lmh6514sqx National Semiconductor Corporation, lmh6514sqx Datasheet - Page 14

• Manufacturer Part Number: lmh6514sqx
• Description: Lmh6514 600 Mhz, Digital Controlled, Variable Gain Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
swing. To drive larger input signals the input common mode
can be forced higher than 1.4V to allow for more swing. An
input common mode of 2.0V will allow an 8 V
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.
At the frequencies where the LMH6514 is the most useful the
input impedance is not 200 Ω and it may not be purely resis-
tive. For many AC coupled applications the impedance can
be easily changed using LC circuits to transform the actual
impedance to the desired impedance.
As shown in Figure 5 a single ended 50Ω source is matched
to the LMH6514 input at 100 MHz. The loss in this circuit is
related to the parasitic resistance in the inductor and capacitor
and the bandwidth is related to the loaded Q of the circuit.
Since the Q, at 1.4 is quite low, the bandwidth is very wide.
(59 MHz 0.3 dB bandwidth). The input match of this circuit is
quite good. It converts the Z
(150 +j0)Ω to (50+j1)Ω. The benefit of LC matching circuits
over a transformer is the ability to match ratios that are not
commonly found on transformers and also the ability to neu-
tralize reactance to present a purely resistive load to the
voltage source.
FIGURE 5. Single Ended Input with LC Matching
(Note capacitor on grounded input)
FIGURE 4. Single Ended Input
AMP
of the amplifier, which is
PP
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maximum
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In Figure 6 the input source resistance is 200Ω differential.
Here the desired input impedance is higher than the amplifier
input impedance, and is differential as well. The amplifier
impedance of (150–j0)Ω is increased to (202–j0.5)Ω. For an
easy way to calculate the L and C circuit values there are
several options for online tools or down-loadable programs.
The following tool might be helpful.
http://www.circuitsage.com/matching/matcher2.html
Excel can also be used for simple circuits; however, the “Anal-
ysis ToolPak” add-in must be installed to calculate complex
numbers.
OUTPUT CHARACTERISTICS
The LMH6514 has the option of two different output configu-
rations. The LMH6514 is an open collector topology. As
shown in Figure 11 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 LMH6514 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 12 will require inductors in order to
be able to develop an output voltage. The 200Ω option as
shown in Figure 13 or Figure 14 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 LMH6514 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 a 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.
FIGURE 6. Differential 200Ω LC Conversion Circuit
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