ful attention to inputs and outputs to avoid large para-

sitic capacitance. Whether or not you use a constant-

impedance board, observe the following design guide-

lines:

• Don’t use wire-wrap boards; they are too inductive.

• Don’t use IC sockets; they increase parasitic capaci-

tance and inductance.

• Use surface-mount instead of through-hole compo-

nents for better high-frequency performance.

• Use a PC board with at least two layers; it should be

as free from voids as possible.

• Keep signal lines as short and as straight as possi-

ble. Do not make 90° turns; round all corners.

Rail-to-Rail Outputs,

Ground-Sensing Input

The input common-mode range extends from

rejection. Beyond this range, the amplifier output is a

nonlinear function of the input, but does not undergo

phase reversal or latchup. The output swings to within

125mV of either power-supply rail with a 2k

Ω load.

Output Capacitive Load and Stability

The MAX4350/MAX4351 are optimized for AC perfor-

mance. They are not designed to drive highly reactive

loads, which decrease phase margin and may produce

excessive ringing and oscillation. Figure 2 shows a cir-

cuit that eliminates this problem. Figure 3 is a graph of

Figure 4 shows how a capacitive load causes exces-

sive peaking of the amplifier’s frequency response if

the capacitor is not isolated from the amplifier by a

resistor. A small isolation resistor (usually 20

Ω to 30Ω)

placed before the reactive load prevents ringing and

oscillation. At higher capacitive loads, AC performance

is controlled by the interaction of the load capacitance

and the isolation resistor. Figure 5 shows the effect of a

27

Ω isolation resistor on closed-loop response.

Coaxial cable and other transmission lines are easily

driven when properly terminated at both ends with their

characteristic impedance. Driving back-terminated

transmission lines essentially eliminates the line’s

capacitance.

Ultra-Small, Low-Cost, 210MHz, Dual-Supply

Op Amps with Rail-to-Rail Outputs

8

_______________________________________________________________________________________

50

Ω

MAX435 _

Figure 2. Driving a Capacitive Load Through an Isolation Resistor

30

25

20

5

10

15

0

CAPACITIVE LOAD (pF)

50

0

100

200

150

250

Figure 3. Isolation Resistance vs. Capacitive Load