Choosing Feedback and Gain Resistors

The MAX4112/MAX4113/MAX4117–MAX4120 are cur-

rent feedback amplifiers. Increasing feedback resistor

values will decrease peaking. Use the input resistor

shows the standard inverting and noninverting configu-

rations. Notice that the gain of the noninverting circuit

(Figure 2b) is 1 plus the magnitude of the inverting

closed-loop gain (Table 1).

DC and Noise Errors

There are several major error sources to consider in

any operational amplifier. These apply equally to the

MAX4112/MAX4113/MAX4117–MAX4120. Offset-error

terms are given by the equation below. Voltage and

current-noise errors are root-square summed and

therefore computed separately. In Figure 3, the total

output offset voltage is determined by:

the input bias currents may flow into or out of the

device. For this reason, there is no benefit to match-

ing the resistance at both inputs.

The equation for total DC error is:

c) The total output-referred noise voltage is:

The MAX4112/MAX4117/MAX4119 have a very low,

2nV/√Hz noise voltage. The current noise at the positive

An example of the DC error calculations, using the

MAX4112 typical data and the typical operating circuit

gives the following:

Calculating total output noise in a similar manner yields:

With a 200MHz system bandwidth, this calculates to

sigma value).

Resistor Types

Surface-mount resistors are the best choice for high-

frequency circuits. They are of similar material to metal-

film resistors, but are deposited using a thick-film

process in a flat, linear manner that minimizes induc-

tance. Their small size and lack of leads also minimizes

parasitic inductance and capacitance, yielding more

predictable performance.

Metal-film resistors with leads are manufactured using

a thin-film process where resistive material is deposited

in a spiral layer around a ceramic rod. Although the

materials used are noninductive, the spiral winding pre-

sents a small inductance (about 5nH) that may have an

adverse effect on high-frequency circuits.

Carbon-composition resistors with leads are manufac-

tured by pouring the resistor material into a mold. This

process yields relatively low-inductance resistors that

are very useful in high-frequency applications, although

they tend to cost more and have more thermal noise

than other types. The ability of carbon-composition

resistors to self-heal after a large current overload

makes them useful in high-power RF applications.

For general-purpose use, surface-mount metal-film

resistors seem to have the best overall performance for

low cost, low inductance, and low noise.

Video Line Driver

The MAX4112/MAX4113/MAX4117–MAX4120 are opti-

mized (gain flatness) to drive coaxial transmission lines

when the cable is terminated at both ends, as shown in

Figure 4. Cable frequency response can cause varia-

tions in the flatness of the signal.

e

1 1

13x10

x50

14x10

x300

2x10

e

9.4nV

Hz

n(OUT)

12

2

12

2

9

2

n(OUT)

/

=+

()

=

−−

−

e

R

R

iR

iR

R

e

n OUT

F

G

nS

nF

G

n

()

||

=+

⎛

⎝

⎜

⎞

⎠

⎟

()

+

−

1

22

2

V

I

R

IR

R

V

1+

R

R

OUT

B

S

B

F

G

OS

F

G

||

=

[]

⎛

⎝

⎜

⎞

⎠

⎟

+

−

Single/Dual/Quad, 400MHz, Low-Power,

Current Feedback Amplifiers

______________________________________________________________________________________

11

MAX4112

MAX4113

MAX4117

MAX4118

MAX4119

MAX4120

Figure 3. Output Offset Voltage