MCP3906 datasheet(15/26 Pages) MICROCHIP

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DS21948C-page 15

MCP3905/06

4.7

FOUT0/1 and HFOUT Output

Frequencies

The thresholds for the accumulated energy are differ-

ent 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 active (real) power is the product of two RMS

inputs, the output frequencies of an AC signal is half

that of the DC equivalent signal, 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 set-

tings.

OUT Hz

()

8.06

REF

()

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

Where:

V0 = the RMS differential voltage on Channel 0

V1 = the RMS differential voltage on Channel 1

G= the PGA gain on Channel 0

(current channel)

FC = the frequency constant selected

VREF = the voltage reference

TABLE 4-3:

OUTPUT FREQUENCY CONSTANT FC FOR FOUT0/1 (VREF =2.4V)

FC (Hz)

(MCLK = 3.58 MHz)

FOUT Frequency (Hz)

with Full-Scale

DC Inputs

FOUT Frequency (Hz)

with Full-Scale

AC Inputs

MCLK/221

1.71

0.74

0.37

MCLK/220

3.41

1.48

0.74

MCLK/219

6.83

2.96

1.48

MCLK/218

13.66

5.93

2.96

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