AD7400 datasheet(13/20 Pages) AD

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AD7400

Rev. A | Page 13 of 20

THEORY OF OPERATION

CIRCUIT INFORMATION

The AD7400 isolated Σ-Δ modulator converts an analog input

signal into a high speed (10 MHz typ), single-bit data stream;

the time average of the modulator’s single-bit data is directly

proportional to the input signal. Figure 22 shows a typical

application circuit where the AD7400 is used to provide

isolation between the analog input, a current sensing resistor,

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 AD7400 is implemented

with a switched capacitor circuit. This circuit implements a

second-order modulator stage that digitizes the input signal

into a 1-bit output stream. The sample clock (MCLKOUT)

provides the clock signal for the conversion process as well as

the output data-framing clock. This clock source is internal on

the AD7400. 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 20).

MODULATOR OUTPUT

+FS ANALOG INPUT

–FS ANALOG INPUT

ANALOG INPUT

Figure 20. Analog Input vs. Modulator Output

A differential signal of 0 V results (ideally) in a stream of ones

and zeros at the MDAT output pin. This output is high 50% of

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

200 mV produces a stream of ones and zeros that are high

81.25% of the time. A differential input of −200 mV produces a

stream of ones and zeros that are high 18.75% of the time.

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

ones. This is the absolute full-scale range of the AD7400, while

200 mV is the specified full-scale range, as shown in Table 9.

Table 9. Analog Input Range

Analog Input

Voltage Input

Full-Scale Range

+640 mV

Positive Full Scale

+320 mV

Positive Specified Input Range

+200 mV

Zero

0 mV

Negative Specified Input Range

−200 mV

Negative Full Scale

−320 mV

To reconstruct the original information, this output needs to be

digitally filtered and decimated. A Sinc3 filter is recommended

because this is one order higher than that of the AD7400

modulator. If a 256 decimation rate is used, the resulting

16-bit word rate is 39 kHz, assuming a 10 MHz internal clock

frequency. Figure 21 shows the transfer function of the AD7400

relative to the 16-bit output.

65535

53248

SPECIFIED RANGE

ANALOG INPUT

12288

–320mV

–200mV

+200mV +320mV

Figure 21. Filtered and Decimated 16-Bit Transfer Characteristic

Σ-Δ

MOD/

ENCODER

INPUT

CURRENT

NONISOLATED

5V/3V

ISOLATED

VDD1

RSHUNT

VIN+

VIN–

GND1

VDD

GND

VDD2

MDAT

SINC3 FILTER

AD7400

MCLKOUT

SDAT

SCLK

MCLK

GND2

DECODER

ENCODER

Figure 22. Typical Application Circuit

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