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LTC1744

1744f

APPLICATIO S I FOR ATIO

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 frequen-

cies) take the following into consideration:

1. Differential drive should be used.

2. Use as large an amplitude as possible; if transformer

coupled use a higher turns ratio to increase the

amplitude.

3. If the ADC is clocked with a sinusoidal signal, filter the

encode signal to reduce wideband noise.

4. Balance the capacitance and series resistance at both

encode inputs so that any coupled noise will appear at

both inputs as common mode noise.

The encode inputs have a common mode range of 1.8V to

VDD. Each input may be driven from ground to VDD for

single-ended drive.

1744 F08a

ENC

VTHRESHOLD = 2V

ENC

0.1

µF

LTC1744

1744 F08b

ENC

130

Ω

3.3V

130

Ω

MC100LVELT22

LTC1744

Ω

Figure 8a. Single-Ended ENC Drive,

Not Recommended for Low Jitter

Figure 8b. ENC Drive Using a CMOS-to-PECL Translator

Maximum and Minimum Encode Rates

The maximum encode rate for the LTC1744 is 50Msps. For

the ADC to operate properly the ENCODE signal should

have a 50% (

±5%) duty cycle. Each half cycle must have

at least 9.5ns for the ADC internal circuitry to have enough

settling time for proper operation. Achieving a precise

50% duty cycle is easy with differential sinusoidal drive

using a transformer or using symmetric differential logic

such as PECL or LVDS. When using a single-ended

ENCODE signal asymmetric rise and fall times can result

in duty cycles that are far from 50%.

At sample rates slower than 50Msps the duty cycle can

vary from 50% as long as each half cycle is at least 9.5ns.

The lower limit of the LTC1744 sample rate is determined

by droop of the sample-and-hold circuits. The pipelined

architecture of this ADC relies on storing analog signals on

small valued capacitors. Junction leakage will discharge

the capacitors. The specified minimum operating fre-

quency for the LTC1744 is 1Msps.

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