AD9910/PCBZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'AD9910/PCBZ' PDF : 60 Pages View PDF
Auxiliary DAC
An 8-bit auxiliary DAC controls the full-scale output current of
the main DAC (IOUT). An 8-bit code word stored in the
appropriate register map location sets IOUT according to the
following equation:
IOUT
=
86.4
RSET
⎜⎝⎛1 +
CODE
96
⎟⎠⎞
where RSET is the value of the RSET resistor (in ohms) and CODE
is the 8-bit value supplied to the auxiliary DAC (default is 127).
For example, with RSET = 10,000 and CODE = 127, then IOUT =
20.07 mA.
INVERSE SINC FILTER
The sampled carrier data stream is the input to the digital-to-
analog converter (DAC) integrated onto the AD9910. The
DAC output spectrum is shaped by the characteristic sin(x)/x
(or sinc) envelope, due to the intrinsic zero-order hold effect
associated with DAC generated signals. The sinc enveloped can
be compensated for because its shape is well known. This
envelope restoration function is provided by the inverse sinc
filter preceding the DAC. The inverse sinc filter is implemented as
a digital FIR filter. It has a response characteristic that very
nearly matches the inverse of the sinc envelope. The response of
the inverse sinc filter is shown in Figure 28 (with the sinc
envelope for comparison).
The inverse sinc filter is enabled using a bit in the register map.
The filter tap coefficients are given in Table 6. The filter
operates by predistorting the data prior to its arrival at the DAC
in such a way as to compensate for the sinc envelope that
otherwise distorts the spectrum.
When the inverse sinc filter is enabled, it introduces an ~3.0 dB
insertion loss. The inverse sinc compensation is effective for
output frequencies up to approximately 40% of
the DAC sample rate.
Table 6. Inverse Sinc Filter Tap Coefficients
Tap No.
Tap Value
1, 7
−35
2, 6
+134
3, 5
−562
4
+6729
In Figure 28, the sinc envelope introduces a frequency
dependent attenuation that can be as much as 4 dB at the
Nyquist frequency (½ of the DAC sample rate). Without the
inverse sinc filter, the DAC output suffers from the frequency
dependent droop of the sinc envelope. The inverse sinc filter
effectively flattens the droop to within ±0.05 dB as shown in
Figure 29, showing the corrected sinc response with the inverse
sinc filter enabled.
1
SINC
0
AD9910
–1
–2
INVERSE
SINC
–3
–4
0
0.1
0.2
0.3
0.4
0.5
FREQUENCY RELATIVE TO DAC SAMPLE RATE
Figure 28. Sinc and Inverse Sinc Responses
–2.8
–2.9
COMPENSATED RESPONSE
–3.0
–3.1
0
0.1
0.2
0.3
0.4
0.5
FREQUENCY RELATIVE TO DAC SAMPLE RATE
Figure 29. DAC Response with Inverse Sinc Compensation
CLOCK INPUT (REF_CLK)
REF_CLK Overview
The AD9910 supports a number of options for producing the
internal SYSCLK signal (that is, the DAC sample clock) via the
REF_CLK input pins. The REF_CLK input can be driven
directly from a differential or single-ended source, or it can
accept a crystal connected across the two input pins. There is
also an internal phase-locked loop (PLL) multiplier that can be
independently enabled. A block diagram of the REF_CLK
functionality is shown in Figure 30. The various input configu-
rations are controlled by means of the XTAL_SEL pin and
control bits in the CFR3 register. Figure 30 also shows how the
CFR3 control bits are associated with specific functional blocks.
Rev. 0 | Page 23 of 60
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