LT5524_15 Datasheet, PDF (11/16 Pages) Linear Technology – Low Distortion IF Amplifier/ADC Driver with Digitally Controlled Gain

LT5524
APPLICATIO S I FOR ATIO
on the LT5524 package must be soldered to a good ground
plane on the PCB.
PGA Function, Linearity and NF
As described in the Circuit Operation section, the LT5524
consists of a variable (step) attenuator followed by a high
gain output amplifier. The overall gain of the LT5524 is
digitally controlled by means of four gain control pins with
internal pull-down. Minimum gain is programmed when
the gain control pins are set low or left floating. In
shutdown mode, these PGA inputs draw <10µA leakage
current, regardless of the applied voltage.
The 6dB and 12dB attenuation steps (PGA2 and PGA3) are
implemented by switching the amplifier inputs to an input
attenuator tap. The 3dB attenuation step (PGA1) changes
the amplifier transconductance. The output IP3 is approxi-
mately independent of the PGA1, PGA2 and PGA3 gain
settings. However, the 1.5dB attenuation step utilizes a
current steering technique that disables the internal linear-
ity compensation circuit, and the OIP3 can be reduced by
as much as 6dB when PGA0 is low. Therefore, to achieve
the LT5524’s highest linearity performance, the PGA0 pin
should be set high.
The LT5524 noise figure is 8.6dB at 100MHz in the maxi-
mum gain state. For the –3dB attenuation setting, the NF
is 9.2dB. The noise figure increases in direct proportion to
the amount of programmed gain reduction for the 1.5dB,
6dB and 12dB steps.
The output noise floor is proportional to the output load
impedance, R_OUT. It is almost constant for PGA1 = high
and for any PGA0, PGA2, PGA3 state. When PGA1 = low,
the output noise floor is 2dB lower (see Typical Perfor-
mance Characteristics).
Other Linearity Considerations
LT5524 linearity is a strong function of signal frequency.
OIP3 decreases about 13dB for every octave of frequency
increase above 100MHz.
As noted in the Circuit Operation section, at any given
frequency and input level, the LT5524 provides a current
output with fairly constant intermodulation distortion fig-
ure in dBc, regardless of the output load value. For higher
R_OUT values, more gain and output power is available, and
better OIP3 figures can be achieved. However, high R_OUT
values are not easily implemented in practice, limited by
the availability of high ratio output impedance transforma-
tion networks.
Linearity can also be limited by the output RC time con-
stant (bandwidth limitations), particularly for high R_OUT
values. A solution is outlined in the Bandpass Applications
section.
The LT5524 linearity degrades when common mode sig-
nal is present. The input transformer center tap should be
decoupled to ground to provide a balanced input differen-
tial signal and to avoid linearity degradation for high
attenuation steps. When the signal frequency is lower than
50MHz, and there is significant common mode signal,
then high attenuation settings may result in degraded
linearity.
At signal frequencies below 100MHz, the LT5524’s inter-
nal linearity compensation circuitry may provide “sweet
spots” with very high OIP3, in excess of +52dBm. This
almost perfect distortion correction cannot be sustained
over the full operating temperature range and with varia-
tions of the LT5524 output load (complex impedance
Z_OUT). Users are advised to rely on data shown in the
Typical Performance Characteristics curves to estimate
the dependable linearity performance.
Wideband Applications
At low frequencies, the value of the decoupling capacitors,
choke inductors and choice of transformer will set the
minimum frequency of operation. Output DC coupling is
possible, but this typically reduces the LT5524’s output
DC bias voltage, and thus the output swing and available
power.
At high frequencies, the output RC time constants set an
upper limit to the maximum frequency of operation in the
case of the wideband output networks presented so far.
For example the LT5524 output capacitance, C_OUT = 1.7pF,
and a pure resistive load, R_OUT = 200Ω, will set the –3dB
bandwidth to about 400MHz. In an actual application, the
R_LOAD • C_LOAD product may be even more restrictive. The
use of wideband output networks will not only limit the
5524f
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