LMH7322SQ National Semiconductor Corporation, LMH7322SQ Datasheet - Page 20

• Manufacturer Part Number: LMH7322SQ
• Description: The ADC14V155 is a high-performance CMOS analog-to-digital converter with LVDS outputs. It is capable of converting analog input signals into 14-Bit digital words at rates up to 155 Mega Samples Per Second (MSPS). Data leaves the chip in a DDR (Dual
• Manufacturer: National Semiconductor Corporation

www.national.com
These cables have a characteristic impedance determined by
their geometric parameters. Widely used impedances for the
coaxial cable are 50Ω and 75Ω. Twisted pair cables have
impedances of about 120Ω to 150Ω.
Other types of transmission lines are the strip line and the
micro strip line. These last types are used on PCB boards.
They have the characteristic impedance dictated by the phys-
ical dimensions of a track placed over a metal ground plane
(see Figure 21).
Differential Microstrip
Line The transmission line which is ideally suited for comple-
mentary signals is the differential microstrip line. This is a
double microstrip line with a narrow space in between. This
means both lines have strong coupling and this determines
the characteristic impedance. The fact that they are routed
above a copper plane does not affect differential impedance,
only CM-capacitance is added. Each of the structures above
has its own geometric parameters, so for each structure there
is different formula to calculate the right impedance. For cal-
culations on these transmission lines visit the National web-
site or order RAPIDESIGNER. At the end of the transmission
line there must be a termination having the same impedance
as that of the transmission line itself. It does not matter what
impedance the line has, if the load has the same value no
reflections will occur. When designing a PCB board with
transmission lines on it, space becomes an important item
especially on high density boards. With a single microstrip
line, line width is fixed for given impedance and a board ma-
terial. Other line widths will result in different impedances.
Advantages of Differential MicrostripLines
Impedances of transmission lines are always dictated by their
geometric parameters. This is also true for differential mi-
crostrip lines. Using this type of transmission line, the distance
FIGURE 21. PBC Lines
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of the track determines the resulting impedance. So, if the
PCB manufacturer can produce reliable boards with low track
spacing the track width for a given impedance is also small.
The wider the spacing, the wider tracks are needed for a spe-
cific impedance. For example two tracks of 0.2 mm width and
0.1 mm spacing have the same impedance as two tracks of
0.8 mm width and 0.4 mm spacing. With high-end PCB pro-
cesses, it is possible to design very narrow differential mi-
crostrip transmission lines. It is desirable to use these to
create optimal connections to the receiving part or the termi-
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 an 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 LMH7322 is fabricated in a 24-
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).
NSC suggests the following evaluation board as a guide for
high frequency layout and as an aid in device testing UL-
V94V-0
551013148-001 Rev A