LMH6515 Datasheet, PDF (14/18 Pages) National Semiconductor (TI) – 600 MHz, Digital Controlled, Variable Gain Amplifier

current gain state is held and subsequent changes to the gain
set pins are ignored. To minimize gain change glitches mul-
tiple 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 glitch-
es 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 con-
tinuous 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 unde-
sirable transients.
The LMH6515 was designed to interface with 3.3V CMOS
logic circuits. If operation with 5V logic is required a simple
voltage divider at each logic pin will allow for this. To properly
terminate 100Ω transmission lines a divider with a 66.5Ω re-
sistor to ground and a 33.2Ω series resistor will properly
terminate the line as well as give the 3.3V logic levels. Care
should be taken not to exceed the 3.6V absolute maximum
voltage rating of the logic pins.
EXPOSED PAD LLP PACKAGE
The LMH6515 is in a thermally enhanced package. The ex-
posed pad is connected to the GND pins. It is recommended,
but not necessary, that the exposed pad be connected to the
supply ground plane. In any case, the thermal dissipation of
the device is largely dependent on the attachment of this pad.
The exposed pad should be attached to as much copper on
the circuit board as possible, preferably external copper.
However, it is also very important to maintain good high speed
layout practices when designing a system board. Please refer
to the LMH6515 evaluation board for suggested layout tech-
niques.
Package information is available on the National web site.
http://www.national.com/packaging/folders/sqa16a.html
INTERFACING TO ADC
The LMH6515 was designed to be used with high speed
ADCs such as the ADC14155. As shown in the Typical Ap-
plication schematic on page 1, AC coupling provides the best
flexibility especially for IF sub-sampling applications. Any re-
sistive networks on the output will also cause a gain loss
because the output signal is developed across the output re-
sistors. The chart Maximum Gain vs. External Load shows the
change in gain when an external load is added.
The inputs of the LMH6515 will self bias to the optimum volt-
age for normal operation. The internal bias voltage for the
inputs is approximately 1.4V. In most applications the
LMH6515 input will need to be AC coupled.
The output common mode voltage is not self biasing, it needs
to be pulled up to the positive supply rail with external induc-
tors as shown in Figure 1. This gives the LMH6515 the
capability for large signal swings with very low distortion on a
single 5V supply. The internal load resistors provide the
LMH6515 with very consistent gain.
A unique internal architecture allows the LMH6515 to be driv-
en by either a differential or single ended source. If driving the
LMH6515 single ended, the unused input should be termi-
nated to ground with a 0.01 µF capacitor. Directly shorting the
unused input to ground will disrupt the internal bias circuitry
and will result in poor performance.
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FIGURE 6. Bandpass Filter
Center Frequency is 140 MHz with a 20 MHz Bandwidth
Designed for 200Ω Impedance
ADC Noise Filter
Figure 6 shows a filter schematic and the following table of
values are for some common IF frequencies. The filter shown
offers a good compromise between bandwidth, noise rejec-
tion and cost. This filter topology is the same as used on the
ADC14V155KDRB High IF Receiver reference design board.
This filter topology works best with the 12 and 14-bit sub-
sampling analog to digital converters shown in the Compati-
ble High Speed Analog to Digital Converters table.
Filter Component Values
Filter Component Values
Fc
75
140 170 250
MHz MHz MHz MHz
BW
40
20
25 Narrow
MHz MHz MHz Band
Components L1, L2 10 µH 10 µH 10 µH 10 µH
L3, L4 390 nH 390 nH 560 nH —
C1, C2 10 pF 3 pF 1.4 pF 47 pF
C3 22 pF 41 pF 32 pF 11 pF
L5 220 nH 27 nH 30 nH 22 nH
R1, R2 100 200 100 499
FIGURE 7. Sample Filter
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