AD7729

–10–

REV. 0

IRxN

QRxN

IRxP

QRxP

10 ... 00

00 ... 00

01 ... 11

ADC CODE

Figure 11. ADC Transfer Function for Differential Operation

IRxN

QRxN

IRxP

QRxP

10 ... 00

00 ... 00

01 ... 11

ADC CODE

Figure 12. ADC Transfer Function for Single-Ended

Operation

Sigma-Delta ADC

The AD7729 receive channels employ a sigma-delta conversion

technique, which provides a high-resolution 15-bit output for

both I and Q channels with system filtering being implemented

on-chip.

The output of the switched-capacitor filter is continuously

sampled at 6.5 MHz (master clock/2), by a charge-balanced

modulator, and is converted into a digital pulse train whose

duty cycle contains the digital information. Due to the high

oversampling rate, which spreads the quantization noise from

band of interest is reduced (Figure 13a). To reduce the quanti-

zation noise still 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 13b).

The digital filter that follows the modulator removes the large

out-of-band quantization noise (Figure 13c), while converting

the digital pulse train into parallel 15-bit-wide binary data. The

15-bit I and Q data, which is in twos complement format, is

made available via a serial port.

QUANTIZATION

NOISE

BAND OF

INTEREST

3.25MHz

a) Effect of High Oversampling Ratio

NOISE

SHAPING

BAND OF

INTEREST

3.25MHz

b) Use of Noise Shaping to Further Improve

SNR

DIGITAL FILTER

CUTOFF FREQUENCY = 100kHz

BAND OF

INTEREST

3.25MHz

c) Use of Digital Filtering to Remove the Out-

of-Band Quantization Noise

Figure 13.

Digital Filter

The digital filters used in the AD7729 receive section carry out

two important functions. Firstly, they remove the out-of-band

quantization noise which is shaped by the analog modulator.

Secondly, they are also designed to perform system level filter-

ing, providing excellent rejection of the neighboring channels.

Digital filtering has certain advantages over analog filtering.

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

process, it can remove noise injected during the conversion

process. Analog filtering cannot do this. Secondly, the digital

filter combines low passband ripple with a steep roll-off, while

also maintaining a linear phase response. This is very difficult to

achieve with analog filters.

However, analog filtering can remove noise superimposed on

the analog signal before it reaches the ADC. Digital filtering

cannot do this and noise peaks riding on signals near full-scale

have the potential to saturate the analog modulator, even

though the average value of the signal is within limits. To allevi-

ate this problem, the AD7729 has overrange headroom built

into the sigma-delta modulator and digital filter which allows

overrange excursions of 100 mV.