AD9789 Analog Devices, AD9789 Datasheet - Page 44

• Manufacturer Part Number: AD9789
• Description: 14-Bit, 2400 MSPS RF DAC with 4-Channel Signal Processing
• Manufacturer: Analog Devices

Specifications of AD9789

• Resolution (bits): 14bit
• Dac Update Rate: 2.4GSPS
• Dac Settling Time: 13ns
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

AD9789
Table 54 shows recommended sum scale values for each QAM
mapper mode. The criteria used to determine the recommended
sum scale values were MER/EVM measurements and spectral
purity. Because clipping results in impulsive noise, it can be
observed in the output spectrum as a transient increase in the
output noise floor. These sum scale values were chosen such
that the transient increases in the noise floor were minimal.
These tests were completed for one, two, three, and four carrier
outputs at approximately 850 MHz. Because clipping can occur
in the RF chain following the DAC, further verification of these
values should be performed at the system level by adding BER
tests to the sum scale selection criteria.
Table 54. Recommended Sum Scale Values for all QAM
Mapper Modes and Channel Count
QAM
Mode
DVB-C
16-QAM
DVB-C
32-QAM
DVB-C
64-QAM
DVB-C
128-QAM
DVB-C
256-QAM
DOCSIS
64-QAM
DOCSIS
256-QAM
Digital 16× Tunable Band-Pass Filter
The digital band-pass filter works in conjunction with a fixed
16× interpolator (see Figure 88). The 16× interpolation filter
creates 16 images of the baseband signal in the Nyquist band
of the DAC. The digital band-pass filter must then be tuned to
reject the 15 undesired images. The center frequency of the
band-pass filter can be placed anywhere from dc to f
tuning word for the band-pass filter center frequency can be
calculated as follows:
The resulting tuning word is a 16-bit value where the most
significant byte is written to Register 0x1D[7:0] and the least
significant byte is written to Register 0x1C[7:0].
The width of the filter’s stop band is fixed at approximately
f
inferred from Figure 89 to Figure 91, mistuning of the BPF
center frequency can result in unwanted images appearing. Care
DAC
Figure 88. Conceptual Block Diagram of 16× Tunable Band-Pass Filter
/64. The effective FLAT pass band is f
BPF
_
1 Channel
48
54
54
80
54
54
54
Center
_
16
Freq
2 Channels
28
34
34
50
34
34
34
Sum Scale Value (Decimal)

f
f
C
CENTER
f
16
DAC
3 Channels
22
26
26
38
26
26
26
DAC

/64. As can be

2
16

DAC
1
4 Channels
16
20
20
30
20
20
20

/2. The
Rev. A | Page 44 of 76
should be taken to appropriately filter the desired signal with
the interpolation filters prior to the input of the BPF..
Figure 89. Band-Pass Filter Response at 200 MHz, f
–100
–100
–10
Figure 90. Band-Pass Filter Response at 1 GHz, f
–20
–40
–60
–80
–20
–40
–60
–80
–2
–4
–6
–8
Figure 91. Band-Pass Filter Pass-Band Detail, f
0
0
0
0
0
0
0.5
0.5
20
FREQUENCY (MHz)
FREQUENCY (GHz)
FREQUENCY (GHz)
1.0
1.0
40
1.5
1.5
60
DAC
DAC
DAC
= 2.4 GHz
2.0
2.0
= 2.4 GHz
= 2.4 GHz
80