Datasheet

MCP3909
DS22025C-page 22  2006-2012 Microchip Technology Inc.
4.8 Active Power F
OUT0/1
and HF
OUT
Output Frequencies
The thresholds for the accumulated energy are differ-
ent for F
OUT0/1
and HF
OUT
(i.e., they have different
transfer functions). The F
OUT0/1
allowed output fre-
quencies are quite low in order to allow superior inte-
gration time (see Section 4.7 “Active Power Low-
Pass Filter and DTF Converter”). The F
OUT0/1
output
frequency can be calculated with the following
equation:
EQUATION 4-1: F
OUT
FREQUENCY
OUTPUT EQUATION
For a given DC input V, the DC and RMS values are
equivalent. For a given AC input signal with 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 fre-
quencies of AC signals are half of the DC inputs ones,
again assuming purely sinusoidal AC signals. The
constant F
C
depends on the F
OUT0
and F
OUT1
digital
settings. Table 4-2 shows F
OUT0/1
output frequencies
for the different logic settings.
The high-frequency output HF
OUT
has lower integra-
tion times and, thus, higher frequencies. The output fre-
quency value can be calculated with the following
equation:
F
OUT
Hz
8.06 V
0
 V
1
 GF
C

V
REF

2
-----------------------------------------------------------=
Where:
V
0
= the RMS differential voltage on
Channel 0
V
1
= the RMS differential voltage on
Channel 1
G = the PGA gain on Channel 0 (current
channel)
F
C
= the frequency constant selected
V
REF
= the voltage reference
TABLE 4-2: ACTIVE POWER OUTPUT FREQUENCY CONSTANT F
C
FOR FOUT0/1 (V
REF
=2.4V)
F1 F0 F
C
(Hz)
F
C
(Hz)
(MCLK = 3.58 MHz)
F
OUT
Frequency (Hz)
with Full-Scale
DC Inputs
F
OUT
Frequency (Hz)
with Full-Scale
AC Inputs
00MCLK/2
21
1.71 0.74 0.37
01MCLK/2
20
3.41 1.48 0.74
10MCLK/2
19
6.83 2.96 1.48
11MCLK/2
18
13.66 5.93 2.96