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Data Sheet

AD7403

THEORY OF OPERATION

CIRCUIT INFORMATION

The AD7403 isolated Σ-Δ modulator converts an analog input

signal into a high speed (20 MHz maximum), single-bit data

stream; the time average single-bit data from the modulator is

directly proportional to the input signal. Figure 27 shows a

typical application circuit where the AD7403 is used to provide

isolation between the analog input, a current sensing resistor or

shunt, and the digital output, which is then processed by a

digital filter to provide an N-bit word.

ANALOG INPUT

The differential analog input of the AD7403 is implemented

with a switched capacitor circuit. This circuit implements a

second-order modulator stage that digitizes the input signal

into a single-bit output stream. The sample clock (MCLKIN)

provides the clock signal for the conversion process as well as

the output data framing clock. This clock source is externally

supplied to the AD7403. The analog input signal is

continuously sampled by the modulator and compared to an

internal voltage reference. A digital stream that accurately

represents the analog input over time appears at the output of

the converter (see Figure 28).

A differential signal of 0 V ideally results in a stream of alter-

nating 1s and 0s at the MDAT output pin. This output is high

50% of the time and low 50% of the time. A differential input of

250 mV produces a stream of 1s and 0s that are high 89.06% of

the time. A differential input of −250 mV produces a stream of

1s and 0s that are high 10.94% of the time.

A differential input of 320 mV ideally results in a stream of all

1s. A differential input of −320 mV ideally results in a stream of

all 0s. The absolute full-scale range is ±320 mV and the specified

full-scale performance range is ±250 mV, as shown in Table 12.

Table 12. Analog Input Range

Analog Input

Voltage Input (mV)

Positive Full-Scale Value

+320

Positive Specified Performance Input

+250

Zero

Negative Specified Performance Input

−250

Negative Full-Scale Value

−320

Σ-Δ

MOD/

ENCODER

NONISOLATED

5V/3V

VDD1

GND1

VIN+

VIN–

GND1

VDD1

VDD

GND

VDD2

MDAT

SINC3 FILTER*

AD7403

MCLKIN

SDAT

SCLK

MCLK

100nF

GND2

DECODER

1nF

10µF

+400V

–400V

220pF

10Ω

5.1V

RSHUNT

10Ω

DECODER

ENCODER

1nF

10µF

GATED

DRIVE

CIRCUIT

FLOATING

POWER SUPPLY

GATED

DRIVE

CIRCUIT

FLOATING

POWER SUPPLY

MOTOR

*THIS FILTER IS IMPLEMENTED

WITH AN FPGA OR DSP

Figure 27. Typical Application Circuit

MODULATOR OUTPUT

+FS ANALOG INPUT

–FS ANALOG INPUT

ANALOG INPUT

Figure 28. Analog Input vs. Modulator Output

Rev. B | Page 15 of 24

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