UM-0085-B09
DT80 Range User Manual
Page 312
RG
Channel Types
The DT80 supports two bridge channel types, which differ by the way in which the excitation voltage V
ex
is determined.
For a
BGV
channel, Vex is measured at the bridge; for
BGI
, the bridge is excited using a known current and then Vex is
calculated from the known current and arm resistance values.
The
BGI
channel type supports two different wiring variants: a true bridge configuration and a 3-wire "simulated bridge"
which has many of the properties of a bridge.
Channel Types
Description
BGV
Voltage-excited Wheatstone bridge
BGI(4W)
Current-excited Wheatstone bridge
BGI(3W)
Current-excited simulated bridge
For a voltage-excited bridge, two separate measurements are therefore required – one to measure V
ex
and one to
measure V
out
. To provide the maximum flexibility in wiring, this is done using two separate channels (one
V
, one
BGV
),
which are linked using the special
BR
channel option, as described below.
For a current-excited bridge, the nominal arm resistance is specified, and the DT80 uses this, along with the known
excitation current, to calculate V
ex
.
Channel Options – BGV
The following channel options are applicable when using BGV channels:
•
the excitation options specify how the bridge is powered:
V
(use internal 4.5V voltage source; default),
E
(external supply connected to EXT* terminal) or
N
(external supply)
•
GL
x
,
ES
n
, as for voltage measurements
•
the
channel factor
specifies an offset in ppm, which will be subtracted from the reading. This can be used to
“zero” the output
•
the
BR
(bridge reference) option must be specified for the voltage (V) channel used to measure Vex. This tells
the DT80 to use the measured voltage as V
ex
for a subsequent BGV channel in the same schedule.
Note:
The Series 4 models provides built-in 16-bit DAC converter at
V/I DAC
terminal. It capable to supply precise output voltages form
10mV to 10V. It can be used as an external excitation power source and it can be connected to the DT80's
EXT*
terminal. The channel
option
E
(external excitation) specifies that the power supply is connected to the DT80's
EXT*
terminal. This will then be automatically
connected to each current source for the duration of a measurement, and then disconnected.
Channel Options – BGI
The following channel options are applicable when using BGI channels:
•
the excitation options specify how the bridge is excited:
II
(use 2.5mA precision current source; default) or
I
(use 200μA source) or
III
(use 2uA precision current source)
•
the wiring options specify the type of bridge:
3W
(simulated bridge; default) or
4W
(true bridge)
•
the
channel factor
specifies the arm resistance, in ohms (default is 350. All arms are assumed to have equal
nominal resistance.
The following sections describe the various bridge wiring configurations.
B1 – 6-Wire BGV Inputs
Note:
Using the
HV
channel type to measure the bridge excitation voltage is not recommended. This channel type is normally not
sufficiently accurate for bridge applications.
Series 4 models provides a built-in 16-bit DAC converter at
V/I DAC
terminal. It capable to supply precise output voltages
from 10mV to 10V
In this configuration the bridge excitation is supplied by an external supply. This power supply can be either:
•
a standard supply, max 3V. This allows the DT80 to measure the actual excitation voltage using the * and #
terminals (
1*V
), as shown in the wiring diagram.
•
a standard supply in the range 3-6V. This is too large to directly measure using the * and # terminals, so instead
we use the bridge as a 2:1 voltage divider. That is, we measure the voltage between the + and # terminals
(
1+V
), then scale the result by 2 to give the actual excitation voltage.