LMH7322_14 Datasheet, PDF (27/33 Pages) Texas Instruments – Dual 700 ps High Speed Comparator with RSPECL Outputs

LMH7322
www.ti.com
SNOSAU8H – MARCH 2007 – REVISED MAY 2011
PCB Layout Considerations and Component Value Selection
High frequency designs require that both active and passive components be selected from those that are
specially designed for this purpose. The LMH7322 is fabricated in a 24-pin WQFN 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. 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. Good practice dictates that a high frequency design
have at least one ground plane, providing a low impedance path for all decoupling capacitors 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 issue 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, particularly 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 minimizes 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 board as a guide for high frequency layout and as an aid in device
testing:
LMH7322EVAL
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