MAX378/MAX379
High-Voltage, Fault-Protected
Analog Multiplexers
6 _______________________________________________________________________________________
_______________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 (R
DS(ON)
) times the multi-
plexer leakage current (I
OUT(ON)
) and the amplifier
bias current (I
BIAS
):
V
ERR
= R
DS(ON)
x (I
OUT(ON)
+ I
BIAS
(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.
Truth Table—MAX378
A2 A1 A0 EN
ON
SWITCH
X
0
0
0
0
1
1
1
1
X
0
0
1
1
0
0
1
1
X
0
1
0
1
0
1
0
1
0
1
1
1
1
1
1
1
1
NONE
1
2
3
4
5
6
7
8
Truth Table—MAX379
A1 A0 EN
ON
SWITCH
X
0
0
1
1
X
0
1
0
1
0
1
1
1
1
NONE
1
2
3
4
Note: Logic “0” = V
AL
≤ 0.8V, Logic “1” = V
AH
≥ 2.4V
MAX420
+15V
-15V
V+
+15V
1M
100k
10k
OUT
OUT
1k
111Ω
IN1
IN1
IN2
IN3
IN4
IN5
V+
THERMOCOUPLE
+15V
-15V
V- GND
DG508A
MAX358
OR
MAX378
MAX378
IN2
STRAIN GUAGE
IN7
+10V
GAIN REFERENCE
IN8
ZERO REFERENCE
IN3
4-20mA LOOP
TRANSMITTER
IN4
IN5
IN6
-15V
V- GND
Figure 6. Typical Data Acquisition Front End
