LTC6945 datasheet(13/24 Pages) LINER

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2271f

LTC2271

APPLICATIONS INFORMATION

CONVERTER OPERATION

The LTC2271 is a low power, 2-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. To minimize the number of data lines

the digital outputs are serial LVDS. Each channel outputs

one bit at a time (1-lane mode), two bits at a time (2-lane

mode) or four bits at a time (4-lane mode). 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

differentially 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.

CSAMPLE

17pF

RON

24Ω

RON

24Ω

VDD

LTC2271

AIN+

2271 F02

CSAMPLE

17pF

VDD

AIN–

ENC–

ENC+

1.2V

10k

1.2V

10k

CPARASITIC

1.8pF

CPARASITIC

1.8pF

10Ω

Figure 2. Equivalent Input Circuit. Only One of Two Analog Channels Is Shown

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 (Figures 4 to 5) has better balance,

resulting in lower A/D distortion.

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