Signal-Line Circuit Protectors

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9

Figure 6 shows a circuit that can give reliable results.

This circuit uses a 100mV voltage source and a low-

voltage-drop ammeter as the measuring circuit, and an

adjustable supply to sweep the analog voltage across

its whole range. The ammeter must have a voltage

drop of less than one millivolt (at any current) for accu-

rate results. (A Keithley Model 617 Electrometer has a

suitable ammeter circuit, appropriate ranges, and a

built-in voltage source designed for this type of mea-

surement.) Measurements are made by setting the

analog voltage, measuring the current, and calculating

the path resistance. The procedure is repeated at

each analog voltage and supply voltage.

It is important to use a voltage source of 100mV or less.

age could cause the OUT pin to go into a fault condi-

tion prematurely.

High-Frequency Performance

In 50

Ω systems, signal response is reasonably flat up

to several megahertz (see

Typical Operating

Characteristics). Above 5MHz, the response has sev-

eral minor peaks, which are highly layout dependent.

Because the path resistance is dependent on the sup-

ply voltage and signal amplitude, the impedance is not

controlled. Adjacent channel attenuation up to 5MHz is

about 3dB above that of a bare IC socket, and is due

entirely to capacitive coupling.

Pulse response is reasonable, but because the imped-

ance changes rapidly, fast rise times may induce ringing

as the signal approaches the fault voltage. At very high

amplitudes (such as noise spikes), the capacitive cou-

pling across the signal pins will transfer considerable

energy, despite the fact that the DC path is a virtual open

circuit.

MAX366

OUT_

4

IN_

V+

V-

V+

V-

PATH RESISTANCE = 100mV/A

100mV

ADJUSTABLE ANALOG VOLTAGE

8

A

Figure 6. Path-Resistance Measuring Circuit

MAX366

OUT1

1

IN1

7

+5V

-5V

OUT2

2

IN2

6

OUT3

3

IN3

5

V+

4

V-

8

P

100k

SWITCHED +5V

OP AMP

Figure 5. Power-Supply Sequencing