LMH6515 Datasheet, PDF (13/18 Pages) National Semiconductor (TI) – 600 MHz, Digital Controlled, Variable Gain Amplifier

input common mode of 2.0V will allow an 8 V_PP maximum
input signal. The trade off for input signal swing is that as the
input common mode is shifted away from the 1.4V internal
bias point the distortion performance will suffer slightly.
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FIGURE 3. Single Ended Input
(Note capacitor on grounded input)
OUTPUT CHARACTERISTICS
The LMH6515 has the option of two different output configu-
rations. The LMH6515 is an open collector topology. As
shown in Figure 8 each output has an on chip 200Ω pull up
resistor. In addition there is an internal 400Ω resistor between
the two outputs. This results in a 200Ω or a 400Ω differential
load in parallel with the external load. The 400Ω option is the
high gain option and the 200Ω provides for less gain. The
200Ω configuration is recommended unless more gain is re-
quired.
The output common mode of the LMH6515 must be set by
external components. Most applications will benefit from the
use of inductors on the output stage. In particular, the 400Ω
option, as shown in Figure 9, will require inductors in order to
be able to develop an output voltage. The 200Ω option as
shown in Figure 10 or Figure 11 will also require inductors
since the voltage drop due to the on chip 200Ω resistors will
saturate the output transistors. It is also possible to use re-
sistors and high voltage power supplies to set the output
common mode. This operation is not recommended, unless
it is necessary to DC couple the output. If DC coupling is re-
quired the input common mode and output common mode
voltages must be taken into account.
Maximum bandwidth with the LMH6515 is achieved by using
the low gain, low impedance output option and using a low
load resistance. With an effective load of 67Ω a bandwidth of
nearly 1 GHz can be realized. As the effective resistance on
the output stage goes up the capacitance of the board traces
and amplifier output stage limit bandwidth in a roughly linear
fashion. At an output impedance of 100Ω the bandwidth is
down to 600 MHz, and at 200Ω the bandwidth is 260 MHz.
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 LMH6515 has a common emitter Class A output
stage and minimizing the amount of current swing in the out-
put devices improves distortion substantially.
The LMH6515 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 two. 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.
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FIGURE 4. Bandwidth Changes Due to Different Inductor
Values
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FIGURE 5. Gain vs. External Load
DIGITAL CONTROL
The LMH6515 has 32 gain settings covering a range of 31 dB.
To avoid undesirable signal transients the LMH6515 should
be powered on at the minimum gain state (all logic input pins
at 0V). The LMH6515 has a 5-bit gain control bus as well as
a latch pin. When the latch pin is low, data from the gain con-
trol pins is immediately sent to the gain circuit (i.e. gain is
changed immediately). When the latch pin transitions high the
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