As seen in Figure 7, the signal input can now be put

into pin 1 of the MAX4506 (new common output for sys-

tem), and outputs can be taken at MAX4508 pins 4 to

7, and 9 to 12. This is the classic demultiplexer opera-

tion. This system now has full protection on both of the

multiplexers’ inputs and outputs.

Measuring Path Resistance

Measuring path resistance requires special techniques,

since path resistance varies dramatically with the IN

and OUT voltages relative to the supply voltages.

Conventional ohmmeters should not be used for the fol-

lowing two reasons: 1) the applied voltage and currents

are usually not predictable, and 2) the true resistance is

a function of the applied voltage, which is dramatically

altered by the ohmmeter itself. Autoranging ohmmeters

are particularly unreliable.

Figure 8 shows a circuit that can give reliable results.

This circuit uses a 100mV voltage source, a low-volt-

age-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 one millivolt (up to the maximum test cur-

rent) for accurate results. A Keithley model 617 elec-

trometer has a suitable ammeter circuit, appropriate

ranges, and a built-in voltage source designed for this

type of measurement. Find the path resistance by set-

ting the analog voltage, measuring the current, and cal-

culating the path resistance. Repeat the procedure at

each analog and supply voltage.

Note: It is important to use a voltage source of 100mV

could cause OUT to go into a fault condition prematurely.

High-Voltage Surge Suppression

These devices are not high-voltage arresters, nor are

they substitutes for surge suppressors. However, the

MAX4506/MAX4507 can fill a vital gap in systems that

use these forms of protection (Figure 9). Although surge

suppressors are extremely fast shunt elements, they

have very soft current knees. Their clamp voltage must

be chosen well above the normal signal levels, because

they have excessive leakage currents as the analog

signal approaches the knee. This leakage current can

interfere with normal operation when signal levels are

low or impedance is high. If the clamp voltage is too

high, the input can be damaged.

Connecting a MAX4506/MAX4507 after a surge sup-

pressor allows the surge-suppressor voltage to be set

above the supply voltage (but within the overvoltage

limits), dramatically reducing leakage effects (Figure 9).

During a surge, the surge suppressor clamps the input

voltage roughly to the ±10V supplies.

Fault-Protected, High-Voltage

Signal-Line Protectors

10

______________________________________________________________________________________

MAX4506

V-

V+

V+

100mV

PATH RESISTANCE = 100mv/A

ADJUSTABLE

ANALOG

VOLTAGE

8

4

OUT1

A

MAX4506

IN3

V-

SURGE SUPPRESSORS

-15V

-10V

V+

+10V

3

4

5

OUT3

IN2

26

OUT2

IN1

17

OUT1

8

OP AMP

Figure 8. Path-Resistance Measuring Circuit

Figure 9. Surge-Suppression Circuit