LMH7220_0805 Datasheet, PDF (18/20 Pages) National Semiconductor (TI) – High Speed Comparator with LVDS Output

phenomena to create optimal connections to the receiving
part or the terminating resistor, in accordance with their phys-
ical 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 an input of
a receiver) must be as short as possible. The allowed length
of such connections varies with the received transients. The
faster the transients the shorter open lines must be to prevent
signal degradation.
PCB LAYOUT CONSIDERATIONS AND COMPONENT
VALUES SELECTION
High frequency designs require that both active- and passive
components are selected that are specially designed for this
purpose. The LMH7220 is fabricated in two different small
packages intended for surface mount design. For reliable high
speed design it is highly recommended also to use small sur-
face 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. Surface mount
devices however are better suited for this purpose. Another
important issue is the PCB itself, which is no longer a simple
carrier for all the parts and a medium to interconnect them.
The PCB becomes a real component itself and consequently
contributes its own high frequency properties to the overall
performance of the circuit. Practice dictates that a high fre-
quency design at least has one ground plane, providing a low
impedance path for all decoupling capacitors and other
ground connections. Care should be taken especially that on-
board transmission lines have the same impedance as the
cables to which they are connected. Most single ended ap-
plications have 50Ω impedance (75Ω for video and cable TV
applications). On PCBs, 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 issue is
that inputs and outputs shouldn’t ‘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, and particu-
larly 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).
NSC suggests the following evaluation boards as a guide for
high frequency layout and as an aid in device testing and
characterization.
LMH730220 / 551012993-002 Rev A
www.national.com
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