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.

To calculate the quarter-bridge:

Measurement SG 1/4 bridge (quarter-bridge) 2/3-wire connection 1: — Connection of the quarter bridge

Explanation:

R1: external quarter-bridge resistor, nominally 120/350/1000 Ω
R2: internal supplementary resistor, is set to the same value as R1 after the CoE setting "Interface", and is therefore also 120, 350 or 1000 Ω
R3, R4: high-resistance internal bridge supplementary resistors, therefore, do not significantly load the internal supply
Rs: switchable shunt resistor
SW: internal switch for 2/3-wire operation; open: 3-wire operation

The strain relationship (µStrain, µε) is as follows:

Measurement SG 1/4 bridge (quarter-bridge) 2/3-wire connection 2:

For the quarter-bridge, N=1 always applies.

The relationship between U_Bridge/U_Exc and ∆R₁ is non-linear:

Measurement SG 1/4 bridge (quarter-bridge) 2/3-wire connection 3: — Relationship between UBridge/UExc and ∆R₁

The ELM350x devices apply internal linearization so that the output is already linearized

Measurement SG 1/4 bridge (quarter-bridge) 2/3-wire connection 4:

since the internal calculation is based on U_Exc'.

Measurement mode	StrainGauge/SG ¼-bridge 120 Ω 2/3-wire
	32 mV/V	8 mV/V	4 mV/V (comp.) ⁵⁾	2 mV/V (comp.) ⁵⁾
Measuring range, nominal	±32 mV/V [corresponds to ±64,000 µε at K=2] 120 ± 15.36 Ω	±8 mV/V [corresponds to ±16,000 µε at K=2] 120 ± 3.84 Ω	±4 mV/V [corresponds to ±8,000 µε at K=2] 120 ± 1.92 Ω	±2 mV/V [corresponds to ±4,000 µε at K=2] 120 ± 0.96 Ω
Measuring range, end value (FSV)	32 mV/V	8 mV/V	4 mV/V	2 mV/V
Measuring range, technically usable	±34.359… mV/V	±8.589… mV/V	±4.294… mV/V	±2.147… mV/V
PDO resolution	24 bit (including sign)
PDO LSB (Extended Range)	0.128 ppm 4.096 nV/V	0.128 ppm 1.024 nV/V	0.128 ppm 0.512 nV/V	0.128 ppm 0.256 nV/V
PDO LSB (Legacy Range)	0.119… ppm 3.814… nV/V	0.119… ppm 0.9535… nV/V	0.119… ppm 0.47675… nV/V	0.119… ppm 0.238375… nV/V

Specific data (preliminary, not valid for ELM3704-1001)

Measurement mode			Measuring bridge/StrainGauge SG ¼-bridge 120 Ω 2/3-wire
			32 mV/V	8 mV/V	4 mV/V (comp.) ⁵⁾	2 mV/V (comp.) ⁵⁾
Basic accuracy: Measuring deviation at 23°C, with averaging, typ. ²⁾		without Offset	< ±0.026 % _FSV < ±260 ppm _FSV < ±8.3 µV/V	< ±0.08 % _FSV < ±800 ppm _FSV < ±6.4 µV/V	< ±0.16 % _FSV < ±1600 ppm _FSV < ±6.4 µV/V	< ±0.32 % _FSV < ±3200 ppm _FSV < ±6.4 µV/V
		incl. Offset	< ±0.1 % _FSV < ±1000 ppm _FSV < ±32.0 µV/V	< ±0.4 % _FSV < ±4000 ppm _FSV < ±32.0 µV/V	< ±0.8 % _FSV < ±8000 ppm _FSV < ±32.0 µV/V	< ±1.6 % _FSV < ±16000 ppm _FSV < ±32.0 µV/V
Offset/Zero point deviation (at 23°C) ⁴⁾		E_Offset	< 960 ppm _FSV	< 3920 ppm _FSV	< 7840 ppm _FSV	< 15680 ppm _FSV
Gain/scale/amplification deviation (at 23°C)		E_Gain	< 160 ppm	< 440 ppm	< 880 ppm	< 1760 ppm
Non-linearity over the whole measuring range		E_Lin	< 200 ppm _FSV	< 650 ppm _FSV	< 1300 ppm _FSV	< 2600 ppm _FSV
Repeatability, over 24 h, with averaging		E_Rep	< 25 ppm _FSV	< 100 ppm _FSV	< 200 ppm _FSV	< 400 ppm _FSV
Noise (without filtering, at 23°C)		E_{Noise, PtP}	< 310 ppm _FSV < 2422 digits < 9.92 µV/V	< 1200 ppm _FSV < 9375 digits < 9.60 µV/V	< 2400 ppm _FSV < 18750 digits < 9.60 µV/V	< 4800 ppm _FSV < 37500 digits < 9.60 µV/V
		E_{Noise, RMS}	< 50 ppm _FSV < 391 digits < 1.60 µV/V	< 200 ppm _FSV < 1563 digits < 1.60 µV/V	< 400 ppm _FSV < 3125 digits < 1.60 µV/V	< 800 ppm _FSV < 6250 digits < 1.60 µV/V
		Max. SNR	> 86.0 dB	> 74.0 dB	> 68.0 dB	> tbd. dB
		Noisedensity@1kHz	< 0.02	< 0.02	< 0.02	< 0.02
Noise (with 50 Hz FIR filter, at 23°C)		E_{Noise, PtP}	< 24 ppm _FSV < 188 digits < 0.77 µV/V	< 72 ppm _FSV < 563 digits < 0.58 µV/V	< 144 ppm _FSV < 1125 digits < 0.58 µV/V	< 288 ppm _FSV < 2250 digits < 0.58 µV/V
		E_{Noise, RMS}	< 4.0 ppm _FSV < 31 digits < 0.13 µV/V	< 12.0 ppm _FSV < 94 digits < 0.10 µV/V	< 24.0 ppm _FSV < 188 digits < 0.10 µV/V	< 48.0 ppm _FSV < 375 digits < 0.10 µV/V
		Max. SNR	> 108.0 dB	> 98.4 dB	> 92.4 dB	> 86.4 dB
Common-mode rejection ratio (without filter) ³⁾			tbd.	tbd.	tbd.	tbd.
Common-mode rejection ratio (with 50 Hz FIR filter) ³⁾			tbd.	tbd.	tbd.	tbd.
Temperature coefficient, typ.		Tc_Gain	< 20 ppm/K	< 48 ppm/K	< 96 ppm/K	< 192 ppm/K
Temperature coefficient, typ.		Tc_Offset	< 50 ppm _FSV /K < 1.60 µV/V/K	< 180 ppm _FSV /K < 1.44 µV/V/K	< 360 ppm _FSV /K < 1.44 µV/V/K	< 720 ppm _FSV /K < 1.44 µV/V/K
Largest short-term deviation during a specified electrical interference test			tbd. %_FSV	tbd. %_FSV	tbd. %_FSV	tbd. %_FSV
Input impedance ±Input 1	Differential		tbd.	tbd.	tbd.	tbd.
Input impedance ±Input 1	CommonMode		tbd.	tbd.	tbd.	tbd.
Input impedance ±Input 2	3-wire
	Differential		tbd.	tbd.	tbd.	tbd.
	CommonMode		tbd.	tbd.	tbd.	tbd.

Measurement mode	StrainGauge/SG ¼-bridge 350 Ω 2/3-wire
	32 mV/V	8 mV/V	4 mV/V (comp.) ⁵⁾	2 mV/V (comp.) ⁵⁾
Measuring range, nominal	±32 mV/V [corresponds to ±64,000 µε at K=2] 350 ± 44.8 Ω	±8 mV/V [corresponds to ±16,000 µε at K=2] 350 ± 11.2 Ω	±4 mV/V [corresponds to ±8,000 µε at K=2] 350 ± 5.6 Ω	±2 mV/V [corresponds to ±4,000 µε at K=2] 350 ± 2.8 Ω
Measuring range, end value (FSV)	32 mV/V	8 mV/V	4 mV/V	2 mV/V
Measuring range, technically usable	±34.359… mV/V	±8.589… mV/V	±4.294… mV/V	±2.147… mV/V
PDO resolution	24 bit (including sign)
PDO LSB (Extended Range)	0.128 ppm 4.096 nV/V	0.128 ppm 1.024 nV/V	0.128 ppm 0.512 nV/V	0.128 ppm 0.256 nV/V
PDO LSB (Legacy Range)	0.119… ppm 3.814… nV/V	0.119… ppm 0.9535… nV/V	0.119… ppm 0.47675… nV/V	0.119… ppm 0.238375… nV/V

Specific data (preliminary, not valid for ELM3704-1001)

Measurement mode		Measuring bridge/StrainGauge SG ¼-bridge 350 Ω 2/3-wire
		32 mV/V	8 mV/V	4 mV/V (comp.) ⁵⁾	2 mV/V (comp.) ⁵⁾
Basic accuracy: Measuring deviation at 23°C, with averaging, typ. ²⁾	without Offset	< ±0.022 % _FSV < ±220 ppm _FSV < ±7.0 µV/V	< ±0.08 % _FSV < ±800 ppm _FSV < ±6.4 µV/V	< ±0.16 % _FSV < ±1600 ppm _FSV < ±6.4 µV/V	< ±0.32 % _FSV < ±3200 ppm _FSV < ±6.4 µV/V
	incl. Offset	< ±0.1 % _FSV < ±1000 ppm _FSV < ±32.0 µV/V	< ±0.4 % _FSV < ±4000 ppm _FSV < ±32.0 µV/V	< ±0.8 % _FSV < ±8000 ppm _FSV < ±32.0 µV/V	< ±1.6 % _FSV < ±16000 ppm _FSV < ±32.0 µV/V
Offset/Zero point deviation (at 23°C) ⁴⁾	E_Offset	< 970 ppm _FSV	< 3920 ppm _FSV	< 7840 ppm _FSV	< 15680 ppm _FSV
Gain/scale/amplification deviation (at 23°C)	E_Gain	< 120 ppm	< 380 ppm	< 760 ppm	< 1520 ppm
Non-linearity over the whole measuring range	E_Lin	< 180 ppm _FSV	< 750 ppm _FSV	< 1500 ppm _FSV	< 3000 ppm _FSV
Repeatability, over 24 h, with averaging	E_Rep	< 25 ppm _FSV	< 100 ppm _FSV	< 200 ppm _FSV	< 400 ppm _FSV
Noise (without filtering, at 23°C)	E_{Noise, PtP}	< 320 ppm _FSV < 2500 digits < 10.24 µV/V	< 1200 ppm _FSV < 9375 digits < 9.60 µV/V	< 2400 ppm _FSV < 18750 digits < 9.60 µV/V	< 4800 ppm _FSV < 37500 digits < 9.60 µV/V
	E_{Noise, RMS}	< 55 ppm _FSV < 430 digits < 1.76 µV/V	< 200 ppm _FSV < 1563 digits < 1.60 µV/V	< 400 ppm _FSV < 3125 digits < 1.60 µV/V	< 800 ppm _FSV < 6250 digits < 1.60 µV/V
	Max. SNR	> 85.2 dB	> 74.0 dB	> 68.0 dB	> 61.9 dB
	Noisedensity@1kHz	< 0.02	< 0.02	< 0.02	< 0.02
Noise (with 50 Hz FIR filter, at 23°C)	E_{Noise, PtP}	< 18 ppm _FSV < 141 digits < 0.58 µV/V	< 72 ppm _FSV < 563 digits < 0.58 µV/V	< 144 ppm _FSV < 1125 digits < 0.58 µV/V	< 288 ppm _FSV < 2250 digits < 0.58 µV/V
	E_{Noise, RMS}	< 3.0 ppm _FSV < 23 digits < 0.10 µV/V	< 12.0 ppm _FSV < 94 digits < 0.10 µV/V	< 24.0 ppm _FSV < 188 digits < 0.10 µV/V	< 48.0 ppm _FSV < 375 digits < 0.10 µV/V
	Max. SNR	> 110.5 dB	> 98.4 dB	> 92.4 dB	> 86.4 dB
Common-mode rejection ratio (without filter) ³⁾		tbd.	tbd.	tbd.	tbd.
Common-mode rejection ratio (with 50 Hz FIR filter) ³⁾		tbd.	tbd.	tbd.	tbd.
Temperature coefficient, typ.	Tc_Gain	< 12 ppm/K	< 50 ppm/K	< 100 ppm/K	< 200 ppm/K
Temperature coefficient, typ.	Tc_Offset	< 30 ppm _FSV /K < 0.96 µV/V/K	< 110 ppm _FSV /K < 0.88 µV/V/K	< 220 ppm _FSV /K < 0.88 µV/V/K	< 440 ppm _FSV /K < 0.88 µV/V/K
Largest short-term deviation during a specified electrical interference test		tbd. %_FSV	tbd. %_FSV	tbd. %_FSV	tbd. %_FSV
Input impedance ±Input 1	Differential	tbd.	tbd.	tbd.	tbd.
Input impedance ±Input 1	CommonMode	tbd.	tbd.	tbd.	tbd.
Input impedance ±Input 2	3-wire
	Differential	tbd.	tbd.	tbd.	tbd.
	CommonMode	tbd.	tbd.	tbd.	tbd.

²) In real bridge measurement, an offset adjustment is usually carried out after installation. The given offset specification of the terminal is therefore practically irrelevant. Therefore, specification values with and without offset are given here. In practice, the offset component can be eliminated by the functions ELM Features and also ELM Features of the terminal or in the controller by a higher-level tare function. The offset deviation of a bridge measurement over time can change, therefore Beckhoff recommends a regular offset adjustment or careful observation of the change.

³) Values related to a common mode interference between SGND and internal ground.

⁴) The offset specification does not apply to 2-wire operation, since the offset is increased on the device side. Therefore, a system-side offset adjustment is recommended, see Tare- or ZeroOffset function. The final targeting basic acuuracy within the 2-wire operation is mainingly dependent by the quality of this system-side offset adjustment.

⁵) The channel measures electrically to 8 mV/V, but displays its measured value scaled to 2 or 4 mV/V. The Compensated function facilitates measurement of low levels even with high offset.

Notice

Transition resistances of the connection contacts

The transition resistance values of the connection contacts affect the measurement. The measuring accuracy can be further increased by a user-side adjustment with the signal connection plugged in.

The temperature sensitivity of the terminal and thus of the measurement setup can be reduced if an external, more temperature-stable supplementary resistor is used for operation of the terminal in half-bridge or even full-bridge mode instead of the internal supplementary resistor for quarter-bridge mode.