Transducers for Measurement Bridges
General Description
These transducers amplify the output signals of resistive measurement bridges and convert them to normalised output voltages (e.g., 0-10 V) or currents (e.g., 4-20 mA). Frequency output is available as an option, max. 20 kHz. In the programmable modules (SCM 90, SIGS 15), switches are used to select the zero point, measurement range, bridge supply voltage and the output. A cut-off frequency (3 Hz - 15 kHz) can also be set in the SCM 90S version.
- For pressure and force cells, strain gauges and other measurement bridges, bipolar input and output (down to -10 V).
- Differential, high-resistance voltage input.
- High precision voltage source to supply the bridge (5 V or 10 V, max. 120 mA). Special design with current source is available on request.
- In the SCM 90/SIGS 15 versions, switches are used to select zero point (max. ±15 mV), measurement range (max. ±127 mV), bridge power supply (5/10 V), output (voltage or current).
- Option: frequency output (max. 20 kHz), galvanic isolation, can be connected to a bus.
- Interference and destruction protection: secure against short circuits and terminal reversal, up to 40 VDC overvoltage protection at all inputs, fulfills all EC-EMC standards (EN50082/IEC 801).
Overview
 |
Modules for DIN-Rails |
| Type |
Output |
Power Supply |
Range |
|
| SCM 70 |
V |
19-30V/±15V |
fixed |
| SCM 82 |
0/4-20mA |
20-30V |
fixed |
|
| SCM 90 |
V/mA |
20-30V |
programmable |
|
 |
Printed Circuit Board/Euro-Card Modules |
| Type |
Output |
Power Supply |
Range |
|
| SIGS 10 |
V |
18-30V/±15V |
fixed |
| SIGS 32 |
0/4-20mA |
18-30V |
fixed |
|
| SIGS 15 |
V/mA |
18-30V |
programmable |
| SIGS 15S |
V/mA |
18-30V |
programmable |
Options
- DC-DC converter for SCM 70-90 (built into the module) for galvanic isolation of the power supply, 1 kV test voltage (3 kV available on request).
- Programmable limit switch for SCM 70-90 (built into the module, finger connectors on the side for relays) for monitoring and control.
- Line break monitoring (also for the bridge supply), output in positive saturation when fault detected.
- Frequency output (max. 20 kHz), galvanically isolated, see Transducers with Frequency Output for more information.
- Other designs (ranges, inputs, outputs, time behavior/filters, noise) and special versions.
When ordering, please specify:
- Module type
- Accuracy class (A, C or D)
- Input range (in mV) and output range (in V or mA, in Hz for frequency output)
- Power supply (24 V standard, 15 V or ±15 V on request)
- Bridge supply (10 V standard)
- For current output: whether 0-20 mA or 4-20 mA
- Current source instead of voltage source for bridge supply
- For programmable modules (SIGS 15/SCM 90), only the type and accuracy class need be specified. Upon request, the module can be precisely adjusted to a specific range.
Technical Data
Specifications (Maximum values at 25°C unless stated otherwise)
| General |
A |
C |
D |
Unit |
| Linearity error in the 50 mV range1 |
0.01 |
0.02 |
0.05 |
% |
| Calibration error, SCM 15/90 (0-64 mV = 0-10 V)1 |
0.05 |
0.1 |
|
% |
| 3 dB-bandwidth, SCM 10/32/70/82, typ.2 3 |
4-15 |
4-15 |
4-15 |
Hz |
| 3 dB-bandwidth SCM 15/90S, programmable4 |
3-15k |
3-15k |
|
Hz |
| Influence of operating voltage (24 VDC), typ. |
0.005 |
0.005 |
0.005 |
%/V |
|
| Input: |
A |
C |
D |
Unit |
| Input current, typ. |
1 |
1 |
30 |
nA |
| Noise 0.1-10 Hz, pp RTI, typ. |
0.5 |
1.0 |
1.5 |
µV |
| Error when switching range, SCM 15/901 |
0.2 |
0.3 |
|
% |
| Common mode input range, unipolar SCM 10/32/70/82 |
1 bis 7 |
1 bis 7 |
1 bis 7 |
V |
| Common mode input range, bipolar und SCM 15/90 |
-7 bis 7 |
-7 bis 7 |
|
V |
|
| Stability of zero point with respect to: |
A |
C |
D |
Unit |
| Temperature (RTI)1 |
0.3 |
1 |
5 |
µV/K |
| Aging, 1 year (RTI)1 |
5 |
10 |
|
µV |
| Aging, 10 years (RTI)1 |
20 |
40 |
|
µV |
|
| Stability of the amplifier with respect to: |
A |
C |
D |
Unit |
| Temperature1 |
25 |
70 |
150 |
ppm/K |
| Aging, 1 year1 |
400 |
800 |
|
ppm |
| Aging, 10 years1 |
1200 |
2500 |
|
ppm |
|
| Voltage output (SIGS 10/15, SCM 70/90) |
A |
C |
D |
Unit |
| Output impedance, typ. |
50 |
50 |
50 |
Ohm |
| Max. output current, typ. |
5 |
5 |
5 |
mA |
| Min. output voltage SCM 10/70 unipolar, typ. |
6 |
6 |
40 |
mV |
| Max. output voltage SCM15/90, typ. |
10.5 |
10.5 |
|
V |
| Min. output voltage SCM 15/90, typ. |
-10.2 |
-10.2 |
|
V |
|
| Current output (SIG 15/32, SCM 82/90) |
A |
C |
D |
Unit |
| Load, max. |
300 |
300 |
300 |
Ohm |
| Load, typ. |
390 |
390 |
390 |
Ohm |
| Max. current (for 24 V supply) |
20.5 |
20.5 |
20.5 |
mA |
|
| Bridge supply: |
A |
C |
D |
Unit |
| Temperature drift1 |
25 |
35 |
70 |
ppm/K |
| Current, max. (for 24 V supply) |
120 |
120 |
120 |
mA |
| Deviation from reference1 |
0.5 |
1 |
2 |
% |
| 1 |
The typical error is two to four times smaller than the quoted maximum error. |
| 2 |
Second order low-pass filter. Much higher frequencies can be delivered on request (up to approx. 10 kHz). |
| 3 |
The response time (to 1% of the final value during a jump in the input signal) varies between approx. 50 ms (for 15 Hz cut-off frequency) and approx. 300ms (for 4 Hz cut-off frequency). |
4 |
The response time (to 1% of the final value during a jump in the input signal) is around 300 ms (standard). In modules with a programmable cut-off frequency (SIGS15S, SCM 90S), it varies between approx. 40us (15 kHz) and 2.1 ms (330 Hz) in the upper range and between 2.1 ms (330 Hz) and approx. 300 ms (3.3 Hz) in the lower range). |
| Temperature range |
°C |
| Recommended |
0/60 |
| Operable |
-20/90 |
Output
- Voltage output: for modules with a fixed range (SIGS 10, SCM 70), switchable between -11 V and +11 V. For programmable modules, selectable between -10 V and +10 V. For modules with a fixed range and unipolar power supply, the minimum output voltage is approx. 6 mV (D-Typ: approx. 40 mV).
- Current output: standard 0-20 mA or 4-20 mA, secure against short circuits; when ordering fixed range modules (SIGS 32, SCM 82), please specify whether 0-20 mA or 4-20 mA.
- Option: frequency output (max. 20 kHz), galvanically isolated, can be connected to a bus, see "Modules with Frequency Output" for more information.
Power Supply
- All DIN-rail modules (SCM 70-90) are designed to cope with unregulated, highly variable, industrial voltages (24 VDC).
- SCM 70/82: standard 24 VDC supply voltage. A bipolar power supply is required for a negative output voltage (e.g., ±15 V). Current requirement without load approx. 5 mA
- SCM 15/90: standard 24 VDC supply voltage. Thanks to the built-in DC-DC converter, a negative output voltage does not require a negative supply voltage. Current requirement without load approx. 28 mA.
- Other supply voltages (also AC) available on request.
Ranges/Programming
- In the programmable versions (SIGS 15/SCM 90), the measurement range is selected via SMD switches (in steps of 1 mV, measurement range max. ±127 mV, zero point max. ±15 mV), intermediate values selectable via potentiometer. Switchable bridge supply (5 V or 10 V). Output switchable (voltage/current), current output switchable between 0-20 mA and 4-20 mA. In the SIGS15S and SCM 90S modules, a potentiometer is used to select a cut-off frequency between 3 Hz and approx. 15 Hz.LI>
- Any input and output range is deliverable (as long as technically feasible) for the fixed range modules (SIGS 10/32, SCM70/82).
Operating Instructions
Connecting a measurement bridge to a DIN-rail module
The above figure shows how to connect a resistive measurement bridge to a DIN-rail module. Note that the impedance of the bridge power supply cables should be as low as possible. The voltage drops could cause a slight reduction in the bridge supply voltage, especially for larger bridge currents.
Connecting a measurement bridge to a printed circuit board module
 |
The above comments are also generally applicable to the printed circuit board modules. |
Testing a bridge amplifier module (SIGS 10/32, SCM 70/82, unipolar)
A voltage calibrator can only be connected to the SIGS 10/32, SCM 70/82 modules with unipolar power supplies, when a DC voltage (common-mode voltage) is used to raise the signal voltage inputs to at least 1 V. This can be achieved, e.g., by connecting a second voltage source between connectors 6 (GND) and 7 (In-). Another possibility is illustrated in the neighbouring diagram. The 10 kOhm resisters reduce the bridge voltage to 5 V, which can then be used as the common-mode voltage. A similar procedure can be carried out for a printed circuit board module.
Note: The signal inputs 7 and 8 must always have a DC path to ground (true differential amplifier). This is automatically the case when a measurement bridge (with bridge supply 5,6) is connected. If sensor fault monitoring is installed (available as an option for modules SIGS 15 and SCM 90), then a voltage equivalent to half the bridge supply voltage must be applied to connector 7 (±20%). For a test, this can, for example, be accomplished as shown in the above diagram.
Programming the SIGS 15 and SCM 90 modules
The programmable switches 1a to 8a and 1b to 8b are located inside the module on the back of the printed circuit board. Carefully remove the plexiglass cover. The printed circuit board can now be removed by pulling gently on the screw terminals.
| Bridge Supply |
Switch 1a |
| 10 V/120 mA max. |
on |
| 5 V/100 mA max. |
off |
| Output (conn. 4) |
Switch 2a |
Switch 3a |
| 0 - 10 V |
on |
off |
| 0 - 20 mA |
off |
off |
| 4 - 20 mA |
off |
on |
| Range of cut-off frequency |
Switch 4a |
| 3 Hz to 340 Hz |
on |
| 330 Hz to 15 kHz |
off |
|
Switch 4a is only active in the versions with a selectable cut-off frequency (SIGS 15S and SCM 90S). |
| Zero point |
Switch |
| -15 mV |
5a on |
| +8 mV |
6a on |
| +4 mV |
7a on |
| +2 mV |
8a on |
| +1 mV |
1b on |
|
The start of the measurement range (zero point) is set using switches 5a - 8a, 1b. Switch 5a shifts the measurement range by -15 mVand switches 6a, 7a, 8a, 1b by +8 mV, +4 mV, +2 mV, +1 mV respectively (all values are relative to the input signal). Example: for a measurement range starting at 10 mV, switches 6a and 8a must be on, for a measurement range starting at -2 mV, switches 5a, 6a, 7a, 1b must be on (-15 +8 +4 +1 = -2 mV). |
| Measurement range |
Switch |
| 64 mV |
2b on |
| 32 mV |
3b on |
| 16 mV |
4b on |
| 8 mV |
5b on |
| 4 mV |
6b on |
| 2 mV |
7b on |
| 1 mV |
8b on |
|
The measurement range is set using switches 2b - 8b. A simple binary code is used (see table). Example: for a measurement range of 10 mV, switches 5b and 7b (8 + 2 = 10 mV) must be on. |
Setting the cut-off frequency in the SIGS 15S and SCM 90S modules
The cut-off frequency is set using a small SMD potentiometer located next to switch b. There are two test holes directly underneath the switch, between which the resistance of the potentiometer can be measured using an ohmmeter. This simplifies the setting of the cut-off frequency. The table below shows the relationship between the cut-off frequency and the potentiometer resistance for the upper range (15 kHz - 330 Hz, switch 4a "off") and for the lower range (340 Hz - 3 Hz, switch 4a auf "on").
The response time (to 1% of the final value by a jump in the input signal) varies in the upper range between approx. 40 us (15 kHz) and 2.1 ms (330 Hz) and in the lower range between 2.1 ms (340 Hz) and approx. 300 ms (3.3 Hz).
| kOhm |
0 | 5 | 20 | 50 | 100 |
| Frequency in kHz |
15 | 4.7 | 1.6 | 0.66 | 0.33 |
| Ohm |
0 | 5 | 20 | 50 | 100 |
| Frequency in Hz |
340 | 70 | 22 | 7 | 3.3 |
Distortions in the output signal at high frequencies
a) Voltage output
A sine curve will be amplified without distortion over the entire range (-10 V to +10 V) up to around 2 kHz. Above this frequency, distortions occur around the zero point and at approx. ±0.8 V (with respect to the output voltage). At larger output amplitudes (> 1 V) the sine shape can also be distorted. A load resistor of between 1k and 10k can practically eliminate the distortions when the output amplitude is not too large. Note, however, the output impedence of approx. 50-70 Ohm.
b) Current output
There is no distortion up to 20 kHz for output current > 2mA (range 0-20 mA) or > 6 mA (range 4-20 mA). At around 2 or 6 mA, respectively, the distortion is limited to a few percent.
Further Information
