dataTaker DT80 User Manual Download

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UM-0085-B09

DT80 Range User Manual

Page 371

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External Sources

Alternatively, an external excitation source may be provided. There are two options here:

•

The source can be connected directly to the sensor. The DT80 then measures the required voltage. As far as

the DT80 is concerned, it is just measuring a voltage; it is not concerned with providing excitation. The N (no

excitation) channel option may therefore be used to indicate this configuration. For example, refer to the bridge

wiring diagram:

B1 – 6-Wire BGV Inputs (P312)

•

A single source can be used to excite multiple channels. In this configuration the external supply is connected to

the

EXT*

terminal, and the return to

AGND

(DT80/ 81) or

EXT#

(DT80 Series 2/ DT85). The DT80 will then

switch this source through to the * terminal of the channel being measured. The excitation current then returns

via the channel's # terminal, through the internal shunt to analog ground, then back to the excitation supply via

the

AGND/ EXT#

terminal. If desired, the 12V power output on the DT85 and DT80 Series 2,3 and 4 can be

connected to

EXT*

as an excitation source, see also

Controlling 12V Power Output (P290)

. Alternatively the

Series 4 models provides built-in 16-bit DAC converter at

V/I DAC

terminal. It capable to supply the output

voltages up to 10V and currents to ~ 25mA.

In the first case the sensor is continuously excited, while in the second it is only excited during an actual measurement

(similar to internal excitation). Exciting a sensor only when needed is beneficial from a power consumption standpoint,

and it will help minimise self-heating issue in sensors such as thermistors. In some cases, however, a sensor may

require a certain warm up time. In this case the

(measurement delay) channel option may be used to extend the

measurement time)

Note:

external excitation cannot be used for channels such as

(resistance) or

BGI

(current excited bridge) where the DT80 needs to

accurately know the excitation current in order to calculate the quantity being measured. For these channel types one of the precision

internal current sources must be used.

3-Wire Compensation

The 3-Wire Compensation circuit is used for 2 and 3 wire resistance measurements. It will therefore be used by default

for any resistance measurement, unless the

option is used to specify a 4-wire measurement.

In a 3-wire measurement, excitation current flows out of the * terminal, through the excite wire, through the resistance

being measured, then back via the return wire to the # terminal. The measurement is then made between the + terminal

(which is shorted to *) and, using a third "sense" wire, the negative end of the resistance being measured. This

measurement will include the voltage drop across the excite wire, as well as that across the unknown resistance. See

– 3-Wire Resistance Inputs (P309)

The 3-wire compensation circuit works by measuring the voltage drop across the return wire (i.e. between the sense

point and the # terminal), multiplying it by 2 (to account for the voltage drop in the excite wire, which is assumed to be

equal to that in the return wire), then using this voltage to offset the negative input of the instrumentation amplifier. This

will effectively subtract the cable's voltage drop, thereby compensating for the effect of the cable resistance.
For a 2-wire measurement there is no sense wire. The return wire connects to the – terminal, so the measurement now

includes both the excite and the return resistance. This can then be manually compensated for by inserting a resistor

equal to the total cable resistance between the – and # terminals. The excitation current will flow through this resistor,

generating a voltage drop which will then offset the amplifier input in a similar way to a 3-wire measurement.

Calibration and Characterisation

There are two main sources of error in an electronic instrument such as the DT80:

•

variations in component values or characteristics due to manufacturing tolerances

•

variations in component values or characteristics due to ambient temperature

During production, the actual values for certain circuit parameters are measured (using test equipment traceable to ISO

standards) and then stored permanently into the DT80 memory. This process is known as

characterisation

. Once the

DT80 has been characterised, the firmware can correct for these variations. These stored values can be listed using the

CHARAC

command (see

CHARAC Command (P277)

Some circuit characteristics are, however, inherently temperature dependent. If the DT80 is to return accurate readings

over the full temperature range then these characteristics must be corrected for as well. This cannot be done in the

factory; it must be done during operation, as the temperature goes up and down. This process is known as

calibration

(not to be confused with characterisation).
On power-up and at regular intervals during operation, the DT80 measures the amplifier's internal "offset voltage". If it is

found to have drifted by a specified amount (3µV by default, can be adjusted using parameter P0) then a

calibration

cycle

is performed. This process takes approximately one second and involves the DT80 measuring several different

internal parameters, such as offset voltages (amplifier output with no input applied) for various input configurations, and

the actual values of the DT80's internal current sources. These readings are then used for all subsequent analog

measurements, until the next calibration cycle occurs. The

TEST

command will force a calibration cycle, and return the

values of most of the measured calibration parameters.