©2008 CADEKA Microcircuits LLC

www.cadeka.com

11

ADVANCE Data Sheet

Recommended Usage

Analog Input

The analog inputs to the CDK1308 is a switched capacitor

track-and-hold amplifier optimized for differential opera-

tion. Operation at common mode voltages at mid supply

is recommended even if performance will be good for the

ranges specified. The CM_EXT pin provides a voltage suit-

able 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 net-

work. The signal source must have sufficiently low output

impedance 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 tran-

sient currents and dampens ringing behavior. A small dif-

ferential shunt capacitor at the chip side of the resistors

may be used to provide dynamic charging currents and

may improve performance. The resistors form a low pass

filter with the capacitor, and values must therefore be de-

termined 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 CDK1308

input specifications.

Figure 3. DC-Coupled Input

Detailed configuration and usage instructions must be

found in the documentation of the selected driver, and

the values given in Figure 3 must be varied according to

the recommendations for the driver.

AC-Coupling

A signal transformer or series capacitors can be used

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

CLK_EXT

N-13

N+1

N+2

N+3

N+4

N+5

N

Figure 1. Timing Diagram

pF