AD9267EBZ datasheet(21/24 Pages) AD

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AD9267

Rev. 0 | Page 21 of 24

APPLICATIONS INFORMATION

FILTERING REQUIREMENT

The need for anti-alias protection often requires one or two

octaves for a transition band, which reduces the usable

bandwidth of a Nyquist converter to between 25% and 50% of

the available bandwidth. A CT Σ-Δ converter maximizes the

available signal bandwidth by forgoing the need for an

antialiasing filter because the architecture possesses inherent

antialiasing. Although a high order, sharp cutoff antialiasing

filter may not be necessary because of the unique characteristics

of the architecture, a low order filter may still be required to

precede the ADC for out-of-band signal handling.

Depending on the application and the system architecture, this

low order filter may or may not be necessary. The signal

transfer function (STF) of a continuous time feedforward ADC

usually contains out-of-band peaks. Because these STF peaks

are typically one or two octaves above the pass-band edge, they

are not problematic in applications where the bulk of the signal

energy is in or near the pass band. However, in applications

with large far-out interferers, it is necessary to either add a filter

to attenuate these problematic signals or to allocate some of the

ADC dynamic range to accommodate them.

Figure 43 shows the normalized STF of the AD9267 CT Σ-Δ

converter. The figure shows out-of-band peaking beyond the

band edge of the ADC. Within the 10 MHz band of interest, the

STF is maximally flat with less than 0.1 dB of gain. Maximum

peaking occurs at 60 MHz with 10 dB of gain. To put this into

perspective, for a fixed input power, a 5 MHz in-band-signal

appears at −5 dBFS, a 25 MHz tone appears at −2 dBFS and

60 MHz tone at +5 dBFS. Because the maximum input to the

ADC is −2 dBFS, large out-of-band signals can quickly saturate

the system. This implies that under these conditions, the digital

outputs of the ADC no longer accurately represents the input.

Refer to the Overrange (OR) Condition section for details on

overrange detection and recovery.

–1

–3

–5

1020

3040

5060

708090

100

FREQUENCY (MHz)

Figure 43. STF

Figure 43 shows the gain profile of the AD9267 and this can be

interpreted as the level in which the signal power should be

scaled back to prevent an overload condition. This is the

ultimate trip point and before this point is reached, the in-band

noise (IBN) slowly degrades. As a result, it is recommended that

the low-pass filter be designed to match the profile of Figure 44,

which shows the maximum input signal for a 3 dB degradation

of in-band noise. The input signal is attenuated to allow only

3 dB of noise degradation over frequency.

The noise performance is normalized to a −2 dBFS in-band

signal. The AD9267 STF and NTF are flat within the band of

interest and should result in almost no change in input level and

IBN. Beyond the bandwidth of the AD9267, out-of-band

peaking adds gain to the system, therefore requiring the input

power to be scaled back to prevent in-band noise degradation.

The input power is scaled back to a point where only 3 dB of

noise degradation is allowed, therefore resulting in Figure 44.

–5

–10

–15

–20

–25

2030

405060

7080

100

FREQUENCY (MHz)

CHEBYSHEV II

FILTER RESPONSE

–40°C

+85°C

+25°C

Figure 44. Maximum Input Level for 3 dB Noise Degradation

An example third-order low-pass Chebyshev II type filter is

shown in Figure 45 and the corresponding magnitude vs.

frequency response of the filter is shown in Figure 44.

180nH

390pF

220pF

39pF

Ω AD9267

Σ-Δ

VIN+

VIN–

Figure 45. Third-Order Low-Pass Chebyshev II Filter

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