LMH6521 Datasheet, PDF (13/24 Pages) Texas Instruments – LMH6521 High Performance Dual DVGA

Application Information
FIGURE 1. LMH6521 Typical Application
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INTRODUCTION
The LMH6521 is a dual, digitally controlled variable gain am-
plifier designed for narrowband and wideband intermediate
frequency sampling applications. The LMH6521 is optimized
for accurate 0.5 dB gain steps with exceptional gain and
phase matching between channels combined with low distor-
tion products. Gain matching error is less than ±0.05 dB and
phase matching error less than ±0.5 degrees over the entire
attenuation range. This makes the LMH6521 ideal for driving
analog-to-digital converters where high linearity is necessary.
shows a typical application circuit.
The LMH6521 has been designed for AC coupled applica-
tions and has been optimized to operate at frequencies
greater than 3 MHz.
BASIC CONNECTIONS
A voltage between 4.75 V and 5.25 V should be applied to the
supply pin labeled +5V. Each supply pin should be decoupled
with a additional capacitance along with some low induc-
tance, surface-mount ceramic capacitor of 0.01uF as close to
the device as possible where space allows.
The outputs of the LMH6521 are low impedance devices that
need to be connected to ground with 1uH RF chokes and re-
quire ac-coupling capacitors of 0.01uF. The input pins are self
biased to 2.5V and should be ac-coupled with 0.01uF capac-
itors as well. The output RF inductors and ac-coupling ca-
pacitors are the main limitations for operating at low
frequencies.
Each channel of the LMH6521 consists of a digital step at-
tenuator followed by a low distortion 26 dB fixed gain amplifier
and a low impedance output stage. The gain is digitally con-
trolled over a 31.5 dB range from +26dB to −5.5dB. The
LMH6521 has a 200Ω differential input impedance and a low
20Ω differential output impedance.
To enable each channel of the LMH6521, the ENA and ENB
pins can be left to float, which internally is connected high with
a weak pull-up resistor. Externally connecting ENA and ENB
to ground will disable the channels of the LMH6521 and re-
duce the current consumption to 17.5mA per channel.
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FIGURE 2. Basic Operating Connection
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