Section 4: Using the Model 9100: RTD Temperature Function 4.15-7
Final Width = 215mm
RTD Temperature Function Action
The UUT drives the current I
R
through R
IN
via the Hi terminal, and draws I
R
out via the
Lo terminal. The value of R
IN
accommodates the value of current I
R
. This first stage acts
as a current-to-voltage converter, whose output is a negative DC voltage of value I
R
x R
IN
,
with respect to the virtual ground at the converter input. This voltage is presented as input
to the gain-control amplifier '-G'
The system gain is set from the temperature selection by the Total Resistance Demand
value (
R
T
) transferred by DAC to control the gain of amplifier '-G'. The final amplifier
is switched by R
STD
and R
SET
in decade values. It generates a negative DC output voltage
V
R
, equal to I
R
x R
T
, across the Hi and Lo terminals; while sourcing the current I
R
drawn
by the UUT from the Lo terminal.
The value of the virtual resistance
R
T
is given by:
R
T
= R
IN
x G x R
STD
/ R
SET
4.15.4.5 4-Wire Connection for RTD Temperature Function
In order to protect the 9100 output, fuses and low-value protection resistors are placed
internally in series with each of the current-carrying wires. Also, as noted earlier, the
external current-carrying wires have their own resistance. The Lead-Impedance
Compensation Bridge is used to compensate for these impedances between the voltage
V
R
and the UUT input.
In Fig. 4.15.2, the 9100 and UUT are shown in 4-wire connection. One pair of wires
passes
I
R
(Hi and Lo at the 9100), while the second pair (sHi and sLo) senses the voltage
at the UUT input terminals.
The bridge receives the sensed voltage from the UUT terminals. If this is not exactly the
same as V
R
, the bridge compensates for any volts drops in the protection circuitry and
interconnecting leads, maintaining V
R
at the UUT input.
It is most important that a four-wire connection be used for RTD Temperature function.
The Model 9105 Leadset, supplied with the 9100, provides a four-wire connection. It is
recommended that the leadset be fitted, using its four short banana leads, whenever the
9100 is being used in RTD Temperature function.
When a four-wire connection is being used, the '
4 WIRE' screen key beneath the 9100
LCD display should be selected (light characters on a dark background). Four-wire
operation is already selected in the default condition of RTD function.
Refer to pages 4.15-2/3.
Caution:
The greatest error exists when 2-wire connections are being used, with 4-wire selected.
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