Datasheet

LTC2258-14
LTC2257-14/LTC2256-14
24
225814fc
For more information www.linear.com/LTC2258-14For more information www.linear.com/LTC2258-14
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,
D13) are inverted before the output buffers. The even bits
(D0, D2, D4, D6, D8, D10, D12), OF and CLKOUT are not
affected. This can reduce digital currents in the circuit
board ground plane and reduce digital
noise, particularly
for ver
y small analog input signals.
When there is a very small signal at the input of the A/D
that is centered around mid-scale, 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.
applicaTions inForMaTion
CLKOUT CLKOUT
OF
D13/D0
D12/D0
•
•
•
D2/D0
D1/D0
D0
225814 F15
OF
D13
D12
D2
D1
D0
RANDOMIZER
ON
D13
FPGA
PC BOARD
D12
•
•
•
D2
D1
D0
225814 F16
D0
D1/D0
D2/D0
D12/D0
D13/D0
OF
CLKOUT
LTC2258-14
Figure 15. Functional Equivalent of Digital Output Randomizer Figure 16. Unrandomizing a Randomized Digital
Output Signal