AD9261 Analog Devices, AD9261 Datasheet - Page 14

• Manufacturer Part Number: AD9261
• Description: 16-Bit, 10 MHz Bandwidth, 30 MSPS to 160 MSPS Continuous Time Sigma-Delta ADC
• Manufacturer: Analog Devices

Specifications of AD9261

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 160MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: 2 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD9261
THEORY OF OPERATION
The AD9261 uses a continuous time Σ-Δ modulator to convert
the analog input to a digital word. The digital word is processed
by the decimation filter and rate-adjusted by the sample rate
converter (see Figure 29). The modulator consists of a continuous
time loop filter preceding a quantizer that samples at f
640 MSPS. This produces an oversampling 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 incom-
plete input cancellation residue is filtered by the loop filter and
is used to form the next quantizer sample.
The quantizer produces a nine-level digital word. The quantization
noise is spread uniformly over the Nyquist band (see Figure 30),
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 31) pushes the in-band
noise out of band; therefore, the amount of quantization noise
in the frequency band of interest is minimal.
The digital decimation filter that follows the modulator removes
the large out-of-band quantization noise (see Figure 32), while
also reducing the data rate from f
PLL is enabled, the sample rate converter generates samples at
the same frequency as the input clock frequency. If the internal
PLL is disabled, the sample rate converter can be programmed
to give an output frequency that is a divide ratio of the modulator
clock. The sample rate converter is designed to attenuate images
outside the band of interest (see Figure 33).
+
–
BAND OF INTEREST
BAND OF INTEREST
LOOP FILTER
MODULATOR
H(f)
Figure 29. Σ-Δ Modulator Overview
Figure 30. Quantization Noise
Figure 31. Noise Shaping
QUANTIZATION NOISE
QUANTIZER
ADC
NOISE SHAPING
MOD
DECIMATION
to f
FILTER
MOD
/16. If the internal
SAMPLE RATE
CONVERTER
SRC
f
f
MOD
MOD
MOD
/2
/2
=
Rev. 0 | Page 14 of 28
ANALOG INPUT CONSIDERATIONS
The continuous time modulator removes the need for an anti-
alias filter at the input to the AD9261. A discrete time converter
aliases signals around the sample clock frequency and its multiples
to the band of interest (see Figure 34). Therefore, an external
antialias filter is needed to reject these signals.
In contrast, the continuous time Σ-Δ modulator used within the
AD9261 has inherent antialiasing. The antialiasing property
results from sampling occurring at the output of the loop filter
(see Figure 35), 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), is a continuous
time transfer function, which introduces poles only in the band
of interest. The signal transfer function, being the product of
NTF(f) and LF(f), only has zeros in alias bands and therefore
suppresses all aliases.
INP UT
LOOP FILTER
BAND OF INTEREST
BAND OF INTEREST
LF(f)
H(z)
DESIRED
INPUT
Figure 32. Digital Filter Cutoff Frequency
Figure 35. Continuous Time Converter
Figure 34. Discrete Time Converter
Figure 33. Sample Rate Converter
f
MOD
DIGITAL FILTER CUTOFF FREQUENCY
f
UNDESIRED
OUT
QUANTIZATION
SIGNAL
/2
f
NOISE
f
MOD
S
/32
ADC
OUTPUT
NTF(f)
L F (f)
f
OUT
f
S
/2
f
f
MOD
MOD
f
MOD
f
MOD
/16
/16
f