LMH7324_0710 Datasheet, PDF (18/20 Pages) National Semiconductor (TI) – Quad 700 ps High Speed Comparator with RSPECL Outputs

nating resistor, in accordance to their physical dimensions.
Seen from the comparator, the termination resistor must be
connected at the far end of the line. Open connections after
the termination resistor (e.g. to the input of a receiver) must
be as short as possible. The allowed length of such connec-
tions varies with the received transients. The faster the tran-
sients, the shorter the open lines must be to prevent signal
degradation.
PCB LAYOUT CONSIDERATIONS AND COMPONENT
VALUE SELECTION
High frequency designs require that both active and passive
components be selected from those that are specially de-
signed for this purpose. The LMH7324 is fabricated in a 32-
pin LLP package intended for surface mount design. For
reliable high speed design it is highly recommended to use
small surface mount passive components because these
packages have low parasitic capacitance and low inductance
simply because they have no leads to connect them to the
PCB. It is possible to amplify signals at frequencies of several
hundreds of MHz using standard through-hole resistors. Sur-
face mount devices however, are better suited for this pur-
pose. Another important issue is the PCB itself, which is no
longer a simple carrier for all the parts and a medium to in-
terconnect them. The PCB becomes a real component itself
and consequently contributes its own high frequency proper-
ties to the overall performance of the circuit. Good practice
dictates that a high frequency design have at least one ground
plane, providing a low impedance path for all decoupling ca-
pacitors and other ground connections. Care should be given
especially that on-board transmission lines have the same
impedance as the cables to which they are connected. Most
single ended applications have 50Ω impedance (75Ω for
video and cable TV applications). Such low impedance, single
ended microstrip transmission lines usually require much
wider traces (2 to 3 mm) on a standard double sided PCB
board than needed for a ‘normal’ trace. Another important is-
sue is that inputs and outputs should not ‘see’ each other. This
occurs if input and output tracks are routed in parallel over the
PCB with only a small amount of physical separation, partic-
ularly when the difference in signal level is high. Furthermore
components should be placed as flat and low as possible on
the surface of the PCB. For higher frequencies a long lead
can act as a coil, a capacitor or an antenna. A pair of leads
can even form a transformer. Careful design of the PCB min-
imizes oscillations, ringing and other unwanted behavior. For
ultra high frequency designs only surface mount components
will give acceptable results. (For more information see
OA-15).
National suggests the following evaluation board as a guide
for high frequency layout and as an aid in device testing:
Device
Package
Evaluation Board
Ordering ID
LMH7324
SQA32A
551013272
This evaluation board can be shipped when a device sample
request is placed with National Semiconductor.
www.national.com
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