LMH6521_16 Datasheet, PDF (11/29 Pages) Texas Instruments – High Performance Dual DVGA

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LMH6521
APPLICATION INFORMATION
SNOSB47D – MAY 2011 – REVISED MARCH 2013
0.01 PF
V_CC
1 PH
40.2: 0.01 PF
RF
200: ½ LMH6521
ADC
0.01 PF
40.2: 0.01 PF
1 PH
LO
6
GAIN 0-5
LATCH
ENABLE
Figure 26. LMH6521 Typical Application
INTRODUCTION
The LMH6521 is a dual, digitally controlled variable gain amplifier 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 distortion 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.
Figure 26 shows a typical application circuit.
The LMH6521 has been designed for AC coupled applications 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 inductance, 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 require ac-coupling capacitors of 0.01uF. The input pins are self biased to 2.5V and should be ac-
coupled with 0.01uF capacitors as well. The output RF inductors and ac-coupling capacitors are the main
limitations for operating at low frequencies.
Each channel of the LMH6521 consists of a digital step attenuator followed by a low distortion 26 dB fixed gain
amplifier and a low impedance output stage. The gain is digitally controlled 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 reduce the current consumption to 17.5mA per channel.
Copyright © 2011–2013, Texas Instruments Incorporated
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