High-Voltage, Fault-Protected

Analog Multiplexers

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9

Digital Interface Levels

The typical digital threshold of both the address lines

and the EN pin is 1.6V, with a temperature coefficient of

about -3mV/°C. This ensures compatibility with 0.8V to

2.4V TTL-logic swings over the entire temperature

range. The digital threshold is relatively independent of

the supply voltages, moving from 1.6V typical to 1.5V

typical as the power supplies are reduced from ±15V to

±5V. In all cases, the digital threshold is referenced to

GND.

The digital inputs can also be driven with CMOS-logic

levels swinging from either V+ to V- or from V+ to GND.

The digital input current is just a few nanoamps of leak-

age at all input voltage levels, with a guaranteed maxi-

mum of 1µA. The digital inputs are protected from ESD

by a 30V zener diode between the input and V+, and

can be driven ±4V beyond the supplies without drawing

excessive current.

Operation as a Demultiplexer

The MAX378/MAX379 will function as a demultiplexer,

where the input is applied to the OUT pin, and the input

pins are used as outputs. The MAX378/MAX379 pro-

vide both break-before-make action and full fault protec-

tion when operated as a demultiplexer, unlike earlier

generations of fault-protected multiplexers.

Channel-to-Channel Crosstalk,

Off Isolation, and Digital Feedthrough

At DC and low frequencies, channel-to-channel

crosstalk is caused by variations in output leakage cur-

rents as the off-channel input voltages are varied. The

MAX378 output leakage varies only a few picoamps as

all seven off inputs are toggled from -10V to +10V. The

output voltage change depends on the impedance level

signal source resistance in most cases, since the load

driven by the MAX378 is usually a high impedance. For

a signal source impedance of 10k

Ω or lower, the DC

crosstalk exceeds 120dB.

Table 2 shows typical AC crosstalk and off-isolation per-

formance. Digital feedthrough is masked by the analog

charge injection when the output is enabled. When the

output is disabled, the digital feedthrough is virtually

unmeasurable, since the digital pins are physically iso-

lated from the analog section by the GND and V- pins.

The ground plane formed by these lines is continued

onto the MAX378/MAX379 die to provide over 100dB

isolation between the digital and analog sections.

Table 1b. MAX379 Charge Injection

+1.7V

0V

-1.7V

+105pC

+73pC

+48pC

±10V

+5V

0V

-5V

+215pC

+135pC

+62pC

±15V

+10V

0V

-10V

+525pC

+180pC

+55pC

±5V

ed analog input level is applied to inputs 1A and 1B; channels

2 through 4 are open circuited. EN = +5V, A1 = 0V, A0 is tog-

gled from 0V to 3V at a 2kHz rate.

+107pC

+74pC

+50pC

+220pC

+139pC

+63pC

+530pC

+185pC

+55pC

Out A

Out B

Injected Charge

-2pC

-1pC

-2pC

-5pC

-4pC

-1pC

-5pC

-5pC

0pC

Differential

A-B

Supply

Voltage

Analog

Input Level

Table 2a. Typical Off-Isolation

Rejection Ratio

OIRR = 20 Log ____________

Frequency

100kHz

500kHz

1MHz

One Channel Driven

74dB

72dB

66dB

All Channels Driven

64dB

48dB

44dB

Table 2b. Typical Crosstalk

Rejection Ratio

at the tabulated frequency is applied to Channel 2. All other

channels are open circuited. Similar crosstalk rejection can be

observed between any two channels.

Frequency

100kHz

500kHz

1MHz

70dB

68dB

64dB

62dB

46dB

42dB