LTC2257-12_15 Datasheet, PDF (24/34 Pages) Linear Technology – 12-Bit, 65/40/25Msps Ultralow Power 1.8V ADCs

LTC2258-12
LTC2257-12/LTC2256-12
Applications Information
Digital Output Randomizer
Interference from the A/D digital outputs is sometimes
unavoidable. Digital interference may be from capacitive or
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
that is centered around midscale, the digital outputs toggle
between mostly 1s and mostly 0s. This simultaneous
switching of most of the bits will cause large currents in the
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
current flow in the ground plane, reducing the digital noise.
CLKOUT
OF
D11
D10
D2
RANDOMIZER
ON
D1
CLKOUT
OF
D11/D0
D10/D0
•
•
•
D2/D0
D1/D0
D0
D0
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Figure 15. Functional Equivalent of Digital Output Randomizer
PC BOARD
CLKOUT FPGA
OF
D11/D0
D10/D0
LTC2258-12
D2/D0
D1/D0
D0
D11
D10
•
•
•
D2
D1
D0
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Figure 16. Unrandomizing a Randomized Digital
Output Signal
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