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For this reason driving very high impedance loads is not rec-
ommended.
Although bandwidth goes down with higher values of load re-
sistance, the distortion performance improves and gain in-
creases. The LMH6514 has a common emitter Class A output
stage and minimizing the amount of current swing in the out-
put devices improves distortion substantially.
The LMH6514 output stage is powered through the collectors
of the output transistors. Power for the output stage is fed
through inductors and the reactance of the inductors allows
the output voltage to develop. In Figure 1 the inductors are
shown with a value of 44.4 nH. The value of the inductors
used will be different for different applications. In Figure 1 the
inductors have been chosen to resonate with the ADC and
the load capacitor to provide a weak band pass filter effect.
For broad band applications higher value inductors will allow
for better low frequency operation. However, large valued in-
ductors will reduce high frequency performance, particularly
inductors of small physical sizes like 0603 or smaller. Larger
inductors will tend to perform better than smaller ones of the
same value even for narrow band applications. This is be-
cause the larger inductors will have a lower DC resistance
and less inter-winding capacitance and hence a higher Q and
a higher self resonance frequency. The self resonance fre-
quency should be higher than any desired signal content by
at least a factor of 2. Another consideration is that the power
inductors and the filter inductors need to be placed on the
circuit board such that their magnetic fields do not cause cou-
pling. Mutual coupling of inductors can compromise filter
characteristics and lead to unwanted distortion products.
30042915
FIGURE 7. Bandwidth Changes Due to Different Inductor
Values
30042906
FIGURE 8. Gain vs. External Load
DIGITAL CONTROL
The LMH6514 has eight gain settings covering a range of
42 dB. To avoid undesirable signal transients the LMH6514
should be powered on at the minimum gain state (all logic
input pins at 0V). The LMH6514 has a 3-bit gain control bus
as well as a Latch pin. When the Latch pin is low, data from
the gain control pins is immediately sent to the gain circuit (i.e.
gain is changed immediately). When the Latch pin transitions
high the current gain state is held and subsequent changes
to the gain set pins are ignored. To minimize gain change
glitches multiple gain control pins should not change while the
latch pin is low. In order to achieve the very fast gain step
switching time of 5 ns the internal gain change circuit is very
fast. Gain glitches could result from timing skew between the
gain set bits. This is especially the case when a small gain
change requires a change in state of three or more gain con-
trol pins. If continuous gain control is desired the Latch pin
can be tied to ground. This state is called transparent mode
and the gain pins are always active. In this state the timing of
the gain pin logic transitions should be planned carefully to
avoid undesirable transients.
The LMH6514 was designed to interface with 3.3V CMOS
logic circuits. If operation with 5V logic is required a simple
voltage divider at each logic pin will allow for this. To properly
terminate 100Ω transmission lines a divider with a 66.5Ω re-
sistor to ground and a 33.2Ω series resistor will properly
terminate the line as well as give the 3.3V logic levels. Care
should be taken not to exceed the 3.6V absolute maximum
voltage rating of the logic pins.
EXPOSED PAD LLP PACKAGE
The LMH6514 is packaged in a thermally enhanced package.
The exposed pad is connected to the GND pins. It is recom-
mended, but not necessary, that the exposed pad be con-
nected to the supply ground plane. In any case, the thermal
dissipation of the device is largely dependent on the attach-
ment of this pad. The exposed pad should be attached to as
much copper on the circuit board as possible, preferably ex-
ternal copper. However, it is also very important to maintain
good high speed layout practices when designing a system
board. Please refer to the LMH6514 evaluation board for sug-
gested layout techniques.
Package information is available on the National web site.
http://www.national.com/packaging/folders/sqa16a.html
15 www.national.com
LMH6514