LTM9002 datasheet(18/34 Pages) LINER

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LTC2157-14/

LTC2156-14/LTC2155-14

21576514fa

CONVERTER OPERATION

The LTC2157-14/LTC2156-14/LTC2155-14 are two-

channel, 14-bit 250Msps/210Msps/170Msps A/D con-

verters that are powered by a single 1.8V supply. The

analog inputs must be driven differentially. The en-

code inputs should be driven differentially for optimal

performance. The digital outputs are double data rate

LVDS. Additional features can be chosen by programming

the mode control registers through a serial SPI port.

ANALOG INPUT

The analog inputs are differential CMOS sample-and-

hold circuits (Figure 2). The inputs must be driven differ-

entially around a common mode voltage set by the VCM

output pin, which is nominally 0.435 • VDD. For the 1.5V

input range, the inputs should swing from VCM – 0.375V

to VCM + 0.375V. There should be 180° phase difference

between the inputs.

The two channels are simultaneously sampled by a

shared encode circuit.

INPUT DRIVE CIRCUITS

Input Filtering

If possible, there should be an RC lowpass filter right at

the analog inputs. This lowpass filter isolates the drive

circuitry from the A/D sample-and-hold switching, and also

limits wide band noise from the drive circuitry. Figure 3

shows an example of an input RC filter. The RC compo-

nent values should be chosen based on the application’s

specific input frequency.

Transformer-Coupled Circuits

Figure 3 shows the analog input being driven by an RF

transformer with the common mode supplied through a

pair of resistors via the VCM pin.

At higher input frequencies a transmission line balun

transformer (Figures 4 and 5) has better balance, resulting

in lower A/D distortion.

2pF

RON

20Ω

RON

20Ω

VDD

LTC2157-14

AIN

21576514 F02

2pF

VDD

AIN

–

ENC–

ENC+

2pF

1.2V

10k

25Ω

4.7Ω

10Ω

0.1μF

10pF

0.1μF

LTC2157-14

0.1μF

T1: MACOM ETC1-1T

21576514 F03

AIN

–

VCM

Figure 2. Equivalent Input Circuit. Only One

of Two Analog Channels Is Shown

Figure 3. Analog Input Circuit Using a Transformer.

Recommended for Input Frequencies from 5MHz to 70MHz

APPLICATIONS INFORMATION

Figure 4. Recommended Front-End Circuit for

Input Frequencies from 15MHz to 150MHz

45Ω

10Ω

4.7Ω

0.1μF

100Ω

0.1μF

T1: MABA

007159-000000

T2: WBC1-1L

21576514 F04

LTC2157-14

AIN

–

VCM

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