Datasheet

LTC6412
14
6412fa
OPERATION
The LTC6412 employs an interpolated, tapped attenuator
circuit architecture to generate the variable-gain charac-
teristic of the amplifi er. The tapped attenuator is fed to a
buffer and output amplifi er 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 ±V
G
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-V
G
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
following table 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)
20 –142
40 –136
70 –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 fl oor 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.