LTC6412 datasheet(14/24 Pages) LINER

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LTC6412

6412f

OPERATION

The LTC6412 employs an interpolated, tapped attenuator

circuit architecture to generate the variable-gain charac-

teristic of the ampliﬁer. The tapped attenuator is fed to a

buffer and output ampliﬁer to complete the differential

signal path shown in the Block Diagram. This circuit

architecture provides good RF input power handling ca-

pability along with a constant output noise and output IP3

characteristic that are desirable for most IF signal chain

applications. The internal control circuitry takes the gain

control signal from the ±VG terminals and converts this

to an appropriate set of control signals to the attenuator

ladder. The attenuator control circuit ensures that the

linear-in-dB gain response is continuous and monotonic

over the gain range for both slow and fast moving input

control signals while exhibiting very little input impedance

variation over gain. These design considerations result

in a gain-vs-VG characteristic with a ±0.1dB ripple and

a 0.5μs gain response time that is slower than a similar

digital step attenuator design.

An often overlooked characteristic of an analog-controlled

VGA is upconverted amplitude modulation (AM) noise

from the gain control terminals. The VGA behaves as a

2-quadrant multiplier, so some minimal care is required to

avoid excessive AM sideband noise generation. The table

below demonstrates the effect of the baseline 20nV/√Hz

equivalent input control noise from the LTC6412 circuit

along with the effect of a higher combined input noise due

to a noisy external control circuit.

CONTROL INPUT TOTAL NOISE

VOLTAGE (nV/√Hz)

PEAK AM NOISE AT 10kHz OFFSET

NEAR MAXIMUM GAIN (dBc/Hz)

–142

–136

–131

100

–128

200

–122

The baseline equivalent 20nV/√Hz input noise is seen to

produce worst-case AM sidebands of –142dBc/Hz which is

near the –147dBm/Hz output noise ﬂoor at maximum gain

for a nominal 0dBm output signal. An input control noise

voltage less than 80nV/√Hz is generally recommended to

avoid measurable AM sideband noise. While op amp control

circuit output noise voltage is usually below 80nV/√Hz,

some low power DAC outputs exceed 150nV/√Hz. DACs

with output noise in the range of 100nV/√Hz to 150nV/√Hz

can usually be accommodated with a suitable 2:1 or 3:1

resistor divider network on the DAC output to suppress the

noise amplitude by the same ratio. Noisy DACs in excess

of 150nV/√Hz should be avoided if minimal AM noise is

important in the application.

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