Datasheet

LTC2261-12
LTC2260-12/LTC2259-12
24
226112fc
For more information www.linear.com/LTC2261-12
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 operation 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 randomizer 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 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
D11/D0
D10/D0
•
•
•
D2/D0
D1/D0
D0
116112 F15
OF
D11
D10
D2
D1
D0
RANDOMIZER
ON
D11
FPGA
PC BOARD
D10
•
•
•
D2
D1
D0
116112 F15
D0
D1/D0
D2/D0
D10/D0
D11/D0
OF
CLKOUT
LTC2261-12
Figure 15. Functional Equivalent of Digital Output Randomizer
Figure 16. Unrandomizing a Randomized Digital
Output Signal