LMH7220_14 Datasheet, PDF (26/34 Pages) Texas Instruments – LMH7220 High Speed Comparator with LVDS Output

LMH7220
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
www.ti.com
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. For reliable high speed design it is highly recommended also 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. Practice dictates that a
high frequency 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 applications 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 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 [SNOA367]).
TI 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
26
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