AD7750

–14–

REV. 0

Registering the Power Output

The low frequency pulse outputs (F1 and F2) of the AD7750

provide the frequency output from the product-to-frequency

conversion. These outputs can be used to drive a stepper motor

or impulse counter.

frequency output is used for calibration purposes. In Mode 2

the output frequency is 16

× F1(2). With a load current of Ib

× 16 from

calculations). If a higher frequency output is required, the FS

frequency is equal to 64

× F1 or 5.8624 Hz at Ib—see Table I.

NO LOAD THRESHOLD OF THE AD7750

The AD7750 will detect when the power drops below a certain

level. When the power (current) drops below a predefined

threshold the AD7750 will cease to generate an output drive for

the stepper motor (F1, F2). This feature of the AD7750 is

intended to reproduce the behavior of Ferraris meters. A

Ferraris meter will have friction associated with the wheel rota-

tion, therefore the wheel will not rotate below a certain power

level. The no load threshold is only implemented in the Magni-

tude Only modes (Modes 1, 2, 5 and 6—see Table I). The

IEC1036 specification includes a test for this effect by requiring

no output pulses during some predetermined time period. This

time period is calculated as:

time period = 60,000/pulses-per-minute

16 times faster than F1 and F2, this time period is 37.5 minutes

(60,000/1,600). The IEC1036 specifications state that the no

load threshold must be less than the start up current level. This

is specified as 0.4% of Ib.

The threshold level for a given design can be easily calculated

given that the minimum output frequency of the AD7750 is

0.00048% of the maximum output frequency for a full-scale

differential dc input. For example if FS = 0, the maximum

output frequency for a full-scale dc input is 2.9 Hz (see Table II)

and the minimum output frequency is, therefore, 1.39

Calculating the Threshold Power (Current)

The meter used in this example is calibrated to 100 PPKWHR,

has an Ib (basic current) of 15 A rms, the line voltage is 220 V

rms and the turns ratio of the CT on Channel 1 is 120:1 with an

2

Ω shunt resistor.

The nominal voltage on Channel 2 of the AD7750 is 255 mV

tude Only Mode (Mode 2) is selected to enable the no load thresh-

old. The gain on Channel 1 is set to 1. The threshold power or

current can be found by using the transfer function in Table I.

F1, F2 = (1.32

× V

1

× V

2

× Gain × F

2

µV rms—

see Calculation 3.

This is equivalent to a line current of:

(37.95

µV/2 Ω ) × 120 = 2.27 mA rms or 0.5 W

or

(2.27 mA/15 A)

× 100% = 0.015% of Ib.

NOTE: The no load threshold as a percentage of Ib will be

different for each value of Ib since the no load in watts is fixed:

0.5 Watts for a 100 PPKWHR meter

5 Watts for a 10 PPKWHR meter

1 Watt for a 100 PPKWHR meter

10 Watts for a 10 PPKWHR meter

Calculation 3

× V

1

× V

2

× Gain × 6.8 Hz) V

REF

2

1.39

× 10–5 Hz = V

1

× 0.2555 × 1 × 6.8)/6.25

µV

EXTERNAL LEAD/LAG COMPENSATION

External phase compensation is often required in a power meter

design to eliminate the phase errors introduced by transducers

and external components. The design restriction on any external

compensating network is that the network must have an overall

low-pass response with a 3 dB point located somewhere between

5 kHz and 6 kHz. The corner frequency of this LPF(s) is much

higher than the band of interest. The reason for this is to mini-

mize its effect on phase variation at 50 Hz due to component

tolerances.

With the antialiasing filters on all channels having the same

corner (–3 dB) frequency, the main contribution to phase error

will be due to the CT. A phase lead in a channel is compensated

by lowering the corner frequency of the antialiasing filter to

increase its associated lag and therefore cancel the lead. A phase

lag in a channel should be compensated by introducing extra lag

in the other channel. This can be done as previously described,

i.e., moving the corner frequency of the antialiasing filters. The

result in this case is that the signal on both channels has the

same amount of phase lag and is therefore in phase at the analog

inputs to the AD7750. The recommended RC values for the

antialiasing filters on the voltage and current channels (see

Antialiasing Components Channels 1 and 2) are R = 1 k

Ω,

C = 33 nF and R = 100

Ω, C = 330 nF respectively. These

values produce a phase lag of 0.6

° through the filters. Varying R

in the antialiasing network from 80

Ω to 100 Ω or 800 Ω to 1 kΩ

produces a phase variation from 0.475

° to 0.6° at 50 Hz. This

allows the user to vary the lag by 0.125

°.