Datasheet

MAX378/MAX379
High-Voltage, Fault-Protected
Analog Multiplexers
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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
at the MAX378 output, which is R
DS(ON)
plus the input
signal source resistance in most cases, since the load
driven by the MAX378 is usually a high impedance. For
a signal source impedance of 10kΩ 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
Test Conditions: C
L
= 1000pF on Out A and Out B; the tabulat-
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
Test Conditions: V
IN
= 20V
P-P
at the tabulated frequency,
R
L
= 1.5kΩ between OUT and GND, EN = 0V.
20V
P-P
OIRR = 20 Log ____________
V
OUT (P-P)
Frequency 100kHz 500kHz 1MHz
One Channel Driven
74dB 72dB 66dB
All Channels Driven
64dB 48dB 44dB
Table 2b. Typical Crosstalk
Rejection Ratio
Test Conditions: Specified R
L
connected from OUT to GND,
EN = +5V, A0 = A1 = A2 = +5V (Channel 1 selected). 20V
P-P
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
F
L
= 1.5k
70dB 68dB 64dB
R
L
= 10k
62dB 46dB 42dB