Fault-Protected, High-Voltage,

Signal-Line Protector

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tance is a function of the applied voltage, which is

dramatically altered by the ohmmeter itself. Autoranging

ohmmeters are particularly unreliable.

Figure 5 shows a circuit that gives 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 entire range. The ammeter must have a voltage drop

of less than 1mV (up to the maximum test current) for

accurate 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. Find the path resistance by setting the ana-

log voltage, measuring the current, and calculating the

path resistance. Repeat the procedure at each analog

and supply voltage.

Note that it is important to use a voltage source of

100mV or less. As shown in Figure 5, this voltage and

prematurely.

MAX4505

V-

V+

V+

100mV

PATH RESISTANCE = 100mV/A

ADJUSTABLE

ANALOG

VOLTAGE

AOUT

A

Figure 5. Path-Resistance Measuring Circuit

TRANSISTOR COUNT: 56

Pin Configurations (continued)

N.C.

N.C.

V-

1

2

8

7

V+

N.C.

N.C.

AIN

AOUT

µMAX

3

4

6

5

MAX4505

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