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2270f

LTC2270

CONVERTER OPERATION

The LTC2270 is a low power, two-channel, 16-bit, 20Msps

A/D converter that is powered by a single 1.8V supply. The

analog inputs must be driven differentially. The encode

input can be driven differentially, or single ended for lower

power consumption. The digital outputs can be CMOS,

double data rate CMOS (to halve the number of output

lines), or double data rate LVDS (to reduce digital noise

in the system.) Many 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 should be driven differen-

tially around a common mode voltage set by the VCM1 or

VCM2 output pins, which are nominally VDD/2. For the 2.1V

input range, the inputs should swing from VCM – 525mV

to VCM + 525mV. There should be 180° phase difference

between the inputs.

The two channels are simultaneously sampled by a shared

encode circuit (Figure 2).

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 wideband noise from the drive circuitry. Figure 3

shows an example of an input RC filter. The RC component

values should be chosen based on the application’s input

frequency.

Transformer Coupled Circuits

Figure 3 shows the analog input being driven by an RF

transformer with a center-tapped secondary. The center

tap is biased with VCM, setting the A/D input at its optimal

DC level. At higher input frequencies a transmission line

balun transformer (Figure 4 to Figure 5) has better balance,

resulting in lower A/D distortion.

CSAMPLE

17pF

RON

24Ω

RON

24Ω

VDD

LTC2270

AIN

2270 F02

CSAMPLE

17pF

VDD

AIN

–

ENC–

ENC+

1.2V

10k

1.2V

10k

CPARASITIC

1.8pF

CPARASITIC

1.8pF

10Ω

25Ω

50Ω

0.1μF

AIN

–

12pF

1μF

VCM

LTC2270

ANALOG

INPUT

0.1μF

1:1

T1: MA/COM MABAES0060

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

2270 F03

Figure 2. Equivalent Input Circuit. Only One of the Two

Analog Channels Is Shown

Figure 3. Analog Input Circuit Using a Transformer.

Recommended for Input Frequencies from 1MHz to 40MHz

APPLICATIONS INFORMATION

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