ADC12DJ3200 Datasheet, PDF (35/111 Pages) Texas Instruments – 6.4-GSPS Single Channel or 3.2-GSPS Dual Channel, 12-bit, RF-Sampling Analog-to-Digital Converter (ADC)

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ADC12DJ3200
SLVSD97 – MAY 2017
The specific frequency-phase pair used for each channel is selected through the NCOA[1:0] or NCOB[1:0] input
pins when CMODE () is set to 1. Alternatively, the selected NCO can be chosen through SPI by CSELA for DDC
A and CSELB for DDC B by setting CMODE to 0 (default). The logic table for NCO selection is provided in
Table 8 for both GPIO and SPI selection options.
NCO Selection
NCO 0 using GPIO
NCO 1 using GPIO
NCO 2 using GPIO
NCO 3 using GPIO
NCO 0 using SPI
NCO 1 using SPI
NCO 2 using SPI
NCO 3 using SPI
Table 8. Logic Table for NCO Selection using GPIO or SPI
CMODE
1
1
1
1
0
0
0
0
NCOx1
0
0
1
1
X
X
X
X
NCOx0
0
1
0
1
X
X
X
X
CSELx[1]
X
X
X
X
0
0
1
1
CSELx[0]
X
X
X
X
0
1
0
1
The frequency for each phase accumulator is programmed independently through the FREQAn, FREQBn (n=0 to
3, see ) and, optionally, NCO_RDIV () settings. The phase offset for each accumulator is programmed
independently through the PHASEAn and PHASEBn (n=0 to 3, see ) register settings.
7.3.5.1.3 Basic NCO Frequency Setting Mode
In basic NCO frequency-setting mode (NCO_RDIV = 0x0000), the NCO frequency setting is set by the 32-bit
register value, FREQAn and FREQBn (n = 0 to 3, see ). The NCO frequency can be calculated using Equation 3.
ƒ_(NCO) = FREQA/Bn × 2^–32 × ƒ_(DEVCLK) (n = 0 – 3)
(3)
NOTE
Changing the register setting after the JESD204B interface is running results in non-
deterministic NCO phase. If deterministic phase is required, the JESD204B link must be
re-initialized after changing the register setting.
7.3.5.1.4 Rational NCO Frequency Setting Mode
In basic NCO frequency mode, the frequency step size is very small and many frequencies can be synthesized,
but sometimes an application requires very specific frequencies that fall between two frequency steps. For
example with ƒ_S equal to 2457.6 MHz and a desired ƒ_(NCO) equal to 5.02 MHz the value for NCO_FREQ is
8773085.867. Truncating the fractional portion results in an ƒ_(NCO) equal to 5.0199995 MHz, which is not the
desired frequency.
To produce the desired frequency, the NCO_RDIV parameter is used to force the phase accumulator to arrive at
specific frequencies without error. First, select a frequency step size (ƒ_(STEP)) that is appropriate for the NCO
frequency steps required. The typical value of ƒ_(STEP) is 10 kHz. Next, program the NCO_RDIV value according
to Equation 4.
NCO _ RDIV
§
¨¨©
¦_{(DEVCLK )}
¦_(STEP)
·
¸¸¹
128
(4)
The result of Equation 4 must be an integer value. If the value is not an integer, adjust either of the parameters
until the result in an integer value.
For example, select a value of 1920 for NCO_RDIV.
NOTE
NCO_RDIV values larger than 8192 can degrade the NCO SFDR performance and are
not recommended.
Now use Equation 5 to calculate the NCO_FREQ register value.
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