MCP3909T-ESLASHSS datasheet(17/44 Pages) MICROCHIP

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 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 HFOUT and maintains the logic ‘1’ until the real

power becomes positive again and HFOUT shows a

pulse.

3.15

Ground Connection (DGND)

DGND 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, DGND must be

connected to the same ground as AGND, 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 (HFOUT)

HFOUT 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 HFC 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 (FOUT0, FOUT1)

FOUT0 and FOUT1 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 FC

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.

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