MAX354/MAX355
The channel-to-channel switching time is typically
180ns, with about 100ns of break-before-make delay.
This 100ns break-before-make delay prevents the
input-to-input short that would occur if two input chan-
nels were simultaneously connected to the output. In a
typical data acquisition system, the dominant delay is
not the switching time of the multiplexer, but is the set-
tling time of the amplifiers and S/H. Another limiting fac-
tor is the RC time constant of the multiplexer R
ON
plus
the signal source impedance multiplied by the load
capacitance on the output of the multiplexer. Even with
low signal source impedances, 100pF of capacitance
on the multiplexer output will approximately double the
settling time to 0.01% accuracy.
Operation with Supply Voltages
Other than ±15V
The main effect of supply voltages other than ±15V is
the reduction in output signal range. The MAX354 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
Operating Characteristics
show R
ON
for +15V and ±5V
power supplies. Maxim tests and guarantees the
MAX354/MAX355 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 MAX354/MAX355 can operate with a single +4.5V
to +30V supply, as well as asymmetrical power sup-
plies such as +15V and -5V. The digital threshold
remains approximately 1.6V above the GND pin, and
the analog characteristics, such as R
ON
, are deter-
mined by the total voltage difference between V+ and
V-. Connect V- to 0V when operating with a +4.5V to
+30V single supply.
The MAX354 digital threshold is relatively independent
of the power-supply voltages, going from 1.6V typical
when V+ is 15V to 1.5V typical when V+ is 5V. This
means that the MAX354/MAX355 operate with standard
TTL-logic levels, even with ±5V power supplies. In all
cases, the threshold of the enable (EN) pin is the same
as the other logic inputs.
Digital Interface Levels
The typical digital threshold of both the address lines
and the enable pin is 1.6V, with a temperature coeffi-
cient of about -3mV/°C. This ensures compatibility with
0.8V to 2.4V TTL-logic swings over the entire tempera-
ture range. The digital threshold is relatively indepen-
dent 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 refer-
enced to the GND pin.
The digital inputs can also be driven with CMOS-logic
levels swinging from either V+ to V- or from V+ to
ground. The digital input current is just a few nanoamps
of leakage at all input voltage levels, with a guaranteed
maximum of 1µA.
Operation as a Demultiplexer
The MAX354/MAX355 function as demultiplexers where
the input is applied to the output pin, and the input pins
are used as outputs. The MAX354/MAX355 provide
both break-before-make action and full fault protection
when operated as demultiplexers, unlike earlier genera-
tions of fault-protected muxes.
Channel-to-Channel Crosstalk,
Off-Isolation, and Digital Feedthrough
At DC and low frequencies the channel-to-channel
crosstalk is caused by variations in output leakage cur-
rents as the off-channel input voltages are varied. The
MAX354 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 MAX354 output, which is R
ON
plus the input
signal source resistance in most cases, since the load
driven by the MAX354 is usually high impedance. For a
signal source impedance of 10kΩ or lower, the DC
crosstalk exceeds 120dB.
Tables 2a and 2b show typical AC crosstalk and off-
isolation performance. Digital feedthrough is masked
by the analog charge injection when the output is
enabled. When the output is disabled, the digital
feedthrough is virtually unmeasureable, since the digi-
tal pins are physically isolated from the analog section
by the GND and V- pins. The ground plane formed by
these lines is continued onto the MAX354/MAX355 die
to provide over 100dB isolation between the digital and
analog sections.
Fault-Protected Analog Multiplexers
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