AD9262 Analog Devices, AD9262 Datasheet - Page 28

• Manufacturer Part Number: AD9262
• Description: 16-Bit, 2.5 MHz/5 MHz/10 MHz, 30 MSPS to 160 MSPS Dual Continuous Time Sigma-Delta ADC
• Manufacturer: Analog Devices

Specifications of AD9262

• Resolution (bits): 16bit
• # Chan: 2
• Sample Rate: 160MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: 2 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD9262
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.
15
13
11
–1
–3
–5
9
7
5
3
1
0
10
20
30
Figure 61. STF
FREQUENCY (MHz)
40
50
60
70
80
90
100
Rev. A | Page 28 of 32
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.
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.
–10
–15
–20
–25
–5
Figure 62. Maximum Input Level for 3 dB Noise Degradation
18pF
5
0
C1
0
Figure 63. Third-Order, Low-Pass Chebyshev II Filter
10
20
180nH
180nH
390pF
390pF
L1
C2
L1
C2
CHEBYSHEVII
FILTER RESPONSE
30
FREQUENCY (MHz)
40
C3
150pF
50
+85°C
VIN+
VIN–
60
–40°C
1kΩ
70
+25°C
AD9262
80
CT-Σ-Δ
90
100