LTC2205-14(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC2205-14
(Rev.:RevA)

14-Bit, 65Msps ADC

Linear Technology 

LTC2205-14

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 applica-

tions with high input frequencies, the low input range will

have improved distortion; however, the SNR will be worse

by up to approximately 2dB. See the Typical Performance

Characteristics section.

Driving the Encode Inputs

The noise performance of the LTC2205-14 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 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-

former 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

LTC2205-14 VDD

TO INTERNAL

ADC CLOCK

DRIVERS

ENC+

VDD 1.6V

6k

33pF

ENC–

VDD 1.6V

6k

220514 F08

Figure 8. Transformer Driven Encode

220514fa

19

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