UM-0085-B09
DT80 Range User Manual
Page 370
RG
Input Termination
The DT80's instrumentation amplifier has a very high input impedance. With such devices, it is necessary to provide a
path to allow the device's inherent "bias current" to flow to ground. If this is not done then the inputs will "float", possibly
causing significant inaccuracies.
For the DT80/ 81, when a measurement is made relative to the # terminal (e.g.
1+V
), the 100Ω shunt resistor between
the # terminal and
AGND
provides a ground path, thereby preventing any problems due to floating inputs. However,
when a measurement is made between the + and – terminals (e.g.
1V
), the shunt resistor will not be connected to the
amplifier input. For such measurements, it is necessary to
terminate
the inputs. This input termination is provided by a
pair of 1MΩ resistors, as shown in the block diagram. These are automatically switched in whenever a voltage
measurement is made using the + and – terminals (e.g.
1V
,
1TK
etc.), or they can be switched in manually using the
T
(terminate) channel option.
The use of these termination resistors will decrease the input impedance of the DT80 to around 1MΩ. That is, it will
increase the load on the voltage source being measured. This will generally only be a problem if the source has a very
high output impedance.
Also note that the ground reference provided by the 1MΩ termination resistors is not as effective as that provided by the
100Ω shunt resistor. For this reason, when performing an independent voltage measurement it is usually preferable to
make the measurement relative to the # terminal rather than the – terminal; that is
1*V
rather than
1V
.
For the DT80 Series 2, 3 and 4 and DT85, these termination resistors are not required, and the
T
option will have no
effect. This is because the 100Ω shunt resistor can be connected to either the – or the # terminal, as shown in the
diagram. This resistor will therefore provide a ground path for all input configurations.
Attenuator
To extend the input voltage range, the DT80 provides a switchable input attenuator. This is a resistive voltage divider
which attenuates the input by approximately 10:1. In Series 4 models this attenuator is set at 16:1 ratio. The attenuator
is enabled by default for the
HV
(high voltage) channel type, or it can be enabled manually using the A channel option.
As indicated in the diagrams, there is no attenuator on the # input. This means that input attenuation is not available for #
terminal measurements (which would typically be current measurements using the internal shunt resistor, e.g.
1#I
or
1#L
).
Furthermore, the excitation switching arrangement on the DT80/81 means that attenuation cannot be used if the logger is
supplying excitation from one of its internal sources. This restriction does not apply to the DT80 Series 2,3 and 4 and
DT85.
Note that when the attenuator is used the DT80's input impedance will decrease to approximately 100kΩ.
Excitation
Some measurements require that the sensor be
excited
in order for a measurable output voltage to be produced. For
example, to measure a resistance an excitation current is passed through the resistance and the resulting voltage drop is
measured.
As shown in the block diagrams, there are some differences between the DT80/81 and the DT80 Series 2,3/DT85 in the
way excitation is switched. These do not, however, affect the way that excitation is used from the user's point of view,
other than the caveat mentioned above regarding attenuation.
Internal Sources
If excitation is required, the DT80 provides four internal excitation sources:
•
precision current source, approx. 2μA (for Series 4 models only) . It can be selected manually using the
III
channel option.
•
precision current source, approx. 213μA. This is the default for the
R
(resistance),
NI
(RTD) and
YS
nn
(thermistor) channel types, or can be selected manually using the I channel option.
•
precision current source, approx. 2.5mA. This is the default for the
BGI
(bridge),
PT
nnn
(RTD) and
CU
(RTD)
channel types, or can be selected manually using the
II
channel option. This setting provides for more
accurate measurements of low resistances, at the expense of a reduced measurement range.
•
voltage source, approx 4.5V. This is the default for the
BGV
(bridge) and IC temperature sensor (e.g.
AD590
,
LM135
etc) channel types, or can be selected manually using the
V
channel option. This output has a 1kΩ
output impedance, so the output voltage will drop if any significant current is drawn.
If one of these sources is selected, it will be switched through to the * terminal of the channel being measured. The
excitation current returns via the channel's # terminal, then through the shunt resistor to analog ground. Note that the
excitation is only connected to the channel's terminals for the duration of the measurement.
For Series 3 models, the 213μA current source may be connected to the + or – terminals, as well as the * terminal. This
allows 2-wire resistance measurements to be performed b and – or between # and any of the other three
terminals.
Содержание DT80
Страница 29: ...UM 0085 B09 DT80 Range User Manual Page 29 RG The DT80 File System P114 ...
Страница 184: ...UM 0085 B09 DT80 Range User Manual Page 184 RG Figure 71 DT80 communications options ...
Страница 185: ...UM 0085 B09 DT80 Range User Manual Page 185 RG Figure 72 DT80 communications options integrated modem models ...