LT5575 datasheet(19/34 Pages) LINER

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21576514fa

LTC2157-14/

LTC2156-14/LTC2155-14

APPLICATIONS INFORMATION

Figure 5. Recommended Front-End Circuit for

Input Frequencies from 150MHz to 900MHz

Figure 6. Front-End Circuit Using a High

Speed Differential Amplifier

Amplifier Circuits

Figure 6 shows the analog input being driven by a high

speed differential amplifier. The output of the amplifier is

AC coupled to the A/D so the amplifier’s output common

mode voltage can be optimally set to minimize distortion.

At very high frequencies an RF gain block will often have

lower distortion than a differential amplifier. If the gain

block is single-ended, then a transformer circuit (Figures

3 and 5) should convert the signal to differential before

driving the A/D. The A/D cannot be driven single-ended.

Reference

The LTC2157-14/LTC2156-14/LTC2155-14 has an internal

1.25V voltage reference. For a 1.5V input range with in-

ternal reference, connect SENSE to VDD. For a 1.5V input

range with an external reference, apply a 1.25V reference

voltage to SENSE (Figure 7).

Encode Input

The signal quality of the encode inputs strongly affects

the A/D noise performance. The encode inputs should

be treated as analog signals—do not route them next to

digital traces on the circuit board.

The encode inputs are internally biased to 1.2V through

10k equivalent resistance (Figure 8). If the common mode

of the driver is within 1.1V to 1.5V, it is possible to drive

the encode inputs directly. Otherwise a transformer or

coupling capacitors are needed (Figures 9 and 10). The

maximum (peak) voltage of the input signal should never

exceed VDD +0.1V or go below –0.1V.

4.7Ω

50Ω

0.1μF

AIN

–

0.1μF

3pF

VCM

LTC2157-14

21576514 F06

INPUT

0.1μF

45Ω

10Ω

100Ω

4.7Ω

0.1μF

T1: MABA

007159-000000

21576514 F05

LTC2157-14

AIN

–

VCM

Figure 7. Reference Circuit

Figure 8. Equivalent Encode Input Circuit

VDD

LTC2157-14

21576514 F08

1.2V

10k

ENC+

ENC–

SCALER/

BUFFER

VREF

2.2μF

SENSE

1.25V

LTC2157-14

21576514 F07

5Ω

ADC

REFERENCE

SENSE

DETECTOR

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