High-Voltage, Fault-Protected

Analog Multiplexers

8

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to-source voltage. The P-channel device (Q2), howev-

the input signal from reaching the output. If the input

off. Similarly, only sub-microamp leakage currents can

flow from the output back to the input, since any volt-

age will turn off either Q1 or Q2.

Figure 9 shows the condition of an OFF channel with

V+ and V- present. As with Figures 7 and 8, either an

N-channel or a P-channel device will be off for any

input voltage from -60V to +60V. The leakage current

with negative overvoltages will immediately drop to a

few nanoamps at +25°C. For positive overvoltages,

that fault current will initially be 40µA or 50µA, decaying

over a few seconds to the nanoamp level. The time

constant of this decay is caused by the discharge of

stored charge from internal nodes, and does not com-

promise the fault-protection scheme.

Figure 10 shows the condition of the ON channel with

V+ and V- present. With input voltages less than ±10V,

all three FETs are on and the input signal appears at the

output. If the input voltage exceeds V+ minus the N-

FET will turn off. For voltages more negative than V-

is typically 3V, the multiplexer’s output swing is limited

to about -12V to +13.5V with ±15V supplies.

The

Typical Operating Characteristics graphs show typi-

cal leakage vs. input voltage curves. Although the max-

imum rated input of these devices is ±65V, the

MAX378/MAX379 typically have excellent performance

up to ±75V, providing additional margin for the unknown

transients that exist in the real world. In summary, the

MAX378/MAX379 provide superior protection from all

fault conditions while using a standard, readily pro-

duced junction-isolated CMOS process.

Switching Characteristics

and Charge Injection

Table 1 shows typical charge-injection levels vs.

power-supply voltages and analog input voltage. Note

that since the channels are well matched, the differen-

tial charge injection for the MAX379 is typically less

than 5pC. The charge injection that occurs during

switching creates a voltage transient whose magnitude

is inversely proportional to the capacitance on the mul-

tiplexer output.

The channel-to-channel switching time is typically 600ns,

with about 200ns of break-before-make delay. This 200ns

break-before-make delay prevents the input-to-input short

that would occur if two input channels were simultaneous-

ly connected to the output. In a typical data acquisition

system, such as in Figure 6, the dominant delay is not the

switching time of the MAX378 multiplexer, but is the set-

tling time of the following amplifiers and S/H. Another limit-

ing factor is the RC time constant of the multiplexer

the load capacitance on the output of the multiplexer.

Even with low signal source impedances, 100pF of capac-

itance on the multiplexer output will approximately double

the settling time to 0.01% accuracy.

Operation with Supply Voltage

Other than ±15V

The main effect of supply voltages other than ±15V is

the reduction in output signal range. The MAX378 limits

the output voltage to about 1.5V below V+ and about 3V

above V-. In other words, the output swing is limited to

+3.5V to -2V when operating from ±5V. The

Typical

for ±15V, ±10V, and ±5V power supplies. Maxim tests

and guarantees the MAX378/MAX379 for operation from

±4.5V to ±18V supplies. The switching delays are

increased by about a factor of 2 at ±5V, but break-

before-make action is preserved.

The MAX378/MAX379 can be operated with a single +9V

to +22V supply, as well as asymmetrical power supplies

such as +15V and -5V. The digital threshold will remain

approximately 1.6V above GND and the analog character-

difference between V+ and V-. Connect V- to 0V when

operating with a +9V to +22V single supply.

This means that the MAX378/MAX379 will operate with

standard TTL-logic levels, even with ±5V power sup-

plies. In all cases, the threshold of the EN pin is the

same as the other logic inputs.

Table 1a. MAX378 Charge Injection

lated analog input level is applied to channel 1; channels 2

through 8 are open circuited. EN = +5V, A1 = A2 = 0V, A0 is

toggled at 2kHz rate between 0V and 3V. +100pC of charge

creates a +100mV step when injected into a 1000pF load

capacitance.

Supply Voltage

Analog Input Level

Injected Charge

±5V

+1.7V

0V

-1.7V

+100pC

+70pC

+45pC

±10V

+5V

0V

-5V

+200pC

+130pC

+60pC

±15V

+10V

0V

-10V

+500pC

+180pC

+50pC