ICS1572 Application Note
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.
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 75Ω for our
transmission line impedance, a value of 91Ω for R1 and R1’
and a value of 430Ω 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
82Ω for R1 and R1’and 820Ω for R2 and R2’would give us a
characteristic impedance of 74.5Ω 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 82Ω pull-up resistors. As this is
controlled by a 4/1 current mirror, 7 mAinto Iprg should set this
current properly. A510Ω resistor from Vdd to Iprg should work
fine.
ICS1572 Output Circuitry
Resistors Rt and Rt’ are shown as series terminating resistors
at the ICS1572 end of the transmission lines. These are not Great care must be used when evaluating high frequency
required for operation, but may be useful for meeting EMI circuits to achieve meaningful results. The 10 pf input capaci-
requirements. Their intent is to interact with the input capaci- tance and long ground lead of an ordinary scope probe will
tance of the RAMDAC and the distributed capacitance of the make any measurements made with it meaningless. A low
transmission line to soften up rise and fall times and conse- capacitance FET probe with a ground connection directly
quently cut some of the high-order harmonic content that is connected to the shield at the tip will be required. A 1GHz
more likely to radiate RF energy. In actual usage they would bandwidth scope will be barely adequate, try to find a faster
most likely be 10 to 20Ω resistors or possibly ferrite beads.
unit.
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