參數資料
型號: DC1370A-K
廠商: Linear Technology
文件頁數: 17/34頁
文件大?。?/td> 0K
描述: BOARD DEMO 40MSPS LTC2257-12
軟件下載: QuikEval II System
設計資源: DC1370A Design Files
標準包裝: 1
系列: *
相關產品: DC718C-ND - DEMO QUIKEVAL-II DATA
LTC2258-12
LTC2257-12/LTC2256-12
24
225812fd
For more information www.linear.com/LTC2258-12
Digital Output Randomizer
Interference from the A/D digital outputs is sometimes
unavoidable.Digitalinterferencemaybefromcapacitiveor
inductive coupling or coupling through the ground plane.
Even a tiny coupling factor can cause unwanted tones
in the ADC output spectrum. By randomizing the digital
output before it is transmitted off chip, these unwanted
tones can be randomized which reduces the unwanted
tone amplitude.
The digital output is “randomized” by applying an
exclusive-OR logic operation between the LSB and all
other data output bits. To decode, the reverse opera-
tion is applied—an exclusive-OR operation is applied
between the LSB and all other bits. The LSB, OF and
CLKOUT outputs are not affected. The output random-
izer is enabled by serially programming mode control
register A4.
Alternate Bit Polarity
Another feature that reduces digital feedback on the circuit
board is the alternate bit polarity mode. When this mode
is enabled, all of the odd bits (D1, D3, D5, D7, D9, D11)
are inverted before the output buffers. The even bits (D0,
D2, D4, D6, D8, D10), OF and CLKOUT are not affected.
This can reduce digital currents in the circuit board ground
plane and reduce digital noise, particularly for very small
analog input signals.
When there is a very small signal at the input of the A/D
thatiscenteredaroundmidscale,thedigitaloutputstoggle
between mostly 1s and mostly 0s. This simultaneous
switchingofmostofthebitswillcauselargecurrentsinthe
ground plane. By inverting every other bit, the alternate bit
polarity mode makes half of the bits transition high while
half of the bits transition low. To first order, this cancels
currentflowinthegroundplane,reducingthedigitalnoise.
applicaTions inForMaTion
CLKOUT
OF
D11/D0
D10/D0
D2/D0
D1/D0
D0
225812 F15
OF
D11
D10
D2
D1
D0
RANDOMIZER
ON
D11
FPGA
PC BOARD
D10
D2
D1
D0
225812 F15
D0
D1/D0
D2/D0
D10/D0
D11/D0
OF
CLKOUT
LTC2258-12
Figure 15. Functional Equivalent of Digital Output Randomizer
Figure 16. Unrandomizing a Randomized Digital
Output Signal
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