MAX378-MAX379 Maxim, MAX378-MAX379 Datasheet - Page 6

• Manufacturer Part Number: MAX378-MAX379
• Description: High-Voltage / Fault-Protected Analog Multiplexers
• Manufacturer: Maxim

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
plexer leakage current (I
bias current (I
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.
High-Voltage, Fault-Protected
Analog Multiplexers
Truth Table—MAX378
Figure 6. Typical Data Acquisition Front End
6
_______________Typical Applications
A2
_______________________________________________________________________________________
X
0
0
0
0
1
1
1
1
V
ERR
= R
= 2.0kΩ x (2nA + 30pA)
= 18.0µV maximum error
ZERO REFERENCE
THERMOCOUPLE
GAIN REFERENCE
STRAIN GUAGE
TRANSMITTER
4-20mA LOOP
A1
DS(ON)
X
0
0
1
1
0
0
1
1
BIAS
+10V
):
x (I
A0
X
0
1
0
1
0
1
0
1
OUT(ON)
OUT(ON)
DS(ON)
IN1
IN2
IN3
IN4
IN5
IN6
IN7
IN8
EN
+ I
0
1
1
1
1
1
1
1
1
-15V
V-
) and the amplifier
BIAS
MAX378
) times the multi-
+15V
V+
(MAX420))
GND
SWITCH
OUT
NONE
ON
1
2
3
4
5
6
7
8
OUT
-15V
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—MAX379
Note: Logic “0” = V
V-
DG508A
MAX358
MAX378
+15V
OR
V+
A1
X
0
0
1
1
GND
IN4
IN5
IN1
IN2
IN3
MAX420
A0
X
0
1
0
1
AL
+15V
-15V
≤ 0.8V, Logic “1” = V
EN
0
1
1
1
1
1M
100k
10k
1k
111
SWITCH
AH
NONE
ON
1
2
3
4
≥ 2.4V