Instructions

UM-0085-B09 DT80 Range User Manual Page 369
RG
Ground Terminals
The DT80's analog section is electrically isolated from the rest of the unit. There are therefore two separate ground
references – digital ground and analog ground.
The DT80's front panel label further reinforces the separation of the analog and digital sections:
Figure 170: The DT80 has two ground systems
Isolating the analog and digital sections means that sensor-to-equipment ground loops (see Ground Loops (P372)) are
unlikely to arise. To preserve the DT80's isolation these grounds should not normally be connected together.
An isolated analog ground also means that the DT80 can reject a large common mode voltage on the input being
measured because its analog ground can "float" up to match the common mode voltage. This means that the common
mode voltage seen by the DT80's amplifier will be small.
 Digital Ground
The DT80's digital ground is connected to the DGND terminals, as well as to the chassis earth point and the various
"common" or "ground" terminals on the communications and power interfaces. Digital ground is the ground reference for:
• DC power input
• all communications links (including serial sensor port), if required
• digital and counter inputs and outputs
• DC power outputs 12V/5V and PWR OUT (DT85 only)
• cable shields
 Analog Ground
The DT80's analog ground is shown by a black triangle symbol in the block diagram.
Most analog measurements will not require any ground reference connection, as the measurement is taken relative to
the channel's – or # terminal. The only case where a ground connection is required is when making current
measurements using the DT80's internal shunt resistor. In this configuration the current to be measured flows in the #
terminal, through the shunt resistor and returns via the analog ground terminal to the external current source. Refer to
the wiring diagrams, e.g. C3 – Independent Current Input using Internal Shunt (P307) for more details.
For the DT80/81, analog ground is permanently connected to the AGND terminals.
For the DT80 Series 2,3 and 4 and DT85, analog ground is connected to the EXT# terminals – but only when needed.
That is, it is only connected during a measurement that uses the internal shunt resistor (e.g. 1#I), or that uses externally
generated excitation (E channel option). Switching the analog ground connection in this way means that a channel which
does not require an analog ground connection (e.g. a thermocouple) can be completely isolated from a channel which
does (e.g. a current loop which uses the internal shunt). With the DT80/81, the thermocouple channel would not be
completely isolated from the current loop channel because the thermocouple channel's # terminal would be connected
(via the shunt resistor) to the AGND terminal and therefore to the ground of the current loop.
For Series 3 models, a permanent AGND terminal is provided as the return path for the switched isolated 5V power
output 5VSW and voltage/current digital converter V/IDAC on Series 4 models.
Input Switching
As shown in the block diagrams, each input terminal is switched via relay contacts. While an analog channel is being
measured, its terminals are switched through to the analog input multiplexer. All other channels have all four of their input
terminals disconnected, so they are completely isolated from the channel being measured.
For the DT80/ 81, the dotted lines between each channel's four relay contacts indicate that the four relay contacts are
always switched together, so all four are either all open or all closed.
For the DT80 Series 2, 3 and 4 and DT85, it can be seen that the contacts are split into two independently controlled
pairs – one for the + and – terminals and one for the * and # terminals. This means that if two independent differential
inputs are connected to the one channel then the two measurements will be fully isolated. That is, when you measure
between + and – (e.g.
1TK) the * and # terminals will be disconnected, and conversely when you measure between *
and # (e.g.
1*TK) the + and – terminals will be disconnected. This isolation improves measurement accuracy in
situations where the two inputs have different common mode voltages.