©2008 CADEKA Microcircuits LLC

www.cadeka.com

11

PRELIMINARY Data Sheet

Recommended Usage

Analog Input

The analog inputs to the CDK2308 is a switched capacitor

track-and-hold amplifier optimized for differential opera-

tion. Operation at common mode voltages at mid sup-

ply is recommended even if performance will be good for

the ranges specified. The CM_EXT pin provides a voltage

suitable as common mode voltage reference. The internal

buffer for the CM_EXT voltage can be switched off, and

driving capabilities can be changed by using the CM_EXT-

BC control input.

Figure 2 shows a simplified drawing of the input network.

The signal source must have sufficiently low output imped-

ance to charge the sampling capacitors within one clock

cycle. A small external resistor (e.g. 22Ω) in series with

each input is recommended as it helps reducing transient

currents and dampens ringing behavior. A small differential

shunt capacitor at the chip side of the resistors may be

used to provide dynamic charging currents and may im-

prove performance. The resistors form a low pass filter

with the capacitor, and values must therefore be deter-

mined by requirements for the application.

Figure 2. Input Configuration

DC-Coupling

Figure 3 shows a recommended configuration for DC-

coupling. Note that the common mode input voltage must

be controlled according to specified values. Preferably, the

CM_EXT output should be used as a reference to set the

common mode voltage.

The input amplifier could be inside a companion chip or

it could be a dedicated amplifier. Several suitable single

ended to differential driver amplifiers exist in the market.

The system designer should make sure the specifications

of the selected amplifier is adequate for the total system,

and that driving capabilities comply with the CDK2308

input specifications.

Figure 3. DC-Coupled Input

Detailed configuration and usage instructions must be

found in the documentation of the selected driver.

AC-Coupling

A signal transformer or series capacitors can be used to

make an AC-coupled input network. Figure 4 shows a

recommended configuration using a transformer. Make

sure that a transformer with sufficient linearity is selected,

CLK_EXT

N-13

+F0

+F2

+F1

+F4

+F

+

Figure 1. Timing Diagram

pF

N-13

N-12

N-11

N-10

N-9

N-8