LMH6515 Datasheet, PDF (18/31 Pages) National Semiconductor (TI) – 600 MHz, Digital Controlled, Variable Gain Amplifier

LMH6515
SNOSAX4C – AUGUST 2007 – REVISED MARCH 2013
www.ti.com
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 resistor 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 WQFN PACKAGE
The LMH6515 is in a thermally enhanced package. The exposed 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 techniques.
Package information is available on the TI Web site.www.ti.com/packaging
INTERFACING TO ADC
The LMH6515 was designed to be used with high speed ADCs such as the ADC14155. As shown in the Typical
Application schematic, AC coupling provides the best flexibility especially for IF sub-sampling applications. Any
resistive networks on the output will also cause a gain loss because the output signal is developed across the
output resistors. 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 voltage 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 inductors as shown in Figure 39. 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 driven by either a differential or single ended source. If
driving the LMH6515 single ended, the unused input should be terminated 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.
5V
680 nH
390 nH ^{3 pF}
41 pF 27 nH 200:
390 nH
3 pF
200
680 nH
ADC C_IN
5V
Figure 44. Bandpass Filter
Center Frequency is 140 MHz with a 20 MHz Bandwidth
Designed for 200Ω Impedance
18
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