ICS1572 datasheet(18/19 Pages) ICST

18 / 19 page

The ICS1572 is incapable of sourcing current, so Vih must be

set by the ratios of these resistors for each of these lines. R1

and R2 are electrically in parallel from an AC standpoint

because Vdd is bypassed to ground through bypass-capacitor

network Cb. If we picked a target impedance of 75

Ω for our

transmission line impedance, a value of 91

Ω for R1 and R1’

and a value of 430

Ω for R2 and R2’ would yield a Thevinin

equivalent characteristic impedance of 75.1W and a Vih value

of VAA-.873 Volts, a margin of 0.127Volts. This may be

adequate; however, at higher frequencies one must contend

with the 10 pF input capacitance of the RAMDAC. Values of

Ω for R1 and R1’and 820Ω for R2 and R2’ would give us a

characteristic impedance of 74.5

Ω and a Vih value of VAA-.45.

With a .55 Volt margin on Vih, this voltage level might be safer.

To set a value for Vil, we must determine a value for Iprg that

will cause the output FET’s to sink an appropriate current. We

desire Vil to be VAA-1.6 or greater. VAA-2 would seem to be a

safe value. Setting up a sink current of 25 milliamperes would

guarantee this through our 82

Ω pull-up resistors. As this is

controlled by a 4/1 current mirror, 7 mA into Iprg should set this

current properly. A 510

Ω resistor from Vdd to Iprg should work

fine.

Resistors Rt and Rt’ are shown as series terminating resistors

at the ICS1572 end of the transmission lines. These are not

required for operation, but may be useful for meeting EMI

requirements. Their intent is to interact with the input capaci-

tance of the RAMDAC and the distributed capacitance of the

transmission line to soften up rise and fall times and conse-

quently cut some of the high-order harmonic content that is

more likely to radiate RF energy. In actual usage they would

most likely be 10 to 20

Ω resistors or possibly ferrite beads.

Cb is shown as multiple capacitors. Typically, a 22

µF tantalum

should be used with separate .1

µF and 220pf capacitors placed

as close to the pins as possible. This provides low series

inductance capacitors right at the source of high frequency

energy. Rd is used to isolate the circuitry from external sources

of noise. Five to ten ohms should be adequate.

Great care must be used when evaluating high frequency

circuits to achieve meaningful results. The 10 pf input capaci-

tance and long ground lead of an ordinary scope probe will

make any measurements made with it meaningless. A low

capacitance FET probe with a ground connection directly

connected to the shield at the tip will be required. A 1GHz

bandwidth scope will be barely adequate, try to find a faster

unit.

ICS1572 Application Note

ICS1572 Output Circuitry

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