AD9957/PCBZ Analog Devices Inc, AD9957/PCBZ Datasheet - Page 26

• Manufacturer Part Number: AD9957/PCBZ
• Description: D/A Converter Evaluation Board
• Manufacturer: Analog Devices Inc
• Series: AgileRF™r

Specifications of AD9957/PCBZ

• Silicon Manufacturer: Analog Devices
• Application Sub Type: Direct Digital Synthesizer
• Kit Application Type: Clock & Timing
• Silicon Core Number: AD9957
• Kit Contents: Board
• Main Purpose: Timing: DDS Modulators
• Embedded: No
• Utilized Ic / Part: AD9957
• Primary Attributes: 14-Bit DAC, 32-Bit Tuning Word Width
• Secondary Attributes: 1GHz, Graphical User Interface
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD9957
In Figure 37, it can be seen that the sinc envelope introduces a
frequency dependent attenuation that can be as much as 4 dB at
the Nyquist frequency (half of the DAC sample rate). Without
the inverse sinc filter, the DAC output also 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 38, which shows the corrected sinc response
with the inverse sinc filter enabled.
OUTPUT SCALE FACTOR (OSF)
In QDUC and interpolating DAC modes, the output amplitude
is controlled using an 8-bit digital multiplier. The 8-bit multiplier
value is called the output scale factor (OSF) and is programmed
via the appropriate control registers. It is available for each of
the eight profiles. The LSB weight is 2
range of 0 to 1.9921875 (2 − 2
factor of 2 to provide a means to overcome the intrinsic loss
through the modulator when operating in the quadrature
modulation mode.
In interpolating DAC mode, the OSF should not be programmed
to exceed unity, as clipping can result. Programming the 8-bit
multiplier to unity gain (0x80) bypasses the stage and reduces
power consumption.
–2.8
–2.9
–3.0
–3.1
Figure 38. DAC Response with Inverse Sinc Compensation
–1
–2
–3
–4
1
0
0
0
INVERSE
COMPENSATED RESPONSE
SINC
SINC
Figure 37. Sinc and Inverse Sinc Responses
FREQUENCY RELATIVE TO DAC SAMPLE RATE
FREQUENCY RELATIVE TO DAC SAMPLE RATE
0.1
0.1
0.2
0.2
−7
). The gain extends to nearly a
−7
0.3
0.3
, which yields a multiplier
0.4
0.4
0.5
0.5
Rev. B | Page 26 of 64
14-BIT DAC
The AD9957 incorporates an integrated 14-bit current-output
DAC. The output current is delivered as a balanced signal using
two outputs. The use of balanced outputs reduces the amount of
common-mode noise at the DAC output, increasing signal-to-
noise ratio. An external resistor (R
DAC_RSET pin and AGND establishes a reference current. The
full-scale output current of the DAC (I
the reference current (see the Auxiliary DAC section that
follows).
Proper attention should be paid to the load termination to keep
the output voltage within the specified compliance range, as
voltages developed beyond this range cause excessive distortion
and can damage the DAC output circuitry.
Auxiliary DAC
The full-scale output current of the main DAC (I
trolled by an 8-bit auxiliary DAC. An 8-bit code word stored in
the appropriate register map location sets I
following equation:
where:
R
CODE is the 8-bit value supplied to the auxiliary DAC (default
is 127).
For example, with R
SET
is the value of the R
I
OUT
=
R
86
SET
4 .
 +


1
SET
= 10,000 and CODE = 127, I
CODE
SET
96
resistor (in ohms).



SET
) connected between the
OUT
) is a scaled version of
OUT
according to the
OUT
OUT
) is con-
= 20.07 mA.
(6)