LMH6515 Datasheet, PDF (17/31 Pages) National Semiconductor (TI) – 600 MHz, Digital Controlled, Variable Gain Amplifier

LMH6515
www.ti.com
SNOSAX4C – AUGUST 2007 – REVISED MARCH 2013
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 39 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 39 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
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 coupling. Mutual coupling of inductors can compromise filter
characteristics and lead to unwanted distortion products.
1
0
1 PH
-1
R_L = 100: TOTAL
-2
-3
470 nH
-4
-5
200 nH
-6
-7
-8
-9
1
10
100
1000 10000
FREQUENCY (MHz)
Figure 42. Bandwidth Changes Due to Different Inductor Values
36
400: INTERNAL WITH 400:
EXTERNAL = 26 dB NET
32
28
INTERNAL LOAD
= 400:
24
INTERNAL LOAD =
200:
20
16
200: INTERNAL WITH 200:
EXTERNAL = 20 dB NET
12
10
100
1k
10k
100k
EXTERNAL DIFFERENTIAL LOAD (:)
Figure 43. 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 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 control 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.
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