Instructions

UM-0085-B09 DT80 Range User Manual Page 305
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Analog Input Isolation
The DT80 uses relay multiplexers to switch input channels through to the instrumentation amplifier one at a time. All four
terminals of each analog input channel remain disconnected from the DT80's electronics, except when that channel is
being measured. This means that each analog input channel is totally isolated from any other input channel.
What about the case where two independent inputs are connected to a single analog input channel?
For the DT80/81 Series 1, these two "independent" inputs are in fact not completely isolated from each other. This is
because for these logger models, all four terminals for a given channel are switched together. So if you are measuring
1V (voltage between + and – input terminals), bear in mind that the * and # terminals will also be connected to the DT80
circuitry.
This may cause problems if the two sensors have significantly different common mode voltages. If this is the case then
they should be connected to separate input channels.
For the DT80 Series 2 and DT85, the two pairs of input terminals (+ - and * #) can be switched independently. This
means that when doing a
1V measurement, the * and # terminals remain isolated. Similarly, when doing a 1*V
measurement, the + and – terminals remain isolated. The upshot of this is that sensor V1 (Figure 114) could have 10V
common mode voltage and sensor V2 could have -2V common mode and both would still be measured correctly. On a
DT80/81, these two sensors would need to be connected to separate channels (e.g.
1V and 2V).
V1 – Shared Terminal Voltage Inputs
In this configuration up to three separate voltage inputs can be connected to one analog input channel. The # terminal
acts as a shared common. See Shared-Terminal Analog Inputs (P23)
Shielded cable may be helpful when the signal has high output impedance or when noise pickup from other cables is a
problem. Ensure that the shield is only connected to ground at one end of the cable, either to a DGND terminal or the
earth point on the side of the DT80 case.
Figure 126: Wiring for shared-terminal voltage input
To measure
Use the command
V1
1*V
V2
1+V
V3
1-V
V2 – Independent Voltage Inputs
In this configuration each voltage measurement is independent of any other (no wires are shared). The trade-off is that at
most two voltages can be measured per analog input channel. For the DT80 Series 2 and DT85, the two measurements
are completely isolated – while measuring V1, V2 is disconnected, and vice versa.
This is not the case, however, for the DT80/81, where both voltage sources will be connected to the logger electronics
(and therefore referenced to the logger's analog ground) when either is being measured. This can cause measurement
errors if the two inputs have significantly different common mode voltages. See Input Switching (P369).
As with shared terminal inputs, a cable shield may be helpful when the signal has high output impedance or when noise
pickup for other cables is a problem. Ensure that the shield is only connected to ground at one end of the cable, either to
a DGND terminal or the earth point on the side of the DT80 case.
Figure 127: Wiring for independent voltage input
To measure
Use the command
V1
1V
V2
1*V