MAX378/MAX379
High-Voltage, Fault-Protected
Analog Multiplexers
8 _______________________________________________________________________________________
to-source voltage. The P-channel device (Q2), howev-
er, has +60V V
GS
and is turned off, thereby preventing
the input signal from reaching the output. If the input
voltage is +60V, Q1 has a negative V
GS
, which turns it
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-
channel threshold voltage (V
TN
), then the N-channel
FET will turn off. For voltages more negative than V-
minus the P-channel threshold (V
TP
), the P-channel
device will turn off. Since V
TN
is typically 1.5V and V
TP
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
R
DS(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 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
Operating Characteristics
graphs show typical R
DS(ON)
,
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-
istics such as R
DS(ON)
are determined by the total voltage
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
Test Conditions: C
L
= 1000pF on multiplexer output; the tabu-
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
