ADE7753ARSRL datasheet(39/60 Pages) AD

39 / 60 page

ADE7753

Rev. C | Page 39 of 60

Watt Gain

The first step of calibrating the gain is to define the line voltage,

base current and the maximum current for the meter. A meter

constant needs to be determined for CF, such as 3200 imp/kWh

or 3.2 imp/Wh. Note that the line voltage and the maximum

current scale to half of their respective analog input ranges in

this example.

The expected CF in Hz is

CFexpected (Hz) =

)

cos(

s/h

3600

(W)

(imp/Wh)

× Load

ant

MeterConst

(34)

where ϕ is the angle between I and V, and cos

is the power

factor.

)

(

The ratio of active energy LSBs per CF pulse is adjusted using

the CFNUM, CFDEN, and WDIV registers.

CFexpected =

)

(

)

(

(s)

CFDEN

CFNUM

WDIV

onTime

Accumulati

LAENERGY

(35)

The relationship between watt-hours accumulated and the

quantity read from AENERGY can be determined from the

amount of active energy accumulated over time with a given

load:

LAENERGY

Time

Accumulati

Load

3600

(s)

(W)

LSB

(36)

where Accumulation Time can be determined from the value in

the line period and the number of half line cycles fixed in the

LINECYC register.

Accumulation time(s) =

(s)

Period

Line

LINECYC

IB ×

(37)

The line period can be determined from the PERIOD register:

Line Period(s) = PERIOD ×

CLKIN

(38)

The AENERGY Wh/LSB ratio can also be expressed in terms of

the meter constant:

(imp/Wh)

)

(

)

(

LSB

ant

MeterConst

WDIV

CFDEN

CFNUM

(39)

In a meter design, WDIV, CFNUM, and CFDEN should be kept

constant across all meters to ensure that the Wh/LSB constant is

maintained. Leaving WDIV at its default value of 0 ensures

maximum resolution. The WDIV register is not included in the

CF signal chain so it does not affect the frequency pulse output.

The WGAIN register is used to finely calibrate each meter. Cali-

brating the WGAIN register changes both CF and AENERGY for

a given load condition.

AENERGYexpected = AENERGYnominal ×

⎟

⎠

⎞

⎜

⎝

⎛ +

WGAIN

(40)

CFexpected (Hz) = CFnominal ×

⎟

⎠

⎞

⎜

⎝

⎛ +

)

(

)

(

WGAIN

CFDEN

CFNUM

(41)

When calibrating with a reference meter, WGAIN is adjusted

until CF matches the reference meter pulse output. If an accurate

source is used to calibrate, WGAIN is modified until the active

energy accumulation rate yields the expected CF pulse rate.

The steps of designing and calibrating the active energy portion

of a meter with either a reference meter or an accurate source

are outlined in the following examples. The specifications for

this example are

Meter Constant:

MeterConstant(imp/Wh)

= 3.2

Base Current:

Ib = 10 A

Maximum Current:

IMAX = 60 A

Line Voltage:

Vnominal = 220 V

Line Frequency:

fl = 50 Hz

The first step in calibration with either a reference meter or an

accurate source is to calculate the CF denominator, CFDEN.

This is done by comparing the expected CF pulse output to the

nominal CF output with the default CFDEN = 0x3F and

CFNUM = 0x3F and when the base current is applied.

The expected CF output for this meter with the base current

applied is 1.9556 Hz using Equation 34.

CFIB(expected)(Hz) =

9556

)

cos(

s/h

3600

220

imp/Wh

200

Alternatively, CFexpected can be measured from a reference meter

pulse output if available.

CFexpected(Hz) = CFref

(42)

The maximum CF frequency measured without any frequency

division and with ac inputs at full scale is 23 kHz. For this

example, the nominal CF with the test current, Ib, applied is

958 Hz. In this example the line voltage and maximum current

scale half of their respective analog input ranges. The line

voltage and maximum current should not be fixed at the

maximum analog inputs to account for occurrences such as

spikes on the line.

CFnominal(Hz) =

MAX

kHz

(43)

CFIB(nominal)(Hz) =

958

kHz

The nominal CF on a sample set of meters should be measured

using the default CFDEN, CFNUM, and WDIV to ensure that

the best CFDEN is chosen for the design.

With the CFNUM register set to 0, CFDEN is calculated to be

489 for the example meter:

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