LTC2205 Linear Technology, LTC2205 Datasheet - Page 24

• Manufacturer Part Number: LTC2205
• Description: (LTC2204 / LTC2205) 65Msps/40Msps
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC2205/LTC2204
PGA Pin
The PGA pin selects between two gain settings for
the ADC front-end. PGA = 0 selects an input range of
2.25V
2.25V input range has the best SNR; however, the distor-
tion 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 oper-
ating point for transformer coupled drive circuits and can
set the logic threshold for single-ended drive circuits.
24
P-P
; PGA = 1 selects an input range of 1.5V
ENCODE
INPUT
0.1µF
ETC1-1T
•
•
Figure 8. Transformer Driven Encode
0.1µF
50Ω
50Ω
100Ω
P-P
. The
ENC –
ENC
33pF
+
LTC2205/
LTC2204
Any noise present on the encode signal will result in ad-
ditional aperture jitter that will be RMS summed with the
inherent ADC aperture jitter.
In applications where jitter is critical (high input frequen-
cies), take the following into consideration:
1. Differential drive should be used.
2. Use as large an amplitude possible. If using trans-
3. If the ADC is clocked with a fi xed frequency sinusoidal
4. Balance the capacitance and series resistance at both
The encode inputs have a common mode range of 1.2V
to 3V. Each input may be driven from ground to V
single-ended drive.
V
V
DD
DD
former coupling, use a higher turns ratio to increase the
amplitude.
signal, fi lter the encode signal to reduce wideband
noise.
encode inputs such that any coupled noise will appear
at both inputs as common mode noise.
1.6V
1.6V
6k
6k
V
DD
TO INTERNAL
ADC CLOCK
DRIVERS
22054 F08
DD
22054fb
for