Output signals of a force sensor or strain gauge are composed of static components (DC voltage components) and dynamic components (oscillations or rapid, pulse-like signal changes).
The static portion may e.g. the weight be on a load cell; dynamic components result e.g. from jerky loads when placing a weight.
Other causes of dynamic loading are, for example, oscillations of the system with the natural frequency from load cell and mass ("mechanical spring" and "weight").
Undesirable dynamic components are e.g. Interference from e.g. 50Hz or 100Hz by other electrical equipment, or by cables routed parallel to the sensor cable. The amplifier electronics themselves also generate dynamic components (noise with a very large number of frequency components ("white noise").
The task of the low-pass filter is to remove the dynamic components in the output signal in order to pass only the desired frequency components. By limiting the so-called "bandwidth", the resolution is improved by e.g. Vibrations are filtered and only the "static" share of a balance is transmitted.
The more dynamic components in a signal, the greater the noise amplitude. As a rule of thumb: 10 times the bandwidth means 3 times the noise amplitude (in fact √10 times with white noise).
Therefore, the bandwidth of the amplifier should be "as low as possible" and "as high as necessary".
At the limit frequency fg, the signal amplitudes are attenuated by 3dB (to approx. 70% amplitude compared to static signals).
For analog measuring amplifiers GSV-1 and GSV-11, the filter is realized as a second order active filter with Bessel characteristics. The attenuation is about -40 dB per decade.
Construction of analog measuring amplifiers
The cutoff frequency is set by different equipping with resistors and capacitors. In addition to the active low-pass filter in the output stage of the measurement amplifier, there are other influencing factors that cause a limitation of the bandwidth. These include e.g. the gain bandwidth product of the differential amplifier in the input stage of the measurement amplifier, but also components for the EMC in the input stage, such as. Chokes and protective diodes.
Table 1 gives an overview of the available bandwidths with GSV-1 and GSV-11:
| Type | Cutoff Frequency | Noise Amplitude max-min | Resolution (2mV/V / Noise-Amplitude) |
|---|---|---|---|
| GSV-11H 010/20/2 | 20 Hz | 200 nV/V | 10000 |
| GSV-11H 010-5/20/2 | 20 Hz | 200 nV/V | 10000 |
| GSV-11H 4-20/20/2 | 20 Hz | 200 nV/V | 10000 |
| GSV-11H 4-20-12/20/2 | 20 Hz | 200 nV/V | 10000 |
| GSV-1H 010/250/2 | 250 Hz | 100 nV/V | 20000 |
| GSV-1H 010/2k5/2 | 2,5 kHz | 300 nV/V | 7000 |
| GSV-1H 010/10k/2 | 10 kHz | 600 nV/V | 3000 |
| GSV-1A8 | 2,5 kHz | 300 nV/V | 7000 |
| GSV-1A4 | 250 Hz | 300 nV/V | 7000 |
| GSV-5L 250 Hz | 250 Hz | 100 nV/V | 20000 |
| GSV-5L 2.5 kHz | 2.5 kHz | 300 nV/V | 7000 |
| GSV-5L 10 kHz | 10 kHz | 600 nV/V | 3000 |
The measuring amplifiers GSV-15L, GSV-6L, GSV-6K, GSV-13i have (only) an analog output. The force sensor or strain gauge signal is digitized with an analog-to-digital converter, processed in a microprocessor, and output as a digitally scaled and digitally filtered signal using a digital-to-analog converter.
The digitization of the input signal takes place at the sampling rate. The update of the output signal via the D / A converter takes place with the data frequency.
| Amplifier Type | Sampling-rate |
data Frequency at Analogue Output |
Noise Amplitude max -min at 2 mV/V input range | Resolution (2 mV/V / Noise Amplitude) |
|---|---|---|---|---|
| GSV-15L | 315 S/s | 315 Hz | 300 nV/V | 7000 |
| GSV-6L, GSV-6K | 25 kS/s | 10 Hz ... 25 kHz kundenseitig konfigurierbar (mit ClickRClackR) | 300 nV/V bei 10Hz | 7000 |
| GSV-13i | 25 kS/s | 10 Hz | 600 nV/V | 3000 |
| GSV-8 | bis 8x 48 kS/s simultan (Summenabtastrate 384 kS/s) | 1 Hz ... 16 kHz kundenseitig konfigurierbar (mit GSVmulti); |
20 nV/V bei 10Hz digital 100 nV/V bei 10Hz analog |
100000 digital 20000 analog |
The measuring amplifiers GSV-2LS, GSV-2AS, GSV-2FSD-DI, GSV-2TSD-DI take a special position. In addition to an interface (USB or RS232 or CANbus), these measuring amplifiers have an analogue output., which is fed to an analog, non-scalable output stage before the A/D converter. Only an automatic zero adjustment is possible.
The analog output of the GSV-2 corresponds to the analog output of the GSV-1. The type GSV-1 is suitable for predominantly analogue applications; The GSV-2 type is suitable for predominantly digital applications.
The measuring amplifier GSV-8 has digital filters FIR up to 14th order and IIR to 4th order. The configuration is possible with the software GSVmulti. With the IIR filter, the characteristics "lowpass", "highpass", "bandpass", "bandstop" can be realized at the analogue output, with the FIR filter lowpass filters up to 14th order are realized.
The same applies to the setting of the data frequency: as low as possible, as high as necessary. Setting the data frequency automatically activates continuous averaging filtering. High-frequency interference components are thereby filtered.
The data frequency must be chosen high enough if e.g. sudden signal changes and peak values are to be detected. In theory (Nyquist-Shannon sampling theorem), the data frequency must be at least twice the signal frequency. In practice, the setting of a data frequency has been proven that is 5 to 10 times the signal frequency.
By configuring the data frequency, the GSV-8 automatically activates an analogue low-pass filter, which precedes the A / D converter as an anti-aliasing filter.
The GSV-8 series measuring amplifiers allow the configuration of digital filters. Setting digital filters requires expert knowledge in digital filter technology. Setting digital filters can be useful, e.g. to implement a tape lock or a tape filter. For the design of digital filters, in particular digital IIR filters of the fourth order, paid software tools or simulation software, such as Matlab, are usually used. Experts in filter technology find in the software GSVmulti a useful tool for the design of an IIR filter of the fourth order.
