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LTC2201

2201f

Digital Output Randomizer

Interference from the ADC digital outputs is sometimes

unavoidable. Interference from the digital outputs may be

from capacitive or inductive coupling or coupling through

the ground plane. Even a tiny coupling factor can result in

discernible unwanted tones in the ADC output spectrum.

By randomizing the digital output before it is transmitted

off chip, these unwanted tones can be randomized, trading

a slight increase in the noise ﬂoor for a large reduction in

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 applied;

that is, an exclusive-OR operation is applied between the

LSB and all other bits. The LSB, OF and CLKOUT output

are not affected. The output Randomizer function is active

when the RAND pin is high.

CLKOUT

D15/D0

D14/D0

D2/D0

D1/D0

RAND = HIGH,

SCRAMBLE

ENABLED

D14

D15

CLKOUT

RAND

2201 F10

LTC2201

Figure 10. Functional Equivalent of Digital Output Randomizer

Output Driver Power

Separate output power and ground pins allow the output

drivers to be isolated from the analog circuitry. The power

supply for the digital output buffers, OVDD, should be tied

to the same power supply as for the logic being driven. For

example, if the converter is driving a DSP powered by a

1.8V supply, then OVDD should be tied to that same 1.8V

supply. In CMOS mode OVDD can be powered with any

logic voltage up to 3.6V. OGND can be powered with any

voltage from ground up to 1V and must be less than OVDD.

The logic outputs will swing between OGND and OVDD.

APPLICATIONS INFORMATION

Figure 11. Descrambling a Scrambled Digital Output

D14

D15

PC BOARD

FPGA

CLKOUT

D15/D0

D14/D0

D2/D0

D1/D0

2201 F11

LTC2201

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