Filter function of the measuring amplifier

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".

Cutoff frequency of the low pass filter

At the limit frequency fg, the signal amplitudes are attenuated by 3dB (to approx. 70% amplitude compared to static signals).

Fig. 1: Frequency response of a low-pass filter

Analog measuring amplifier

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 cut-off frequency
GSV-11H 010/20/2 20 Hz
GSV-11H 010-5/20/2 20 Hz
GSV-11H 4-20/20/2 20 Hz
GSV-11H 4-20-12/20/2 20 Hz
GSV-1H 010/250/2 250 Hz
GSV-1H 010/2k5/2 2,5 kHz
GSV-1H 010/10k/2 10 kHz
GSV-1A8 2,5 kHz
GSV-1A4 250 Hz

 

Measuring amplifier with digital signal processing

Measuring amplifier with (configurable) analog output

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.

Measuring Amplifier Samplingrate

Data Frequency at the Analog Output

GSV-15L 315 S/s 315 Hz
GSV-6L, GSV-6K 25 kS/s 10 Hz ... 25 kHzconfigurable by customer (with ClickRClackR)
GSV-13i 25 kS/s 10 Hz ...25 kHz Factory configurable
GSV-8 bis 8x 48 kS/s simultan (Summenabtastrate 384 kS/s) 1 Hz ... 16 kHz customer-configurable (with GSVmulti);

Digital amplifiers with non-configurable analog output

Special features include the measuring amplifiers GSV-2LS, GSV-2AS, GSV-2FSD-DI, GSV-2TSD-DI. In addition to an interface (USB or RS232 or CANbus), these measuring amplifiers have an analogue output, which is routed 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. For predominantly analogous applications, the type GSV-1 is suitable; For predominantly digital applications, the type GSV-2 is suitable.

Digital amplifiers with (configurable) analogue output and configurable digital filters

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.

Configuration of the data frequency

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.