AD7779 datasheet(32/97 Pages) AD

32 / 97 page

AD7779

Data Sheet

Rev. 0 | Page 32 of 97

THEORY OF OPERATION

The AD7779 is an 8-channel, simultaneously sampling, low

noise, 24-bit -Δ ADC with integrated digital filtering per

channel and SRC.

The AD7779 offers two operation modes: high resolution mode,

which offers up to 16 kSPS, and low power mode, which offers

up to 8 kSPS. In low power mode, the specifications are guaranteed

up to 4 kSPS, with performance degradation expected at ODRs

higher than 4 kSPS.

The AD7779 employs a -Δ conversion technique to convert

the analog input signal into an equivalent digital word. The

overview of the -Δ technique is that the modulator samples

the input waveform and outputs an equivalent digital word at

the input clock frequency, fCLKIN.

Due to the high oversampling rate, this technique spreads the

quantization noise from 0 to fCLKIN/2 (in the case of the AD7779,

fCLKIN relates to the external clock); therefore, the noise energy

contained in the band of interest is reduced (see Figure 82). To

further reduce the quantization noise, a high order modulator is

employed to shape the noise spectrum so that most of the noise

energy is shifted out of the band of interest (see Figure 83). The

digital filter that follows the modulator removes the large out of

band quantization noise (see Figure 84).

For more information on basic and advanced concepts of -Δ

ADCs, see MT-022 and MT-023.

Digital filtering has certain advantages over analog filtering.

Because digital filtering occurs after the analog-to-digital

conversion process, it can remove noise injected during the

conversion. Analog filtering cannot remove noise injected

during conversion.

QUANTIZATION NOISE

fICLK\2

BAND OF INTEREST

Figure 82.

-∆ ADC Operation, Reduction of Noise Energy Contained in the

Band of Interest (Linear Scale X-Axis)

fICLK\2

NOISE SHAPING

BAND OF INTEREST

Figure 83.

-∆ ADC Operation, Majority of Noise Energy Shifted Out of the

Band of Interest (Linear Scale X-Axis)

fICLK\2

BAND OF INTEREST

DIGITAL FILTER CUTOFF FREQUENCY

Figure 84.

-∆ ADC Operation, Removal of Noise Energy from the Band of

Interest (Linear Scale X-Axis)

The -Δ ADC starts the conversions of the input signal after the

supplies generated by the internal LDOs become stable. An

external signal is not required to generate the conversions.

ANALOG INPUTS

The AD7779 can be operated in bipolar or unipolar modes and

accepts true differential, pseudo differential, and single-ended

input signals, as shown in Figure 85 through Figure 88.

Table 14 summarizes the maximum differential input signal and

dynamic range for the different input modes.

Table 14. Input Signal Modes

Input Signal Mode

PGA Gain

Maximum Differential Signal

Maximum Peak-to-Peak Signal

True differential

All gains

±(VREF/PGAGAIN)

2 × VREF/PGAGAIN

Pseudo differential

All gains

±(VREF/PGAGAIN)

2 × VREF/PGAGAIN

Single-ended

All gains

VREF/PGAGAIN

AD7779 Datasheet(PDF) 32 Page - Analog Devices

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