Connecting Signals
4-13
Note:
Although input isolation minimizes the flow of noise current, it
cannot eliminate it entirely. Even isolated devices couple to ground
through finite capacitance. Consider coupling of a 5 volt, 8MHz ISA-bus
signal to the analog ground of the DAS-TC/B through a capacitance of
only 1pF. This coupling can cause roughly a 200
µ
A noise current to flow
in the connection between the analog ground and the ground of the object
to which the thermocouple is connected. If this current flows through the
negative lead of your thermocouple, instead of through a separate wire as
in Figure 4-8, a noise-voltage is generated in the lead. If you are using a
24 AWG Type K (Chromel-Alumel) thermocouple with 10-foot leads, a
200
µ
A noise current generates roughly a 1mV noise voltage across the
Alumel lead (resistance = ~ 0.44
Ω
/ft). Any of this noise that is rectified in
the input circuit, or that otherwise ends up as unwanted pulses at the
output of the voltage-to-frequency converter, results in a temperature
error.
Connecting Other Signal Sources
The connection principles are similar for signals other than thermocouple
signals, as discussed below.
Connecting a Floating Signal Source
If any of your signal sources are completely isolated, connect them as
shown in Figure 4-9.
Figure 4-9. Connecting a Floating Signal Source, Method 1
Channel n High
Channel n Low
Optional Shield
Es
STA-TC/B
or
STP-TC/B
Chassis
GND
Input
Amplifier
(Chassis GND via
S-1800 cable for
STA-TC/B)
B GND
NC
Es
Signal
Source
+
-
