Output Circuitry

The dot clock signals CLK and CLK- are typically the highest

frequency signals present in the workstation. To minimize

problems with EMI, crosstalk, and capacitive loading extra

care should be taken in laying out this area of the PC board.

The ICS1572 is packaged in a 0.3”-wide 20-pin SOIC package.

This permits the clock generator, crystal, and related compo-

nents to be laid out in an area the size of a postage stamp. The

ICS1572 should be placed as close as possible to the RAM-

DAC. The CLK and CLK- pins are running at VHF frequen-

cies; one should minimize the length of PCB trace connecting

them to the RAMDAC so that they don’t become radiators of

RF energy.

At the frequencies that the ICS1572 is capable of, PC board

traces may be long enough to be a significant portion of a

wavelength of that frequency. PC traces for CLK and CLK-

should be treated as transmission lines, not just interconnecting

wires. These lines can take two forms: microstrip and stripline.

A microstrip line is shown below:

Essentially, the microstrip is a copper trace on a PCB over a

ground plane. Typically, the dielectric is G10 glass epoxy. It

differs from a standard PCB trace in that its width is calculated

to have a characteristic impedance. To calculate the charac-

teristic impedance of a microstrip line one must know the width

and thickness of the trace, and the thickness and dielectric

constant of the dielectric. For G10 glass epoxy, the dielectric

of dielectric constant. For G10 propagation, delay is calculated

to be 1.77 ns/ft.

Stripline is the other form a PCB transmission line can take. A

buried trace between ground planes (or between a power plane

and a ground plane) is common in multi-layer boards.

Attempting to create a workstation design without the use of

multi-layer boards would be adventurous to say the least, the

issue would more likely be whether to place the interconnect

on the surface or between layers. The between layer approach

would work better from an EMI standpoint, but would be more

difficult to lay out. A stripline is shown below:

Using 1oz. copper (0.0015” thick) and 0.040” thickness G10,

a 0.010” trace will exhibit a characteristic impedance of 75

Ω

in a stripline configuration.

logical “0.” Worst case input capacitance is 10 pF.

Output circuitry for the ICS1572 is shown in the following

diagram. It consists of a 4/1 current mirror, and two open drain

output FETs along with inverting buffers to alternately enable

each current-sinking driver. Both CLK and CLK- outputs are

connected to the respective CLOCK and CLOCK* inputs of

the RAMDAC with transmission lines and terminated in their

equivalent impedances by the Thevenin equivalent impedances

of R1 and R2 or R1’ and R2’.

ICS1572 Application Information

Output Circuit Considerations for the ICS1572

17