ltc2258cuj-12 Linear Technology Corporation, ltc2258cuj-12 Datasheet - Page 23

• Manufacturer Part Number: ltc2258cuj-12
• Description: 12-bit, 65/40/2 5msps Ultralow Power 1.8v Adcs
• Manufacturer: Linear Technology Corporation

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 exclu-
sive-OR logic operation between the LSB and all other
data output bits. To decode, the reverse operation is ap-
plied—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.
Figure 15. Functional Equivalent of Digital Output Randomizer
D0
RANDOMIZER
ON
CLKOUT
D11
D10
OF
D2
D1
•
•
•
225812 F15
CLKOUT
OF
D11/D0
D10/D0
D2/D0
D1/D0
D0
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 alter-
nate bit polarity mode makes half of the bits transition
high while half of the bits transition low. To fi rst order,
this cancels current fl ow in the ground plane, reducing
the digital noise.
Figure 16. Unrandomizing a Randomized Digital
Output Signal
LTC2257-12/LTC2256-12
PC BOARD
LTC2258-12
CLKOUT
D11/D0
D10/D0
D2/D0
D1/D0
OF
D0
FPGA
LTC2258-12
•
•
•
D11
D10
D2
D1
D0
225812 F15
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225812p