LTM9002 datasheet(13/28 Pages) LINER

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215812fa

LTC2158-12

For more information www.linear.com/LTC2158-12

CONVERTER OPERATION

The LTC2158-12 is a two-channel, 12-bit 310Msps

A/D converter 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 1.32V

input range, the inputs should swing from VCM – 0.33V to

VCM + 0.33V. 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

circuitryfromtheA/Dsample-and-holdswitching,andalso

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

VDD

LTC2158-12

AIN+

215812 F02

2pF

VDD

AIN–

ENC–

ENC+

2pF

1.2V

10k

4.7

0.1µF

10pF

0.1µF

LTC2158-12

0.1µF

T1: MACOM ETC1-1T

1:1

215812 F03

AIN+

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

4.7

0.1µF

100

0.1µF

T2: MABA

007159-000000

T1: WBC1-1L

215812 F04

LTC2158-12

AIN+

AIN–

VCM

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