15

LTC1749

1749f

Input Range

The LTC1749 performance may be optimized by adjusting

the ADC’s input range to meet the requirements of the

application. For lower input frequency applications

(<40MHz), the highest input range of

±1.125V(2.25V)will

provide the best SNR while maintaining excellent SFDR.

For higher input frequencies (>80MHz), a lower input

range will provide better SFDR performance with a reduc-

tion in SNR.

The input range of the ADC is determined as

APPLICATIO S I FOR ATIO

PGA gain. Table 1 shows the input range of the ADC versus

the state of the two pins, PGA and SENSE.

Driving the Encode Inputs

The noise performance of the LTC1749 can depend on the

encode signal quality as much as on the analog input. The

ENC/ENC inputs are intended to be driven differentially,

primarily for immunity from common mode noise sources.

Each input is biased through a 6k resistor to a 2V 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 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

single-ended drive.

SENSE

2V

1V

4.7

µF

10k

1

µF

10k

1749 F06a

LTC1749

SENSE

2V

5V

2.5k

6

4

1, 2

4.7

µF

1

µF

1

µF

10k

0.1

µF

1749 F06b

LTC1749

LT1790-1.25

Figure 6a. 2V Range ADC

Figure 6b. 2V Range ADC with External Reference

Table 1

PGA

INPUT RANGE

COMMENTS

Best Noise, SNR = 71.8dB. Good SFDR, >80dB Up to 100MHz

Improved High Frequency Distortion. SNR = 70.5dB. SFDR > 80dB Up to 250MHz

0

= GND

Reduced Internal Reference Mode with PGA = 0. Provides Similar Input Range as

1

= GND

Smallest Possible Input Span. Useful for Improved Distortion at Very High

Frequencies, But with Reduced Noise Performance. SNR = 69dB

0

2

Adjustable Input Range with Better Noise Performance. SNR = 71.8dB with

1

1.2

Adjustable Input Range with Better High Frequency Distortion. SNR = 70.5dB with