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2270f

LTC2270

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, D13,

D15) are inverted before the output buffers. The even bits

(D0, D2, D4, D6, D8, D10, D12, D14), 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 1’s and mostly 0’s. 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. This cancels current flow in

the ground plane, reducing the digital noise.

The digital output is decoded at the receiver by inverting

the odd bits (D1, D3, D5, D7, D9, D11, D13, D15.) The

alternate bit polarity mode is independent of the digital

output randomizer – either, both or neither function can

be on at the same time. The alternate bit polarity mode is

enabled by serially programming mode control register A4.

APPLICATIONS INFORMATION

CLKOUT

D15/D0

D14/D0

D2/D0

D1/D0

2270 F15

D15

D14

RANDOMIZER

D15

FPGA

PC BOARD

D14

2270 F16

D1/D0

D2/D0

D14/D0

D15/D0

CLKOUT

LTC2270

Figure 15. Functional Equivalent of Digital Output Randomizer

Figure 16. Decoding a Randomized Digital Output Signal

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