LINEAR LTC6412 User's Manual

DESIGN FEATURES L

GA IN (d B)

Analog VGA Simplifies Design and Outperforms Competing

Gain Control Methods by Walter Strifler

Introduction Variable gain amplifiers (VGAs) are

SHDN EN GNDVCCVCCVCCVCC GND

widely used in communications and imaging applications such as cel-

REFERENCE AND BIAS CONTROL

lular radio, satellite receivers, global

+IN +OUT

positioning, radar, and ultrasound ap-

GA IN (d B)

plications. Most of these applications

BUFFER/ involve transmit and receive signals • • • OUTPUT AMPLIFIER

of varying amplitude that need to be

–IN –OUT

managed within the constraints of the

DECL1

overall system design. On the transmit

ATTENUATOR REFERENCE AND

side, the signal amplitude is usually VCM DECL2

CONTROL BIAS CONTROL

adjusted near a maximum limit im-

EXPOSED posed by the transmit power amplifier +VG VREF –VG GND GND GND GND GND PAD

or below a power limit imposed by the receivers or reflectors of the signal. On

Figure 1. Block diagram of the LTC6412

the receive side, the signal amplitude is usually amplified and tailored to take vs digital control are also discussed. tally controlled VGA. While this trend optimum advantage of the demodula- This is followed by a brief introduction statement is largely true, it overlooks tor or ADC that decodes the signal. In to the important design and perfor- important distinctions between the both the transmit and receive case, the mance features of the LTC6412 along two types of VGA control.

GA IN ON FO RM AN CE RR OR (d B)

optimum signal gain targets change with a discussion of a few application The digitally controlled VGA is over time and temperature, so most examples. a natural choice when the system systems share a common require- parameters that determine optimum

ment of controlling signal amplitude Analog vs Digital Control gain are known to the digital control through the use of adjustable gain of VGAs system and are readily available across

stages commonly known as variable The vast majority of modern com- a data bus. This information is piped gain amplifiers. munication and imaging equipment to the data inputs of the VGA, and the This article introduces the LTC6412, contains significant digital hardware desired gain is step-adjusted during Linear Technology’s first high fre- in the form of microprocessors, con- noncritical periods in the time-slotted quency, analog-controlled VGA—now trollers, memory, data busses and the signal. added to Linear Technology’s existing like, so the choice of analog vs digital The digital control scenario is the portfolio of digitally controlled VGAs. system control would seem to be a goal of most system designs, but The design considerations for analog forgone conclusion in favor of the digi- it leaves many application gaps for

FREQ = 140MHz FREQ = 140MHz

–VG: NEGATIVE +VG: POSITIVE –40°C 25°C SLOPE MODE SLOPE MODE

GMIN –10

85°C GMAX GMIN

FREQUENCY (MHz) +VG OR –VG VOLTAGE (V) –VG VOLTAGE (V)

Figure 2. LTC6412 gain vs frequency over gain Figure 3. Differential gain vs control voltage Figure 4. LTC6412 gain conformance error vs control range over temperature for the LTC6412 control voltage over temperature

Linear Technology Magazine • September 2009