MCP3909-I/SS Microchip Technology, MCP3909-I/SS Datasheet - Page 87

• Manufacturer Part Number: MCP3909-I/SS
• Description: IC POWER METERING-1 PHASE 24SSOP
• Manufacturer: Microchip Technology

Specifications of MCP3909-I/SS

• Package / Case: 24-SSOP (0.200", 5.30mm Width)
• Input Impedance: 390 KOhm
• Measurement Error: 0.1%
• Voltage - I/o High: 2.4V
• Voltage - I/o Low: 0.85V
• Current - Supply: 2.3mA
• Voltage - Supply: 4.5 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Meter Type: Single Phase
• Operating Temperature Range: - 40 C to + 85 C
• Mounting Style: SMD/SMT
• Supply Voltage Range: 4.5V To 5.5V
• Digital Ic Case Style: SSOP
• No. Of Pins: 24
• Interface Type: Serial, SPI
• Supply Voltage Max: 5.5V
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: MCP3909EV-MCU16 - EVALUATION BOARD FOR MCP3909MCP3909RD-3PH1 - REF DESIGN MCP3909 3PH ENGY MTR
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

C.5
© 2009 Microchip Technology Inc.
MEASURING FREQUENCY
There are many ways to measure frequency, with the most common being counting the
signal cycle. In this method, a counter increments each time a zero-crossing is
detected. Based on the counts, the width of a cycle can be measured. If the zero-cross-
ing is accurate and the counter precision is high enough, cycle counting can be a
simple and practical method. But if the input signal has large harmonic components,
causing distortion around zero-crossing, then this approach may produce large errors.
Another method is to analyze and process the sampled data and calculate the frequen-
cies. Analysis may be carried out in time domain, such as digital differential ND and
interpolation method; or may be carried out in frequency domain after DFT transforma-
tion, such as gravity center method, spectrum zoom method and phase difference
method, among which the phase difference method is the most common one. It is not
sensitive to signal distortion around zero-crossing points.
The basic idea of the phase difference method is: if the rough range of to-be-measured
signal frequency is known, then we may assume a frequency that is close to the actual
frequency and then acquire an array of samples based on the assumed frequency. In
the sampled data, the phase of the 1st cycle and the subsequent N-th cycle are meau-
red and their difference may be calculated. Then the phase difference may be used to
calculate the difference between the actual freqency and the assumed frequence, thus
figuring out the actual frequency.
If the frequency f
Δf << f, then from Equation C-27, the fundamental signal can be expressed as:
EQUATION C-45:
If:
EQUATION C-46:
EQUATION C-47:
EQUATION C-48:
0
to be measured is known to be a definite value f, i.e., f
U
1
t ( )
u
u
a
b
=
=
=
=
=
u
-- -
T
2
2
-- -
T
c1
-- -
T
2
2
-- -
T
⋅
⋅
⋅
sin
⋅
T
∫
0
T
∫
0
T
∫
0
T
∫
0
U
U
u
u
(
c1
1
c1
1
ω
Power Calculation Theory
t ( )
t ( )
sin
sin
⋅
t
cos
sin
+
(
(
2
T
2
(
ϕ
(
π
π
ω
ω
u1
=
f
f
0
t
0
t
t
)
t
)
)
1
-- -
t d
f
t d
+
+
=
ϕ
ϕ
u
u1
u1
c1
)
)
cos
sin
sin
(
(
(
2
2
2
π
π
π
f
ft
ft
0
)
t
)
t d
t d
+
ϕ
u1
)
DS51723A-page 87
0
= f + Δf,