ω components is minimal.

However, these components are not totally removed,

since realized low-pass filters are never ideal. This will

create a small jitter in the output frequency. Averaging

the output pulses with a counter or a MCU in the

application will then remove the small sinusoidal

content of the output frequency and filter out the

remaining 2

ω ripple.

HFOUT is intended to be used for calibration purposes

due to its instantaneous power content. The shorter

integration period of HFOUT demands that the 2ω

component be given more attention. Since a sinusoidal

signal average is zero, averaging the HFOUT signal in

steady-state conditions will give the proper real energy

value.

4.7

FOUT0/1 and HFOUT Output

Frequencies

The thresholds for the accumulated energy are

different for FOUT0/1 and HFOUT (i.e., they have

different transfer functions). The FOUT0/1 allowed

output frequencies are quite low in order to allow

superior integration time (see Section 4.6 “Low-Pass

Filter and DTF Converter”). The FOUT0/1 output

frequency can be calculated with the following

equation:

EQUATION 4-1:

FOUT FREQUENCY

OUTPUT EQUATION

For a given DC input V, the DC and RMS values are

equivalent. For a given AC input signal with peak-to-

peak amplitude of V, the equivalent RMS value is

V/sqrt(2), assuming purely sinusoidal signals. Note

that since the real power is the product of two RMS

inputs, the output frequencies of AC signals are half of

the DC inputs ones, again assuming purely sinusoidal

AC signals. The constant FC depends on the FOUT0

and FOUT1 digital settings. Table 4-3 shows FOUT0/1

output frequencies for the different logic settings.

-40

-35

-30

-25

-20

-15

-10

-5

0.1

100

1000

Frequency (Hz)

OUT Hz

()

8.06

REF

()2

-----------------------------------------------------------

Where:

V0 is the RMS differential voltage on Channel 0

V1 is the RMS differential voltage on Channel 1

G is the PGA gain on Channel 0 (current channel)

FC is the frequency constant selected

VREF is the voltage reference

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