Datasheet

 2006-2012 Microchip Technology Inc. DS22025C-page 17
MCP3909
3.13 Oscillator (OSC1, OSC2)
OSC1 and OSC2 provide the master clock for the
device. A resonant crystal or clock source with a similar
sinusoidal waveform must be placed across these pins
to ensure proper operation. The typical clock frequency
specified is 3.579545 MHz. However, the clock fre-
quency can be within the range of 1 MHz to 4 MHz
without disturbing measurement error. Appropriate
load capacitance should be connected to these pins for
proper operation.
A full-swing, single-ended clock source may be con-
nected to OSC1 with proper resistors in series to
ensure no ringing of the clock source due to fast
transient edges.
3.14 Serial Data Output or Negative
Power Output Logic Pin (NEG)
This dual function pin can act as either the serial data
output for SPI communication or NEG. NEG detects the
phase difference between the two channels and will go
to a logic ‘1’ state when the phase difference is greater
than 90° (i.e., when the measured real power is nega-
tive). The output state is synchronous with the rising
edge of HF
OUT
and maintains the logic ‘1’ until the real
power becomes positive again and HF
OUT
shows a
pulse.
3.15 Ground Connection (D
GND
)
D
GND
is the ground connection to internal digital
circuitry (SINC filters, multiplier, HPF, LPF,
digital-to-frequency converter and oscillator). To
ensure accuracy and noise cancellation, D
GND
must be
connected to the same ground as A
GND
, preferably
with a star connection. If a digital ground plane is
available, it is recommended that this device be tied to
this plane of the Printed Circuit Board (PCB). This
plane should also reference all other digital circuitry in
the system.
3.16 High-Frequency Output (HF
OUT
)
HF
OUT
is the high-frequency output of the device and
supplies the instantaneous real-power information. The
output is a periodic pulse output, with its period propor-
tional to the measured real power, and to the HF
C
con-
stant defined by F0, F1 and F2 pin logic states. This
output is the preferred output for calibration due to
faster output frequencies, giving smaller calibration
times. Since this output gives instantaneous real
power, the 2 ripple on the output should be noted.
However, the average period will show minimal drift.
3.17 Frequency Output (F
OUT0
, F
OUT1
)
F
OUT0
and F
OUT1
are the frequency outputs of the
device that supply the average real-power information.
The outputs are periodic pulse outputs, with its period
proportional to the measured real power, and to the F
C
constant, defined by F0 and F1 pin logic states. These
pins include high-output drive capability for direct use
of electromechanical counters and 2-phase stepper
motors. Since this output supplies average real power,
any 2 ripple on the output pulse period is minimal.