UM-0085-B09
DT80 Range User Manual
Page 304
RG
The auto-ranging process may therefore cause the time taken to sample a channel to be increased on occasion. To
avoid this, the gain can be locked on a particular setting, using the
GL
x
channel options.
Note:
auto-ranging does not affect the attenuator setting. Each channel definition command specifies (either implicitly or explicitly)
whether the attenuators should be on or off.
The following table summarises all possible gain/attenuator options
Channel definition
Attenuators
Input range
Units
1V
off
auto 30mV, 300mV, 3V
mV
1V(GL30MV)
off
30mV
mV
1V(GL300MV)
off
300mV
mV
1V(GL3V)
off
3V
mV
1V(GL50V)
error
1V(A)
on
auto 300mV, 3V, 50V
mV
1V(A,GL30MV)
error
1V(A,GL300MV)
on
300mV
mV
1V(A,GL3V)
on
3V
mV
1V(A,GL50V)
on
50V
mV
1HV
on
auto 300mV, 3V, 50V
V
1HV(GL30MV)
error
1HV(GL300MV)
on
300mV
V
1HV(GL3V)
on
3V
V
1HV(GL50V)
on
50V
V
1HV(NA)
off
auto 30mV, 300mV, 3V
V
1HV(NA,GL30MV)
off
30mV
V
1HV(NA,GL300MV)
off
300mV
V
1HV(NA,GL3V)
off
3V
V
1HV(NA,GL50V)
error
Warning:
Maximum input voltage on any analog input is ±50VDC (±30VDC in Series 3 or older), relative to the AGND/EXT# terminal. If
this is exceeded then permanent damage may occur.
Input Configurations
By definition, a voltage is a measurement between two points. Multiple separate voltage measurements can share a
common reference point, or each measurement can have an independent reference point.
The reference point, be it shared or unshared, need not be at ground potential. All voltage measurements made by the
DT80 are therefore differential measurements – they measure only the difference in voltage between the two terminals.
Another way of looking at it is that the DT80 will reject (ignore) the terminals' common mode voltage – that is, the voltage
that is common to both terminals. So if a channel's + terminal is at 7V (relative to the DT80's analog ground) and the –
terminal is at 5V then the 1V channel will return a value of 2V. The common mode voltage (5V) has been rejected.
Important The DT80 can effectively remove the unwanted common mode component from the input signals provided that
the common mode limits for each terminal is not exceeded (max. ±3.5V/35V for attenuators off/on, relative to the DT80's
analog ground). Note that because the DT80's analog ground is isolated, it can normally "float" up to match whatever
common mode voltage is present on the sensor being measured, thereby keeping the common mode voltage seen by
the DT80's amplifier within limits.
Shared-Terminal Inputs
In a shared-terminal configuration, a sensor’s "return" or "negative" wire is usually connected to the channel's # terminal,
as shown in
V1 – Shared Terminal Voltage Inputs(P305)
. The remaining sensor wire (the "positive" or "signal") is
connected to any of the channel’s other three terminals.
Each of the DT80's analog inputs can therefore support up to three shared terminal voltage inputs.
For shared-terminal inputs, the channel number is given a suffix indicating the terminal to which the positive wire is
connected. For example, the channel definition
1+V
specifies a shared-terminal voltage input applied to channel 1
between the + and # terminals (and likewise
1*V
and
1-V
).
Independent Analog Inputs
An independent or unshared” input is one that connects to its own terminals and does not share any of those terminals
with any other inputs As shown in
V2 – Independent Voltage Inputs(P305)
, each analog input can support up to two
independent voltage inputs – one between the + and – terminals (
1V
) and one between * and # (
1*V
).
For the DT80 Series 2 and DT85, these two independent inputs (i.e.
1V
and
1*V
) operate identically. For the DT80/81
however, they have slightly different characteristics. In particular, referencing a measurement to the # terminal (i.e. the
1*V
configuration) provides a better ground reference, so in most cases it is preferred. See
for
more details.