1GHz, Low-Power, SOT23,

Current-Feedback Amplifiers with Shutdown

______________________________________________________________________________________

11

To realize the full AC performance of these high-speed

amplifiers, pay careful attention to power-supply

bypassing and board layout. The PC board should

have at least two layers: a signal and power layer on

one side and a large, low-impedance ground plane on

the other. The ground plane should be as free of voids

as possible, with one exception: the inverting input pin

(IN-) should have as low a capacitance to ground as

possible. This means that there should be no ground

connected to it. With multilayer boards, locate the

ground plane on a layer that incorporates no signal or

power traces.

Whether or not a constant-impedance board is used, it

is best to observe the following guidelines when

designing the board:

1) Do not use wire-wrapped boards (they are too

inductive) or breadboards (they are too capacitive).

2) Do not use IC sockets. IC sockets increase reac-

tance.

3) Keep signal lines as short and straight as possible.

Do not make 90° turns; round all corners.

4) Observe high-frequency bypassing techniques to

maintain the amplifier’s accuracy and stability.

5) In general, surface-mount components have shorter

bodies and lower parasitic reactance, giving better

high-frequency performance than through-hole com-

ponents.

The bypass capacitors should include a 10nF ceramic,

surface-mount capacitor between each supply pin and

the ground plane, located as close to the package as

possible. Optionally, place a 10µF tantalum capacitor

at the power-supply pins’ point of entry to the PC board

to ensure the integrity of incoming supplies. The power-

supply trace should lead directly from the tantalum

sitic inductance, keep PC traces short and use surface-

mount components. The N.C. pins should be

connected to a common ground plane on the PC board

to minimize parasitic coupling.

If input termination resistors and output back-termina-

tion resistors are used, they should be surface-mount

types, and should be placed as close to the IC pins as

possible. Tie all N.C. pins to the ground plane to mini-

mize parasitic coupling.

Choosing Feedback and Gain Resistors

As with all current-feedback amplifiers, the frequency

response of these devices depends critically on the

internal compensation capacitor to form the dominant

increases the pole frequency and the -3dB bandwidth,

but also increases peaking due to interaction with other

peaking and bandwidth.

Table 1 shows optimal values for the feedback resistor

MAX4228. Note that the MAX4224/MAX4227/MAX4228

offer superior AC performance for all gains except unity

gain (0dB). These values provide optimal AC response

using surface-mount resistors and good layout tech-

niques. Maxim’s high-speed amplifier evaluation kits

provide practical examples of such layout techniques.

Stray capacitance at IN- causes feedback resistor

decoupling and produces peaking in the frequency-

response curve. Keep the capacitance at IN- as low as

possible by using surface-mount resistors and by

avoiding the use of a ground plane beneath or beside

these resistors and the IN- pin. Some capacitance is

unavoidable; if necessary, its effects can be counter-

consistency over a wide range of production lots.

Table 1. Optimal Feedback Resistor

Networks

MAX4223/MAX4225/MAX4226

2

6

200

200

380

115

GAIN

(dB)

(

Ω)

(

Ω)

0.1dB

BW

(MHz)

GAIN

(V/V)

-3dB

BW

(MHz)

5

14

100

25

235

65

2

6

470

470

600

200

5

14

240

62

400

90

10

20

130

15

195

35

MAX4224/MAX4227/MAX4228

*

For the MAX4223EUT, this optimal value is 470

Ω.

1

0

560*

Open

1000

300