AD7723 datasheet(19/33 Pages) AD

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AD7723

Rev. C | Page 18 of 32

CIRCUIT DESCRIPTION

The AD7723 ADC employs a Σ-Δ conversion technique to

convert the analog input into an equivalent digital word. The

modulator samples the input waveform and outputs an

equivalent digital word at the input clock frequency, fCLKIN.

Due to the high oversampling rate that spreads the quantization

noise from 0 to fCLKIN/2, the noise energy contained in the band

of interest is reduced (Figure 27A). 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 (Figure 27B).

The digital filter that follows the modulator removes the large

out-of-band quantization noise (Figure 27C) while also

reducing the data rate from fCLKIN at the input of the filter to

fCLKIN/32 or fCLKIN/16 at the output of the filter, depending on the

state on the MODE1/MODE2 pins in parallel interface mode or

the SLDR pin in serial interface mode. The AD7723 output data

rate is a little over twice the signal bandwidth, which guarantees

that there is no loss of data in the signal band.

Digital filtering has certain advantages over analog filtering.

First, since digital filtering occurs after the A/D conversion, it

can remove noise injected during the conversion process.

Analog filtering cannot remove noise injected during

conversion. Second, the digital filter combines low pass-band

ripple with a steep roll-off while also maintaining a linear phase

response.

NOISE SHAPING

QUANTIZATION NOISE

DIGITAL FILTER CUTOFF FREQUENCY

fCLKIN/2

BAND OF INTEREST

fCLKIN/2

Figure 27. Sigma-Delta ADC

The AD7723 employs four or five finite impulse response (FIR)

filters in series. Each individual filter’s output data rate is half

that of the filter’s input data rate. When data is fed to the

interface from the output of the fourth filter, the output data

rate is fCLKIN/16 and the resulting oversampling ratio (OSR) of

the converter is 16. Data fed to the interface from the output of

the fifth filter results in an output data rate of fCLKIN/32 and a

corresponding OSR for the converter of 32. When an output

data rate (ODR) of fCLKIN/32 is selected, the digital filter

response can be set to either low-pass or band-pass. The band-

pass response is useful when the input signal is band limited

because the resulting output data rate is half that required to

convert the band when the low-pass operating mode is used. To

illustrate the operation of this mode, consider a band-limited

signal, as shown in Figure 28A. This signal band can be

correctly converted by selecting the (low-pass) ODR = fCLKIN/16

mode, as shown in Figure 28B. Note that the output data rate is a

little over twice the maximum frequency in the frequency band.

Alternatively, the band-pass mode can be selected, as shown in

Figure 28C. The band-pass filter removes unwanted signals

from dc to just below fCLKIN/64. Rather than outputting data at

fCLKIN/16, the output of the band-pass filter is sampled at

fCLKIN/32. This effectively translates the wanted band to a

maximum frequency of a little less than fCLKIN/64, as shown in

Figure 28D. Halving the output data rate reduces the workload

of any following signal processor and also allows a lower serial

clock rate to be used.

BAND LIMITED SIGNAL

0dB

ODR

LOW-PASS FILTER RESPONSE

SAMPLE

IMAGE

LOW-PASS FILTER. OUTPUT DATA RATE = fCLKIN/16

0dB

SAMPLE

IMAGE

BAND-PASS FILTER

RESPONSE

BAND-PASS FILTER

fCLKIN/16

LOW-PASS FILTER. OUTPUT DATA RATE =

fCLKIN/32

ODR

SAMPLE

IMAGE

FREQUENCY

TRANSLATED

INPUT SIGNAL

0dB

fCLKIN/16

Figure 28. Band-Pass Operation

The frequency response of the three digital filter operating

modes is shown in Figure 29, Figure 30, and Figure 31.

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