AD7750

–13–

REV. 0

age attenuated by a factor of 860.

Calculation 1

100 PPKWHR = 0.02777 Hz/kW.

Ib of 15 A rms and line voltage of 220 V = 3.3 k

Ω. Hence, the

output frequency is given by 3.3

× 0.02777 Hz = 0.0916 Hz.

Calculation 2

k = (1.32

× V

1

× V

2

× Gain)/V

REF

0.01347 = (1.32

× 0.25 × V

2

× 1)/6.25.

Figure 21 below shows how the design equations from the previ-

ous page are implemented.

Measuring the Load Current

The load current is converted to a voltage signal for Channel 1

using a CT (Current Transformer). A 15 A rms load should

produce a 250 mV rms signal on Channel 1. A CT with a turns

ratio of 120 and a shunt resistor of 2

Ω. will carry out the neces-

sary current to voltage conversion. The CT and its shunt resis-

tance should be placed as close as possible to the AD7750. This

will improve the accuracy of the meter at very small load cur-

rents. At small load currents the voltage levels on Channel 1 are

in the order of 10 mV and the meter is more prone to error due

to stray signal “pick up.” When measuring power the HPF in

the current channel must be switched on. This is done by con-

to ground. This can be achieved as shown in Figure 21 below,

centertap on the CT secondary to ground.

Measuring the Line Voltage

When the AD7750 is biased around the live wire as shown in

Figure 21, the task of measuring the line voltage is greatly

simplified. A resistor divider attenuates the line voltage and

provides a single-ended input for Channel 2. The component

values of the divider are chosen to give the correct rating (e.g.,

100 PPKWHR) for the meter. See the design equations on the

previous page. For this design an attenuation ratio of 860:1 is

required.

Antialiasing Components Channels 1 and 2

The AD7750 is basically two ADCs and a digital multiplier. As

with any ADC, a LPF (Low-Pass Filter) should be used on the

analog inputs to avoid out of band signal being aliased into the

band of interest. In the case of a Class 1 meter the band of

interest lies in the range 48 Hz to 1 kHz approximately. The

components R3, R4, R6, R7, C5, C6, C9 and C10 make up the

LPFs on each of the four analog inputs. Note that although

Power Supply Circuit

The AD7750 operates from a single power supply of 5 V

± 5%

but still accommodates input signals in the range

±1 V. Because

the AD7750 doesn’t require dual supplies the number of exter-

nal components for the power supply is reduced. One of the

most important design goals for the power supply is to ensure

that the ripple on the output is as low as possible. Every analog

or mixed signal IC is to a greater or lesser extent susceptible to

power supply variations. Power supply variations or ripple, if

large enough, may affect the accuracy of the device when mea-

suring small signals. The plot in Figure 20 shows the ripple

associated with the circuit in Figure 21. The ripple is in the

region of 10 mV peak to peak.

TIME

ms

5.065

5.060

5.050

600

620

5.055

640

660

680

700

720

740

760

780

800

Figure 20. Power Supply Ripple

C1

CT

120:1

R1

*BIASING AROUND THE LIVE WIRE PATENTED BY SCHLUMBERGER.

XTAL 3.57954MHz

C2

R2

Z1

D1

R4

R3

C5

C6

1

R6

C9

C10

C12

C11

C8

C7

MOV

SOURCE

LOAD

D2

D3

D4

R7

R8

R9

14

13

12

11

17

16

15

20

19

18

10

9

8

1

2

3

4

7

6

5

TOP VIEW

(Not to Scale)

AD7750

M

MODE2

*

1

VR1

R5

PULSE COUNTER

IR DIODE FOR

CALIBRATION

REVERSE POLARITY

INDICATOR

D5

C4

C3

Figure 21. Suggested Class 1 Meter Implementation