LMH6515 Datasheet, PDF (16/31 Pages) National Semiconductor (TI) – 600 MHz, Digital Controlled, Variable Gain Amplifier

LMH6515
SNOSAX4C – AUGUST 2007 – REVISED MARCH 2013
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INPUT CHARACTERISTICS
The LMH6515 input impedance is set by internal resistors to a nominal 200Ω. Process variations will result in a
range of values as shown in the 5V Electrical Characteristics table. At higher frequencies parasitics will start to
impact the impedance. This characteristic will also depend on board layout and should be verified on the
customer’s system board.
At maximum gain the digital attenuator is set to 0 dB and the input signal will be much smaller than the output. At
minimum gain the output is 12 dB or more smaller than the input. In this configuration the input signal size may
limit the amplifier output amplitude, depending on the output configuration and the desired output signal voltage.
The input signal cannot swing more than 0.5V below the negative supply voltage (normally 0V) nor should it
exceed the positive supply voltage. The input signal will clip and cause severe distortion if it is too large. Because
the input stage self biases to approximately 1.4V the lower supply voltage will impose the limit for input voltage
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_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.
INTERNAL BIAS = 1.4V
C₁
Vin
R_IN = R₁ || 200
R₁
C₂
5V
LMH6515
(Note capacitor on grounded input)
5
GAIN 1-5
LATCH
Figure 41. Single Ended Input
OUTPUT CHARACTERISTICS
The LMH6515 has the option of two different output configurations. The LMH6515 is an open collector topology.
As shown in Figure 46 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 required.
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 47, will require
inductors in order to be able to develop an output voltage. The 200Ω option as shown in Figure 48 or Figure 49
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 resistors 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 required 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 recommended.
Although bandwidth goes down with higher values of load resistance, the distortion performance improves and
gain increases. The LMH6515 has a common emitter Class A output stage and minimizing the amount of current
swing in the output devices improves distortion substantially.
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