AD9262EBZ datasheet(29/33 Pages) AD

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AD9262

Rev. A | Page 28 of 32

APPLICATIONS INFORMATION

FILTERING REQUIREMENT

The need for antialias protection often requires one or two

octaves for a transition band, which reduces the usable band-

width of a Nyquist converter to between 25% and 50% of the

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

ble signal bandwidth by forgoing the need for an anti-aliasing

filter because the architecture possesses inherent anti-aliasing.

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 trans-

fer 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 61 shows the normalized STF of the AD9262 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 represent the input.

See the Overrange (OR) Condition section for details on over-

range detection and recovery.

FREQUENCY (MHz)

102030405060708090

100

–1

–3

–5

Figure 61. STF

Figure 61 shows the gain profile of the AD9262, and this can be

interpreted as the level at which the signal power should be

scaled back to prevent an overload condition. This is the ulti-

mate 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 62,

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 AD9262 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 AD9262, 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 the response

shown in Figure 62.

–25

–20

–15

–10

–5

1020

3040

5060

708090

100

FREQUENCY (MHz)

+25°C

+85°C

–40°C

CHEBYSHEVII

FILTER RESPONSE

Figure 62. Maximum Input Level for 3 dB Noise Degradation

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

shown in Figure 63. Table 24 summarizes the components

and manufacturers used to build the circuit.

1kΩ AD9262

CT-Σ-Δ

VIN+

VIN–

180nH

390pF

180nH

390pF

18pF

150pF

Figure 63. Third-Order, Low-Pass Chebyshev II Filter

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