ADC31RF80 Datasheet, PDF (36/136 Pages) Texas Instruments – 3-GSPS Telecom Receiver and Feedback Device

ADC31RF80
SBAS860 – AUGUST 2017
www.ti.com
Example: f_S = 2949.12 MSPS, Divide-by-4 (LMFS = 8411), K = 16
SYSREF = 2949.12 MSPS / LCM (4 ,64, 16) = 46.08 MHz / N
Operate SYSREF at 2.88 MHz (effectively divide-by-1024, N = 16)
For proper device operation, disable the SYSREF signal after the JESD synchronization is established.
8.3.4 DDC Block
The ADC31RF80 provides a sophisticated on-chip, digital down converter (DDC) block that can be controlled
through SPI register settings and the general-purpose input/output (GPIO) pins. The DDC block supports two
basic operating modes: receiver (RX) mode with single- or dual-band DDC and wide-bandwidth observation
receiver mode.
Figure 93 shows that the ADC channel is followed by two DDC chains consisting of the digital filter along with a
complex digital mixer with a 16-bit numerically-controlled oscillator (NCO). The NCOs allow accurate frequency
tuning within the Nyquist zone prior to the digital filtering. One DDC chain is intended for supporting a dual-band
DDC configuration in receiver mode and the second DDC chain supports the wide-bandwidth output option for
the observation configuration. At any given time, either the single-band DDC, the dual-band DDC, or the
wideband DDC can be enabled. Furthermore, three different NCO frequencies can be selected on that path and
are quickly switched using the SPI or the GPIO pins to enable wide-bandwidth observation in a multi-band
application.
NCO 1,
16 Bits
NCO 2,
16 Bits
NCO 3,
16 Bits
fOUT / 4
IQ data
Real[ ] ^{Wideband Real Output}
GPIO
LPF
2,3
LPF
2
Wideband IQ Output
ADC
3 GSPS
IQ data, 3 GSPS
LPF
N/2
LPF
2
RX1 IQ Output
IQ data
Real[ ]
fOUT / 4
RX1 Real Output ^JESD204B
IQ 3 GSPS
LPF
N/2
LPF
2
RX2 IQ Output
NCO 4,
16 Bits
IQ data
Real[ ]
SYSREF
NOTE: Red traces show SYSREF going to the NCO blocks.
fOUT / 4
Figure 93. DDC Chains Overview
RX2 Real Output
Additionally, the decimation filter block provides the option to convert the complex output back to real format at
twice the decimated, complex output rate. The filter response with a real output is identical to a complex output.
The band is centered in the middle of the Nyquist zone (mixed with f_OUT / 4) based on a final output data rate of
f_OUT.
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