13

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

For the 2.1V input range, the inputs should swing from

phase difference between the inputs.

17pF

24Ω

24Ω

LTC2271

2271 F02

17pF

1.2V

10k

1.2V

10k

1.8pF

1.8pF

10Ω

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

DC level. At higher input frequencies a transmission line

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

resulting in lower A/D distortion.