LMH6514_14 Datasheet, PDF (18/29 Pages) Texas Instruments – LMH6514 600 MHz, Digital Controlled, Variable Gain Amplifier

LMH6514
SNOSB06 – JANUARY 2008
www.ti.com
OUTPUT CHARACTERISTICS
The LMH6514 has the option of two different output configurations. The LMH6514 is an open collector topology.
As shown in Figure 52 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 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 53 will require
inductors in order to be able to develop an output voltage. The 200Ω option as shown in Figure 54 or Figure 55
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 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. 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 LMH6514 has a common emitter Class A output stage and minimizing the amount of current
swing in the output 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 42 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 42 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 inductors 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 because 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 frequency 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 coupling. Mutual coupling of inductors can compromise filter
characteristics and lead to unwanted distortion products.
1
0
1 PH
-1
-2
-3
470 nH
-4
-5
200 nH
-6
-7
-8
R_L = 100: TOTAL
-9
1
10
100
1000
FREQUENCY (MHz)
Figure 48. Bandwidth Changes Due to Different Inductor Values
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