AD7722

–13–

REV. 0

CIRCUIT DESCRIPTION

The AD7722 ADC employs a sigma-delta conversion technique

that converts the analog input into a digital pulse train. The

analog input is continuously sampled by a switched capacitor

modulator at twice the rate of the clock input frequency, 2

×

the 1’s density of the bit stream at the output of the sigma-delta

modulator. The modulator outputs a bit stream at a data rate

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

band of interest is reduced (Figure 24a). To reduce the quanti-

zation noise further, 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 provides three main

functions. The filter performs sophisticated averaging on the

1 bit samples from the output of the modulator, while removing

the large out of band quantization noise (Figure 24c). Lastly the

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 conver-

sion. Second, the digital filter combines low pass-band ripple

with a steep roll off, while also maintaining a linear phase

response.

BAND OF INTEREST

DIGITAL FILTER CUTOFF FREQUENCY

WHICH EQUALS 97.65kHz (12.5MHz)

BAND OF INTEREST

QUANTIZATION NOISE

BAND OF INTEREST

NOISE SHAPING

a.

b.

c.

Figure 24. Sigma-Delta ADC

The AD7722 employs two Finite Impulse Response (FIR) filters

in series. The first filter is a 384 tap filter that samples the output

and decimates by 2. The implementation of this filter architec-

ture results in a filter with a group delay of 42 conversions (84

conversions for settling to a full-scale step).

The digital filter provides 6 dB of attenuation at a frequency

of 12.5 MHz, the digital filter has a pass-band frequency of

90.625 kHz, a cutoff frequency is 96.92 kHz and stop-band

frequency of 104.6875 kHz.

Due to the sampling nature of the digital filter, the filter does

not provide any rejection at integer multiples of its input

sampling frequency. The filter response in Figure 25a shows the

unattenuated frequency bands occurring at n

× f

n = 1, 2, 3. . . . At these frequencies, there are frequency bands

± f

either side of n

× f

output. Out of band signals coincident with any of the filter

images are aliased into the pass band. However, due to the

AD7722’s high oversampling ratio, these bands occupy only a

small fraction of the spectrum, and most broadband noise is

filtered. This means that the antialias filtering requirements in

front of the AD7722 are considerably reduced versus a conven-

tional converter with no on-chip filtering. Figure 25b shows the

frequency response of an antialias filter. With a –3 dB corner

Depending on the application, however, it may be necessary to

provide additional antialias filtering prior to the AD7722 to

eliminate unwanted signals from the frequency bands the digital

filter passes. It may also be necessary in some applications to

provide analog filtering in front of the AD7722 to ensure that

differential noise signals outside the band of interest do not

saturate the analog modulator.

0dB

Figure 25a. Digital Filter Frequency Response

OUTPUT

DATA RATE

0dB

ANTIALIAS FILTER

RESPONSE

REQUIRED

ATTENUATION

Figure 25b. Frequency Response of Antialias Filter