Datasheet

© 2009 Microchip Technology Inc. DS21948E-page 19
MCP3905/06
4.7 F
OUT0/1
and HF
OUT
Output
Frequencies
The thresholds for the accumulated energy are
different for F
OUT0/1
and HF
OUT
(i.e., they have differ-
ent transfer functions). The F
OUT0/1
allowed output
frequencies are quite low in order to allow superior
integration time (see Section 4.6 “Low-Pass Filter
and DTF Converter”). The 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 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 F
C
depends on the
F
OUT0
and F
OUT1
digital settings. Table 4-3 shows
F
OUT0/1
output frequencies for the different logic
settings.
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 4-3: OUTPUT FREQUENCY CONSTANT FC 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