Engineering Application Sheet E032 - SY014 Integral Loadcell Amplifier

General Limitations

1. A loadcell with an integral amplifier will not perform as well as the same type of loadcell without the amplifier. The overall performance will be a combination loadcell performance and the amplifier performance.
2. The integral amplifier limits the operating temperature range of the loadcell to 0 to 50°C.
3. The incorrect supply voltage or connections can damage a loadcell with an integral amplifier unlike a basic strain gauge bridge that is inherently electrically robust.
4. Re-calibration of the loadcell is difficult because the trimmers on the amplifier cannot normally be adjusted without dismantling the loadcell.
5. The span and zero drift performance are generally not as good as an external amplifier.
6. Permanent zero changes caused by over loading the loadcell can cause problems because of the limited signal range. The new zero plus the working signal may exceed 20mA. This takes the output outside the linear operating range of the amplifier.
7. The typical amplifier non-linearity of 0.05% of full range can be significant when the amplifier is used with a loadcell that has good non-linearity.
8. The measurement resolution is limited in bi-directional applications because the 16mA output range has to be split between compression and tension by setting zero at 12mA.
9. The amplifiers are too large to fit into a lot of our loadcell families.
10. Loadcells are often mounted in areas where physical damage is possible. If this occurs to a loadcell with an integral amplifier it will be a more expensive item to repair than a passive loadcell.

Selection Criteria

1. Use an external amplifier if possible. The performance is likely to be better and access to the zero and span adjustments will be possible. One reason often given for using integral amplifiers is to avoid problems with long loadcell cables but practical experience shows that passive loadcells work well with long cables even in electrically noisy environments. One manufacturer of very high precision loadmeters specifies a maximum cable length of 500m for a single loadcell.
2. If an integral amplifier must be used the first choice should be a three wire unit as this will be more stable. This is partly because the working current drawn by the electronics and the strain gauge bridge forms part of the 4mA zero current. This current will change with temperature causing apparent zero drift. The supply voltage to a two wire amplifier changes with signal level because of the voltage drop across the loop resistance. This generally worsens the performance of two wire amplifiers. The two wire SY014 amplifier requires a bridge resistance of at least 2000ohm.
3. Pick a loadcell with 2mV/V output and low zero drift specification if possible.
4. Avoid bi-directional calibration if possible.
5. Avoid non-standard scaling if possible as this is likely to increase the drift and can cause operational problems. If the scaling requires full range output of the amplifier for 25% of full range on a 2mV/V loadcell the signal will only be 0.5mV/V. An SY014 with this input range will have a zero drift approximately four times greater than the drift of a 2mV/V unit.
6. Consider how the system will deal with zero changes. As mentioned above zero changes can result in outputs that exceed the linear range of the amplifier.
7. It is important to be realistic about the overall performance particularly if wide temperature variations are possible during normal operation of the loadcell.

Installing A Loadcell With An Integral Amplifier

1. Read the connection sheet that is supplied with the loadcell.
2. The power supply must be within the range stated on the connection sheet at all times.
3. The power supply must be current limited to 1A or less to reduce the chance of damage caused by reverse supply polarity. In two wire systems the loop resistance may be sufficient to limit the current. Experience has shown that modules are most often damaged by bench tests using a multi-meter and an unsuitable power supply. This is because the multi-meter will have a very low resistance on its current ranges and will not limit the current adequately if the supply polarity is reversed.
4. Always check the wiring before switching on the power for the first time.
5. Unlike a basic strain gauge bridge reversing the supply polarity will not reverse the loadcell output.
6. Three wire amplifiers can normally be connected to data loggers that have a four wire current input by connecting the two 0V connections on the input together.
7. High voltage insulation tests must never be made on loadcells with integral amplifiers.
8. Incorrect external wiring or the application of voltages outside of those given on the connection sheet are not covered by warranty.
9. The body of the loadcell should always be earthed to reduce the effect of electrical interference.
10. Try to avoid placing the loadcell close to sources of interference.
11. Site the loadcell where the ambient temperature never exceeds the 0 to 50°C range.

Operation Of Two Wire 4 to 20mA Outputs

1. The loadcell should be allowed to warm up before taking measurements. Allow at least 30 minutes, more time may be required for large loadcells. This may need testing after the loadcell is installed.
2. The best accuracy will be obtained if the loadcell is operated using a 250Ω loop resistance and a 24V power supply. Changes in the supply voltage and loop resistance will change the output current.
3. The zero drift is typically 0.012mA/°C with a 2mV/V loadcell with a 0.005%RL/°C bridge zero drift. This could increase by a factor of two with a 1mV/V loadcell. If the bridge zero drift was 0.03%RL/°C this could add 0.005mA/°C to the drift on a 1mV/V loadcell.
4. Zero drift in two wire amplifiers is difficult to predict because there are a lot of variables that affect the drift so the values given here are subject to large variations.

Operation Of Three Wire 4 to 20mA Outputs

1. The loadcell should be allowed to warm up before taking measurements. Allow at least 30 minutes, more time may be required for large loadcells. This may need testing after the loadcell is installed.
2. While the three wire circuit has better immunity to supply voltage and loop resistance changes it is still better to avoid operating at the extremes of the specification.
3. The zero drift is typically 0.003mA/°C with a 2mV/V loadcell with a 0.005%RL/°C bridge zero drift. This could increase by a factor of two with a 1mV/V loadcell. If the bridge zero drift was 0.03%RL/°C this could add 0.005mA/°C to the drift on a 1mV/V loadcell.
4. The span drift is typically 0.002mA/°C with a 2mV/V loadcell. This could increase by a factor of two with a 1mV/V loadcell.

Other Related Documents

MN0016 SY014-2 Two Wire 4 to 20mA Output Amplifier - Operating Information
MN0017 SY014-3 Three Wire 4 to 20mA Output Amplifier - Operating Information
MN0042 SY014-3 Three Wire 4 to 20mA Output Amplifier F218-Z0899/0900 - Operating Information
MN0059 SY014-3 Three Wire 4 to 20mA Output Amplifier For F256-Z3241 - Operating Information
MN0062 SY014-3 Three Wire 4 to 20mA Output Amplifier For F256-Z3258 - Operating Information
E012/0102 Engineering Application Sheet E012: Electrical
Copies of these documents can be obtained from our sales department.

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