Measurement Sg 1/4 Bridge (Quarter-Bridge) 2/3-Wire Connection - Beckhoff EPP3504-0023 Short Manual

3.2.8 Measurement SG 1/4 bridge (quarter-bridge) 2/3-wire
connection
Notes
• Quarter-bridge measurement in 2-wire operation is not recommended in practice. The normal copper
supply cables with their own resistance (e.g. ~17 mΩ/m with 1 mm² wire) and their very high
temperature sensitivity (~4000 ppm/K, ~0.4%/K) have a considerable effect on the calculation and can
only be corrected by continuous offset and gain adjustment. Only 3-wire operation should be used.
• Specifications apply to 5 V excitation.
The specification deteriorates at lower excitation voltage; Beckhoff does not have detailed information
on this.
If a lower excitation voltage is desired for reasons of sensor self-heating, the excitation voltage can be
temporarily switched on/off for non-continuous measurements (clocked operation). Switching on/off
must be done from the controller via ADS access to the CoE 0x80n0:02.
• Specifications only apply when using wire end sleeves and for cross-sections of 0.5 mm² or more. For
smaller cross-sections, increased transition resistance is to be expected.
• Avoid repeated insertion/extraction of the push-in connectors in quarter-bridge operation since this may
increase the transition resistance.
• Integrated supply: 2...5V adjustable, max. supply/excitation 21 mA (internal electronic overload
protection).
Note: effectively only half the voltage is present at the quarter-bridge due to the internally switched
bridge supplement.
• Data valid from production week 21/2019 and for EPP3504-0023: HW04
By a user-side adjustment with a connected bridge sensor, the measurement uncertainity related to gain and
offset error can be significant reduced.
The integrated switcheable shunt resistor can be used to generate a predictable detuning or, in case of
deviation, a correction factor.
To calculate the quarter-bridge:
EPP3504-0023 Version: 1.2
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