Datasheet

LMH6559

www.ti.com

SNOSA57C –APRIL 2003–REVISED MARCH 2013

PCB Board Layout And Component Selection

Sound practice in the area of high frequency design requires that both active and passive components be used

for the purposes for which they were designed. 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. Surface mount resistors and capacitors are smaller and therefore parasitics are of lower value and

therefore have less influence on the properties of the amplifier. 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 board becomes a real

component itself and consequently contributes its own high frequency properties to the overall performance of

the circuit. Sound practice dictates that a design have at least one ground plane on a pcb which provides 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 - 50Ω for

most applications and 75Ω in case of video and cable TV applications. Such transmission lines usually require

much wider traces on a standard double sided PCB board than needed for a 'normal' trace. Another important

issue is that inputs and outputs must not 'see' each other. This occurs if inputs and outputs are routed together

over the pcb with only a small amount of physical separation, particularly when there is a high differential in

signal level between them. 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 avoids oscillations or other unwanted behaviors. For ultra

high frequency designs only surface mount components will give acceptable results. (for more information see

OA-15 (Literature Number SNOA367).

TI suggests the following evaluation boards as a guide for high frequency layout and as an aid in device testing

and characterization.

Device Package Evaluation Board Part Number

LMH6559MA SOIC CLC730245

LMH6559MAX SOIC CLC730245

LMH6559MF SOT-23 CLC730136

LMH6559MFX SOT-23 CLC730136

These free evaluation boards are shipped when a device sample request is placed with Texas Instruments.

POWER SEQUENCING OF THE LMH6559

Caution should be exercised in applying power to the LMH6559. When the negative power supply pin is left

floating it is recommended that other pins, such as positive supply and signal input should also be left

unconnected. If the ground is floating while other pins are connected the input circuitry is effectively biased to

ground, with a mostly low ohmic resistor, while the positive power supply is capable of delivering significant

current through the circuit. This causes a high input bias current to flow which degrades the input junction. The

result is an input bias current which is out of specification. When using inductive relays in an application care

should be taken to connect first both power connections before connecting the bias resistor to the input.

Product Folder Links: LMH6559