Measurement SG 1/2 bridge (half bridge) 3/5-wire connection

The nominal/technical measuring range is specified in "mV/V"; the maximum permitted supply voltage is 5 V. The maximum nominal measuring range that can be used for the bridge voltage is therefore ±16 mV/V ⋅ 5 V = ±80 mV; the internal circuits are designed for the 160 mV of the full bridge measurement.

The internal measurement is ratiometric, i.e. the feed voltage and the bridge voltage are not measured absolutely, but as a ratio.

The integrated supply can be used as power supply. An external supply is permitted, as long as 5 V is not exceeded.

The following is the specification given for the 5 wire connection. External line resistances are compensated by the 5 wire connection and the half bridge is detected directly from the measuring channel.
In the 3 wire connection, the terminal generally has the same specification, but its view of the connected half bridge is clouded by the unclear and temperature-dependent lead resistances within cables and connectors. In this respect, the overall system "half bridge + leads + measurement channel" will practically not achieve specification values given below.

The lead resistances (cables, connectors, ...) have an effect especially on the gain error, also depending on the temperature. The gain error can be estimated by:

(R_+uv (1 + ∆T ⋅ Tc_Cu) + R_-uv(1 + ∆T ⋅ Tc_Cu) )/R_nom with Tc_Cu~3930 ppm/K, R_nom

e.g. 350 Ω and R_+uv or R_-uv lead resistances respectively.

The use of the measurement channel in the 5 wire connection is recommended.

To calculate the R_1/2 half bridge:

Measurement SG 1/2 bridge (half bridge) 3/5-wire connection 1:

R_3/4 are the internal switchable supplementary resistors of the terminal. They have a high resistance of a few kΩ compared to R_1/2 and thus do not significantly load the internal supply.

Other half-bridge configurations (e.g. R_1/4 or R_1/3 variable) cannot be connected.

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

Measurement SG 1/2 bridge (half bridge) 3/5-wire connection 2:

N should be chosen based on the mechanical configuration of the variable resistors (Poisson, 2 active uniaxial, …). The channel value (PDO) is interpreted directly [mV/V].

Common data

Measurement mode	StrainGauge/SG 1/2-Bridge 3/5-wire
	16 mV/V	2 mV/V
Integrated power supply	1…5V adjustable, max. supply/Excitation 21 mA (internal electronic overload protection) therefore 120R SG: up to 2.5 V 350R SG: up to 5.0 V
Measuring range, nominal	-16 … 16 mV/V	-2 … 2 mV/V
Measuring range, end value (full scale value)	16 mV/V	2 mV/V
Measuring range, technically usable	-17.179 … 17.179 mV/V	-2.147 … 2.147 mV/V
PDO resolution	24 bit (including sign)
PDO LSB (Extended Range)	0.128 ppm
PDO LSB (Legacy Range)	0.119… ppm

Note: specifications apply for 3.5 V SG excitation and symmetric 350R SG.

Specific data ELM3702-0101 (preliminary data in cursive format)

Measurement mode		Measuring bridge/SG/StrainGauge/ SG 1/2-Bridge 3/5-wire
Measurement mode		16 mV/V	2 mV/V
Basic accuracy: Measuring deviation at 23°C, with averaging, typ. ²⁾	without Offset	< ±0.0145 %_FSV < ±145 ppm_FSV < ±2.32 µV/V	< ±0.09 %_FSV < ±900 ppm_FSV < ±tbd.
	incl. Offset	< ±0.041 %_FSV < ±410 ppm_FSV < ±6.56 µV/V	< ±0.27 %_FSV < ±2700 ppm_FSV < ±tbd.
Extended basic accuracy: Measuring deviation at 0…55°C, with averaging, typ. ^{2) 6)}	without Offset	< ±0.053 %_FSV < ±530 ppm_FSV < ±8.48 µV/V	< ±tbd. %_FSV < ±tbd. ppm_FSV < ±tbd.
	incl. Offset	< ±0.0655 %_FSV < ±655 ppm_FSV < ±10.48 µV/V	< ±tbd. %_FSV < ±tbd. ppm_FSV < ±tbd.
Offset/Zero point deviation (at 23°C)	E_Offset	< 385 ppm_FSV	< 2550 ppm_FSV
Gain/scale/amplification deviation (at 23°C)	E_Gain	< 80 ppm	< 500 ppm
Non-linearity over the whole measuring range	E_Lin	< 120 ppm_FSV	< 740 ppm_FSV
Repeatability, over 24 h, with averaging	E_Rep	< 20 ppm_FSV	< 120 ppm_FSV
Temperature coefficient, typ.	Tc_Gain	< 5 ppm/K	< tbd. ppm/K
Temperature coefficient, typ.	Tc_Offset	< 15 ppm_FSV/K < 0.24 µV/V/K	< tbd. ppm_FSV/K < tbd.
Common-mode rejection ratio (without filter) ³⁾	DC:	tbd. typ.	tbd. typ.
	50 Hz:	tbd. typ.	tbd. typ.
	1 kHz:	tbd. typ.	tbd. typ.
Common-mode rejection ratio (with 50 Hz FIR filter) ³⁾	DC:	tbd. typ.	tbd. typ.
	50 Hz:	tbd. typ.	tbd. typ.
	1 kHz:	tbd. typ.	tbd. typ.
Noise (without filtering, at 23°C)	E_{Noise, PtP}	< 500 ppm _FSV < 3906 digits < 8.00 µV/V	< 4000 ppm _FSV < 31250 digits < 8.00 µV/V
	E_{Noise, RMS}	< 85 ppm _FSV < 664 digits < 1.36 µV/V	< 660 ppm _FSV < 5156 digits < 1.32 µV/V
	Max. SNR	> 81.4 dB	> 63.6 dB
	Noisedensity@1kHz	< 19.23	< 18.67
Noise (with 50 Hz FIR filter, at 23°C)	E_{Noise, PtP}	< 35 ppm _FSV < 273 digits < 0.56 µV/V	< 280 ppm _FSV < 2188 digits < 0.56 µV/V
	E_{Noise, RMS}	< 6.0 ppm _FSV < 47 digits < 0.10 µV/V	< 46.0 ppm _FSV < 359 digits < 0.09 µV/V
	Max. SNR	> 104.4 dB	> 86.7 dB
Largest short-term deviation during a specified electrical interference test		tbd.	tbd.
Input impedance ±Input 1 (internal resistance)		Differential typ. tbd.	Differential typ. tbd.
Input impedance ±Input 1 (internal resistance)		CommonMode typ. tbd.	CommonMode typ. tbd.
Input impedance ±Input 2 (internal resistance)		3-wire: No usage of this input in this mode	3-wire: No usage of this input in this mode
		Differential typ. tbd.	Differential typ. tbd.
		CommonMode typ. tbd.	CommonMode typ. 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.

⁶) Calculated value according to equation in chapter "General information on measuring accuracy/measurement uncertainty" for quick estimation of usability over the specified ambient temperature range in operation (T_ambient). In real use, for example at a relatively constant ambient temperature T_ambient, a lower (better) achievable uncertainty is attained. A specific calculation according to chapter "General information on measuring accuracy/measurement uncertainty" is recommended, especially if the instrument allows a wider ambient temperature range in operation than 0...55 °C.

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.

	Validity of property values The resistor of the bridge is positioned parallel to the internal resistor of the terminal and leads to an offset shifting respectively. The Beckhoff factory calibration will be carried out with the half bridge 350 Ω, thus the values specified above are directly valid for the 350 Ω half bridge. By connection of another dimensioned half-bridge is to: perform a balancing (offset correction) by the terminal itself or the control/PLC on application side or the abstract offset error have to be entered into the balancing parameter S0 of the terminal. Example: a 350 Ω half bridge correlates by the compensated effect of the input resistor (2 MΩ) during factory calibration 0.26545 %_FSV (16 mV/V), that corresponds to 20738 digits.