AD9267EBZ datasheet(13/24 Pages) AD

13 / 24 page

AD9267

Rev. 0 | Page 13 of 24

THEORY OF OPERATION

The AD9267 uses a continuous time Σ-Δ modulator to convert

the analog input to a digital word. The modulator consists of a

continuous time loop filter preceding a quantizer (see Figure 27),

which samples at fMOD = 640 MSPS. This produces an oversam-

pling ratio (OSR) of 32 for a 10 MHz input bandwidth. The output

of the quantizer is fed back to a DAC that ideally cancels the

input signal. The incomplete input cancellation residue is filtered

by the loop filter and is used to form the next quantizer sample.

H(f)

LOOP FILTER

QUANTIZER

–

ADC

MODULATOR

Figure 27. Σ-Δ Modulator Overview

The quantizer produces a nine-level digital word. The quantiza-

tion noise is spread uniformly over the Nyquist band (see

Figure 28) but the feedback loop causes the quantization noise

present in the nine-level output to have a nonuniform spectral

shape. This noise shaping technique (see Figure 29) pushes the

in-band noise out of band; therefore, the amount of quantiza-

tion noise in the frequency band of interest is minimal.

QUANTIZATION NOISE

fMOD/2

BAND OF INTEREST

Figure 28. Quantization Noise

NOISE SHAPING

BAND OF INTEREST

fMOD/2

Figure 29. Noise Shaping

ANALOG INPUT CONSIDERATIONS

The continuous time modulator removes the need for an anti-

alias filter at the input to the AD9267. A discrete time converter

aliases signals around the sample clock frequency and its

multiples to the band of interest (see Figure 30). An external

antialias filter is needed to reject these signals.

DESIRED

INPUT

UNDESIRED

SIGNAL

ADC

fS/2

Figure 30. Discrete Time Converter

In contrast, the continuous time Σ-Δ modulator used within the

AD9267 has inherent antialiasing. The antialiasing property

results from sampling occurring at the output of the loop filter

(see Figure 31), and thus aliasing occurs at the same point in the

loop as quantization noise is injected; aliases are shaped by the

same mechanism as quantization noise. The quantization noise

transfer function, NTF(f), has zeros in the band of interest and

in all alias bands because NTF(f) is a discrete time transfer

function, whereas the loop filter transfer function, LF(f),

introduces poles only in the band of interest because LF(f) is a

continuous time transfer function. The signal transfer function,

being the product of NTF(f) and LF(f), only has zeros in all

alias bands and therefore suppresses all aliases.

LF(f)

H(z)

QUANTIZATION

NOISE

INP UT

OUTPUT

LOOP FILTER

fMOD

NTF(f)

L F (f)

Figure 31. Continuous Time Converter

Input Common Mode

The analog inputs of the AD9267 are not internally dc biased.

In ac-coupled applications, the user must provide this bias

externally. Setting the device such that VCM = AVDD is

recommended for optimum performance. The analog inputs

are 500 Ω resistors and the internal reference loop aims to

develop 0.5 V across each input resistor (see Figure 32). With

0 V differential input, the driver sources 1 mA into each

analog input.

TO LOOP FILTER

STAGE 2

DAC

AVDD – 0.5V

500

Ω

500

Ω

VIN+x

VIN–x

FROM QUANTIZER

2.3V

1.8V

1.3V

2.3V

1.8V

1.3V

Figure 32. Input Common Mode

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