AD7405 datasheet(13/20 Pages) AD

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Data Sheet

AD7405

THEORY OF OPERATION

CIRCUIT INFORMATION

The AD7405 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 21 shows a

typical application circuit where the AD7405 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 AD7405 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 external on

the AD7405. 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 22).

A differential input signal of 0 V ideally results in a differential

stream of alternating 1s and 0s at the MDAT± output pins. 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 10.

Table 10. 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

Figure 21. Typical Application Circuit

Figure 22. Analog Input vs. Modulator Output

Σ-Δ

MOD/

ENCODER

NONISOLATED

5V/3V

VDD1

GND1

VIN+

VIN–

GND1

VDD1

VDD

GND

VDD2

MDAT+

MDAT–

MDAT

SINC3 FILTER*

AD7405

MCLKIN+

SDAT

SCLK

MCLKIN–

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

MODULATOR OUTPUT

+FS ANALOG INPUT

–FS ANALOG INPUT

ANALOG INPUT

Rev. A | Page 13 of 20

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