LTC2217 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2217' PDF : 32 Pages View PDF
APPLICATIONS INFORMATION
PC BOARD
CLKOUT
FPGA
LTC2217
OF
D15 ⊕ D0
D14 ⊕ D0
D2 ⊕ D0
D1 ⊕ D0
D0
D15
D14
•
•
•
D2
D1
D0
LTC2217
Internal Dither
The LTC2217 is a 16-bit ADC with a very linear transfer
function; however, at low input levels even slight imperfec-
tions in the transfer function will result in unwanted tones.
Small errors in the transfer function are usually a result
of ADC element mismatches. An optional internal dither
mode can be enabled to randomize the input location on
the ADC transfer curve, resulting in improved SFDR for
low signal levels.
As shown in Figure 15, the output of the sample-and-hold
amplifier is summed with the output of a dither DAC. The
dither DAC is driven by a long sequence pseudo-random
number generator; the random number fed to the dither DAC
is also subtracted from the ADC result. If the dither DAC
is precisely calibrated to the ADC, very little of the dither
signal will be seen at the output. The dither signal that does
leak through will appear as white noise. The dither DAC is
calibrated to result in typically less than 0.5dB elevation
in the noise floor of the ADC as compared to the noise
floor with dither off, when a suitable input termination is
provided (see Demo Board schematic DC996B).
2217 F14
Figure 14. Descrambling a Scrambled Digital Output
LTC2217
AIN+
ANALOG
S/H
INPUT
AMP
AIN–
16-BIT
PIPELINED
ADC CORE
DIGITAL
SUMMATION
OUTPUT
DRIVERS
CLKOUT
OF
D15
•
•
•
D0
CLOCK/DUTY
CYCLE
CONTROL
PRECISION
DAC
MULTIBIT DEEP
PSEUDO-RANDOM
NUMBER
GENERATOR
ENC + ENC –
2217 F15
DITH
DITHER ENABLE
HIGH = DITHER ON
LOW = DITHER OFF
Figure 15. Functional Equivalent Block Diagram of Internal Dither Circuit
2217f
25
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