LTC2204CUK View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2204CUK' PDF : 36 Pages View PDF
LTC2205/LTC2204
APPLICATIONS INFORMATION
PGA Pin
The PGA pin selects between two gain settings for
the ADC front-end. PGA = 0 selects an input range of
2.25VP-P; PGA = 1 selects an input range of 1.5VP-P. The
2.25V input range has the best SNR; however, the distortion
will be higher for input frequencies above 100MHz. For
applications with high input frequencies, the low input
range will have improved distortion; however, the SNR
will be worse by up to approximately 2dB to 6dB. See the
typical performance curves section.
Driving the Encode Inputs
The noise performance of the LTC2205/LTC2204 can depend
on the encode signal quality as much as on the analog input.
The encode inputs are intended to be driven differentially,
primarily for noise immunity from common mode noise
sources. Each input is biased through a 6k resistor to a
1.6V bias. The bias resistors set the DC operating point
for transformer coupled drive circuits and can set the logic
threshold for single-ended drive circuits.
Any noise present on the encode signal will result in
additional aperture jitter that will be RMS summed with
the inherent ADC aperture jitter.
In applications where jitter is critical (high input
frequencies), take the following into consideration:
1. Differential drive should be used.
2. Use as large an amplitude possible. If using transformer
coupling, use a higher turns ratio to increase the
amplitude.
3. If the ADC is clocked with a fixed frequency sinusoidal
signal, filter the encode signal to reduce wideband
noise.
4. Balance the capacitance and series resistance at both
encode inputs such that any coupled noise will appear
at both inputs as common mode noise.
The encode inputs have a common mode range of 1.2V
to 3V. Each input may be driven from ground to VDD for
single-ended drive.
0.1μF
ENCODE
ETC1-1T
INPUT
••
50Ω
100Ω
50Ω
0.1μF
LTC2205/
VDD
LTC2204
ENC+
VDD 1.6V
6k
33pF
ENC–
VDD 1.6V
6k
TO INTERNAL
ADC CLOCK
DRIVERS
22054 F08
Figure 8. Transformer Driven Encode
22054fc
24
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