MAX378 Datasheet, PDF (6/12 Pages) Maxim Integrated Products – High-Voltage, Fault-Protected Analog Multiplexers

High-Voltage, Fault-Protected
Analog Multiplexers
Truth Table—MAX378
A2
A1
A0
EN
X
X
X
0
0
0
0
1
0
0
1
1
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
1
1
1
1
1
ON
SWITCH
NONE
1
2
3
4
5
6
7
8
Truth Table—MAX379
A1
A0
EN
ON
SWITCH
X
X
0
NONE
0
0
1
1
0
1
1
2
1
0
1
3
1
1
1
4
Note: Logic “0” = VAL ≤ 0.8V, Logic “1” = VAH ≥ 2.4V
THERMOCOUPLE
STRAIN GUAGE
4-20mA LOOP
TRANSMITTER
+10V
GAIN REFERENCE
ZERO REFERENCE
+15V
IN1
V+
OUT
IN2
IN3
IN4 ^MAX378
IN5
IN6
IN7
IN8
V-
GND
-15V
+15V
MAX420
-15V
+15V
V+
IN1
IN2
OUT
DG508A
MAX358 ^IN3
OR
MAX378 ^IN4
IN5
V-
GND
-15V
1M
100k
10k
1k
111Ω
Figure 6. Typical Data Acquisition Front End
_______________Typical Applications
Figure 6 shows a typical data acquisition system
using the MAX378 multiplexer. Since the multiplexer
is driving a high-impedance input, its error is a func-
tion of its own resistance (RDS(ON)) times the multi-
plexer leakage current (IOUT(ON)) and the amplifier
bias current (IBIAS):
VERR = RDS(ON) x (IOUT(ON) + IBIAS (MAX420))
= 2.0kΩ x (2nA + 30pA)
= 18.0µV maximum error
In most cases, this error is low enough that preamplifi-
cation of input signals is not needed, even with very
low-level signals such as 40µV/°C from type J thermo-
couples.
In systems with fewer than eight inputs, an unused chan-
nel can be connected to the system ground reference
point for software zero correction. A second channel
connected to the system voltage reference allows gain
correction of the entire data acquisition system as well.
A MAX420 precision op amp is connected as a pro-
grammable-gain amplifier, with gains ranging from 1 to
10,000. The guaranteed 5µV unadjusted offset of the
MAX420 maintains high signal accuracy, while program-
mable gain allows the output signal level to be scaled to
the optimum range for the remainder of the data acqui-
sition system, normally a Sample/Hold and A/D. Since
the gain-changing multiplexer is not connected to the
external sensors, it can be either a DG508A multiplexer
or the fault-protected MAX358 or MAX378.
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