LT5524 datasheet(11/16 Pages) AD

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LT5524

5524f

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, ROUT. 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

ROUT values, more gain and output power is available, and

better OIP3 figures can be achieved. However, high ROUT

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 ROUT

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

ZOUT). 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, COUT = 1.7pF,

and a pure resistive load, ROUT = 200Ω, will set the –3dB

bandwidth to about 400MHz. In an actual application, the

RLOAD • CLOAD product may be even more restrictive. The

use of wideband output networks will not only limit the

APPLICATIO S I FOR ATIO

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