LMH6401_16 Datasheet, PDF (19/48 Pages) Texas Instruments – LMH6401 DC to 4.5 GHz, Fully-Differential, Digital Variable-Gain Amplifier

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9 Detailed Description
LMH6401
SBOS730A – APRIL 2015 – REVISED MAY 2015
9.1 Overview
The LMH6401 is a very high-performance, differential I/O, digitally-controlled variable gain amplifier (DVGA). The
device is optimized for radio frequency (RF), intermediate frequency (IF), or high-speed time-domain applications
with 3-dB bandwidths up to 4.5 GHz. The device is ideal for dc- or ac-coupled applications requiring a variable
gain stage when driving an analog-to-digital converter (ADC).
The LMH6401 is best suited to optimize system linearity and noise performance over the entire gain range in the
RF and IF bands. Operating on a nominal 5-V supply or ±2.5-V split supplies, the device consists of an
attenuator stage followed by a fixed-gain amplifier to provide voltage gain control from –6 dB to 26 dB in 1-dB
steps (as shown in the Functional Block Diagram section) with an overall 32-dB gain range. The variable gain
control for the device is offered through the digital serial peripheral interface (SPI) register. The device has a
unique attenuator ladder architecture providing dynamic range improvements where the overall noise figure (NF)
remains relatively constant for the first 5-dB attenuator steps, with NF degrading proportional to the attenuator
steps on the sixth step. This behavior repeats over the entire gain range; see Figure 26.
The device has a differential input impedance of 100-Ω and is intended to be driven differentially by a matched
100-Ω differential source impedance for the best linearity and noise performance. The LMH6401 has two on-chip,
10-Ω resistors, one on each output (as shown in the Functional Block Diagram section). For most load
conditions, the 10-Ω resistors are only a partial termination. Consequently, external termination resistors are
required in most applications. See Table 11 for common load values and the matching resistors.
The LMH6401 supports a common-mode reference input (VOCM) pin to align the amplifier output common-mode
with the subsequent stage (ADC) input requirements. The output common-mode of the LMH6401 is self-biased
to mid-supply when the VOCM pin is not driven externally. The device can be operated on a power-supply
voltage range of 4.0 V to 5.25 V and supports both single- and split-supply operation. For correct digital
operation, the positive supply must not be below 2 V for ground reference logic. A power-down feature is also
available through the SPI register and the external PD pin.
9.2 Functional Block Diagram
VS+
ZIN ~ 100 Ÿ
INM
INP
CS, SCK, SDI
SDO
PD
0-dB to 32-dB
Attenuation in 1-dB
Steps
Gain
Control
POR
SPI Decoder
Power Down
Buffer
Fixed Gain (AV)
= 26 dB
Thermal
Feedback Gain
and Frequency
Control Circuit
VS+
VOCM ^-
Error
Amplifier
+
Copyright © 2015, Texas Instruments Incorporated
VS-
Product Folder Links: LMH6401
10 Ÿ
10 Ÿ
OUTM
OUTP
VS+
50 NŸ
50 NŸ
VS-
VOCM
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