AD652KP datasheet(22/28 Pages) AD

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AD652

Rev. C | Page 22 of 28

ISOLATED FRONT END

In some applications, it may be necessary to have complete

galvanic isolation between the analog signals being measured

and the digital portions of the circuit. The circuit shown in

Figure 32 runs off a single 5 V power supply and provides a self-

contained, completely isolated analog measurement system. The

power for the AD652 SVFC is provided by a chopper and a

transformer, and is regulated to 15 V.

Both the chopper frequency and the AD652 clock frequency are

125 kHz, with the clock signal being relayed to the SVFC

through the transformer. The frequency output signal is relayed

through an opto-isolator and latched into a D flop. The chopper

frequency is generated from an AD654 VFC, and is frequency

divided by two to develop differential drive for the chopper

transistors, and to ensure an accurate 50% duty cycle. The pull-

up resistors on the D flop outputs provide a well-defined high

level voltage to the choppers to equalize the drive in each

direction. The 10 µH inductor in the 5 V lead of the transformer

primary is necessary to equalize any residual imbalance in the

drive on each half cycle, and thus prevent saturation of the core.

The capacitor across the primary resonates the system so that

under light loading conditions on the secondary, the wave shape

is sinusoidal and the clock frequency is relayed to the SVFC. To

adjust the chopper frequency, disconnect any load on the

secondary and tune the AD654 for a minimum in the supply

current drawn from the 5 V supply.

A-TO-D CONVERSION

In performing an A-to-D conversion, the output pulses of a

VFC are counted for a fixed-gate interval. To achieve maximum

performance with the AD652, the fixed-gate interval should be

generated using a multiple of the SVFC clock input. Counting

in this manner eliminates any errors due to the clock (whether

it be jitter, drift with time or temperature, and so on) since it is

the ratio of the clock and output frequencies that is being

measured.

The resolution of the A-to-D conversion measurement is

determined by the clock frequency and the gate time. If, for

instance, a resolution of 12 bits is desired and the clock

frequency is 1 MHz (resulting in an AD652 FS frequency of

500 kHz) the gate time is:

()

192

sec

8192

4096

MHz

–

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

Freq

Clock

Freq

Where N is the total number of codes for a given resolution.

Figure 33 shows the AD652 SVFC as an A-to-D converter in

block diagram form.

VIN

AD652

COUNTER

µP

INPUT

GATE

fOUT

CLOCK

÷2N

Figure 33. Block Diagram of SVFC A-to-D Converter

To provide the ÷2N block, a single-chip counter such as the

4020B can be used. The 4020B is a 14-stage binary ripple

counter that has a clock and master reset for inputs, and

buffered outputs from the first stage and the last 11 stages. The

output of the first stage is fCLOCK ÷ 21 = fCLOCK/2, while the output

of the last stage is fCLOCK ÷ 214 = fCLOCK/16384. Therefore, using

this single chip counter as the ÷2N block, 13-bit resolution can

be achieved. Higher resolution can be achieved by cascading D-

type flip flops or another 4020B with the counter.

Table 4

shows the relationship between clock frequency and gate

time for various degrees of resolution. Note that if the variables

are chosen such that the gate times are multiples of 50 Hz,

60 Hz, or 400 Hz, normal mode rejection (NMR) of those line

frequencies occur.

Table 4.

Resolution

Clock

Conversion or Gate Time (ms)

Typical Linearity (%)

Comments

12 Bits

4096

81.92 kHz

100

0.002

50 Hz, 60 Hz,400 Hz NMR

12 Bits

4096

2 MHz

4.096

0.01

12 Bits

4096

4 MHz

2.048

0.02

4 Digits

10000

200 kHz

100

0.002

50 Hz, 60 Hz, 400 Hz NMR

14 Bits

16384

327.68 kHz

100

0.002

50 Hz, 60 Hz, 400 Hz NMR

14 Bits

16384

1.966 MHz

16.66

0.01

60 Hz NMR

14 Bits

16384

1.638 MHz

0.01

50 Hz NMR

4½ Digits

20000

400 kHz

100

0.002

50 Hz, 60 Hz, 400 Hz NMR

16 Bits

65536

655.36 kHz

200

0.002

50 Hz, 60 Hz, 400 Hz NMR

16 Bits

65536

4 MHz

32.77

0.02

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