AD7723 datasheet(15/23 Pages) AD

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AD7723

–15–

REV. 0

CLOCK FREQUENCY – MHz

225

150

101520

200

175

125

100

AIDD (HALF_POWER = 1)

DIDD

AIDD (HALF_POWER = 0)

Figure 21. Power Consumption vs. CLKIN Frequency

–150

0E+0

600E+3

100E+3

200E+3

300E+3

400E+3

500E+3

–25

–50

–75

–100

–125

SNR = –86.19dB

SNR&D = –85.9dB

THD = –96.42dB

SFDR = –99.61dB

2ND HARMONIC = –100.98dB

3RD HARMONIC = –99.61dB

AIN = 100kHz

MEASURED BW = 460kHz

FREQUENCY – Hz

Figure 22. 16K Point FFT (Output Data Rate = 1.2 MHz)

–120

0E+0

300E+3

50E+3

100E+3

150E+3

200E+3

250E+3

–20

–40

–60

–80

–100

SNR = –89.91dB

SNR&D = –89.7dB

THD = –101.16dB

SFDR = –102.1dB

2ND HARMONIC = –102.1dB

3RD HARMONIC = –110.3dB

AIN = 50kHz

MEASURED BW = 300kHz

–140

–160

FREQUENCY – Hz

Figure 23. 16K Point FFT (Output Data Rate = 600 kHz)

CIRCUIT DESCRIPTION

The AD7723 ADC employs a sigma-delta conversion technique

to convert the analog input into an equivalent digital word. The

modulator samples the input waveform and outputs an equiva-

lent digital word at the input clock frequency, fCLKIN.

Due to the high oversampling rate, which spreads the quantiza-

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

band of interest is reduced (Figure 24a). 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 24b).

The digital filter that follows the modulator removes the large

out-of-band quantization noise, (Figure 24c) while also reduc-

ing 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 MODE1/2 pins in parallel interface mode or the pin

SLDR 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.

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

can remove noise injected during the conversion process. Ana-

log filtering cannot remove noise injected during conversion.

Secondly, the digital filter combines low passband 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

(a)

(b)

(c)

Figure 24. 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 Over Sampling 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 re-

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

pass response is useful when the input signal is band limited

since the resulting output data rate is half that required to con-

vert 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 25a. This signal band can be correctly

converted by selecting the (low pass) ODR = fCLKIN/16 mode, as

shown in Figure 25b. Note that the output data rate is a little

over twice the maximum frequency in the frequency band. Alterna-

tively the band-pass mode can be selected as shown in Figure 25c.

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

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