LT5514 datasheet(16/20 Pages) LINER

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LT5514

5514f

For example, for ROUT = 200Ω, L1, L2 = 33nH results in

500MHz bandwidth.

The series inductor can extend the application bandwidth,

but it provides no improvement in linearity performance.

Series inductance may also produce peaking in the AC

response. This can be the case when (high Q) choke

inductors are used in an output interface such as in

Figure 5, and the PCB trace (connection) to the load is too

long. Since the LT5514’s output impedance is relatively

high, the PCB trace acts as a series inductor. The most

direct solution is to shorten the connection lines by

placing the driver closer to the load. Another solution to

flatten the AC response is to place resistance close to the

LT5514 outputs. In this way the connection line behaves

more like a terminated transmission line, and the AC

peaking due to the capacitive load can be removed.

Bandpass Applications

For narrow band IF applications, the LT5514’s output

capacitance and the application load capacitance can be

incorporated as part of an LC impedance transformation

network, giving improved linearity performance for signal

frequencies greater than 100MHz. Figure 8 is an example

of such a network.

The network consists of two parallel resonant LC tank

circuits critically coupled by capacitors C1 and C2. The

ROUT to RLOAD transformation ratio in this particular

implementation is 2. The choice of impedance transfor-

mation ratio is more flexible than in the wideband case.

The LC network is a bandpass filter, a useful feature in

many applications.

A variety of bandpass matching network configurations

are conceivable, depending on the requirements of the

particular application. The design of these networks is

facilitated by the fact that the LT5514 outputs are not

destabilized by reactive loading.

Note that these LC networks may distort the output signal

if their amplitude and phase response exhibit nonlinear

behavior. For example, if resistors R1 and R2 in Figure 5

are replaced with LC resonant tank circuits, then severe

OIP3 degradation may occur (e.g., 4dB to 6dB at 200MHz).

Low Output Noise Floor Applications

In some applications the maximum output noise floor is

specified. The LT5514 output noise floor is elevated above

the available noise power (–174dBm/Hz into 50

Ω) by the

NF + Gain. Consequently, reduction of the LT5514’s power

gain is the only way to reduce the output noise floor.

In fixed gain applications, the LT5514 can be set to 3dB

attenuation relative to maximum gain. As shown in the

Typical Performance Characteristics, this gives a 2.8dB

reduction in the output noise floor with no loss of linearity.

In general, the output noise floor can be reduced by

decreasing ROUT (and hence power gain), at the cost of

reduced OIP3.

In some situations, it may be feasible to use two LT5514

parts in parallel. In this case, the effective gm doubles,

APPLICATIO S I FOR ATIO

–

100

0.33

µF

1:2

DUT

LT5514

ROUT

200

Ω

IN+

IN–

0.1

µF

PGA0 PGA1 PGA2 PGA3

RSRC

Ω

VSRC

VCC

ENA

ENB

RLOAD

100

Ω

2.2pF

GAIN = 33dB

OIP3 (LOAD) = +41dBm

UP TO 9dBm PER TONE

1dB BANDWIDTH:

fL = 130MHz

fU = 220MHz

12pF

56nH

0.1

µF

NOTE:

C3 + CLOAD = 12pF

C4 + CLOAD = 12pF

5.6pF

12pF

56nH

RLOAD

Ω

CLOAD

5514 F08

CLOAD

VOSUP

RLOAD

Ω

TC2-1T

Figure 8. Bandpass Output Transformation Network Example

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