AD7750

–12–

REV. 0

produces a 90 ms wide pulse at a frequency that is proportional

to the product of Channel 1 and Channel 2 or the instanta-

neous voltage on Channel 1 or Channel 2. The output fre-

quencies are given in Table I in the Determining the Output

Frequencies of the AD7750 section of this data sheet. As in

F1

F2

0V

0V

0V

Figure 18. Timing Diagram for Frequency Outputs

VOLTAGE REFERENCE

The AD7750 has an on-chip temperature compensated band-

gap voltage reference of 2.5 V with a tolerance of

±250 mV.

The temperature drift for the reference is specified at 50 ppm/

°C.

It should be noted that this reference variation will cause a

frequency output variation from device to device for a given set

of input signals. This should not be a problem in most applica-

tions since it is a straight gain error that can easily be removed

at the calibration stage.

REVERSE POLARITY INDICATOR

When the AD7750 is operated in a Magnitude Only mode of

operation (i.e., Modes 1, 2, 5 and 6), and the polarity of the

power changes, the logic output REVP will go high. However,

the REVP pin is only activated when the there is pulse output

on F1 or F2. Therefore, if the power being measured is low, it may

be some time before the REVP pin goes logic high even though the

polarity of the power is reversed. Once activated the REVP output

will remain high until the AD7750 is powered down.

APPLICATIONS INFORMATION

Designing a Single Phase Class 1 Energy Meter (IEC 1036)

The AD7750 Product-to-Frequency Converter is designed for

use in a wide range of power metering applications. In a typical

power meter two parameters are measured (i.e., line voltage and

current) and their product obtained. The real power is then

obtained by low-pass filtering this product result. The line

voltage can be measured through a resistor divider or voltage

transformer, and the current can be sensed and converted to

a voltage through a shunt resistor, current transformer or hall

effect device.

The design methodology used in the following example is to use

the upper end of the current channel dynamic range, i.e., Chan-

nel 1 of the AD7750. The assumption here is that the signal on

the voltage channel will remain relatively constant while the

signal on the current channel will vary with load. Using the

upper end of the dynamic range of Channel 1 will improve the

meter accuracy with small load currents. Hence an error of less

than 1% from 4% Ib to 400% Ib will be easier to achieve.

We will assume the design of a Class 1 meter. The specification

(IEC1036) requires that the meter have an error of no greater

the basic current

commodates signals with a crest factor of 2. The crest factor is

of sqrt(2) = 1.414 and an undistorted triangular waveform has a

crest factor of sqrt(3) = 1.73. Using a gain of 1 on Channel 1

the maximum differential signal which can be applied to Chan-

nel 1 is

±2 V—See Analog Input Ranges section. With a crest

factor of 2 the maximum rms signal on Channel 1 is, therefore,

pearing on Channel 1 is therefore 10 mV rms.

Load Current

Channel 1

4% Ib

10 mV rms

Ib

250 mV rms

400 Ib

1 V rms

2

0. 2

0. 02

0. 002

1

0.01

400% Ib

4% Ib

Figure 19. Use the Upper End of the Dynamic Range of

Channel 1 (Current)

Calculations for a 100 PPKWHR Meter

The AD7750 offers a range of maximum output frequencies—

see Table I and Table II. In the Magnitude Only modes of

operation the two maximum output frequencies are 1.45 Hz

and 2.9 Hz. The signal on the voltage channel (Channel 2) is

scaled to achieve the correct output pulse frequency for a given

load (e.g., 100 PPKWHR). The relationship between the input

signals and the output frequency is given by the equation:

Freq = k

× F

MAX

where k = (1.32

× V

1

× V

2

× Gain)/V

REF

2

(2.5 V

± 8%).

To design a 100 PPKWHR meter with Ib = 15 A rms and a line

voltage of 220 V rms the output pulse frequency with a load

current of Ib is 0.0916 Hz (See Calculation 1 below).

Therefore, 0.0916 Hz = k

× 6.8 Hz (Mode 2) or k = 0.01347.

to 0.25 V rms (remember 400% Ib = 1 V rms) and, therefore,

tion 2). This means that the nominal line voltage (220 V rms)

needs to be attenuated by approximately 860, i.e., 220/0.255.