Strain Gauge Amplifier with Digital and Analog Output

Load cell amplifiers (or load cell signal conditioners) provide regulated excitation voltage to the strain gage circuit and convert the mv/V output signal into another form of signal that is more useful to the user. The signal generated by the strain gage bridge is low strength signal and may not work with other components of the system, such as PLC, data acquisition modules (DAQ), computers, or microprocessors. Thus, strain gauge load cell signal conditioner functions include excitation voltage, noise filtering or attenuation, signal amplification, and output signal conversion.

Those are the most common force sensor amplifier outputs:

• Analog output: voltage, current or differential outputs (0-10VDC, ± 10VDC, ± 5VDC, 4-20 mA);
• Digital output (SPI, USB) performed by load cell ADC (analog to digital converter);
• Serial communication (RS232, RS485).

Load cell signal conditioner amplifier also support applications where it is necessary to integrate force and torque sensors with Arduino or Raspberry pi microcontrollers.

The function of a load cell amplifier circuit is to capture the signal from the load cell or torque sensor and convert it into a higher level of an electrical signal. In order to do so, the mV/V low amplitude output of the load cell goes thru several different signal conditioning steps:

Excitation Voltage:

Full-bridge load cells or torque sensors require an excitation voltage to feed the strain gage Wheatstone bridge and generate their output signal as a ratio of the input excitation voltage. Thus, you need to establish if your DAQ or PLC can support the sensor’s input voltage or excitation voltage requirements. Simply put, an unstable excitation voltage input leads to an unstable load cell output. In case you you need a load cell amplifier for PLC or DAQ and they do not provide a stable input excitation voltage, the amplifier will be the excitation voltage source to ensure the sensor provides a reliable and consistent output signal. For example, FUTEK’s USB Family digital signal conditioners can provide excitation for amplified sensors, up to 24VDC, all off the USB 2.0 5VDC supply.

Filtering:

Analog sensor signals are susceptible to electrical noise and/or residual ripple voltage, which can distort or skew measurements. Noise needs to be filtered out before you can capture an accurate signal. DAQs and PLCs designed to interface directly with full-bridge sensors will include pass band and other forms of signal conditioning and filtration. In a low noise load cell signal conditioner, electronic filters eliminate some effects on accuracy by removing electrical noise and ripple effect above and below the analog sensor’s signal range, resulting in a low signal to noise ratio. For example, FUTEK IAA Family Analog signal conditioners has bandwidth selection feature, used to set the bandwidth from 100 Hz to 50,000 Hz, allowing for noise filtering according to the load cell or torque sensor application.

Amplification:

A full-bridge strain gage sensor can output a signal in the nanovolt through millivolt range. When your DAQ or PLC is limited to measuring volts, you will need an strain gage amplifier to convert millivolts to a larger signal. Some PLCs and DAQs come with built-in amplification; others will require an external amplifier. What if your existing DAQ or PLC does not provide built-in amplification, signal conditioning, and a stable power source for sensor excitation? In that case, you will need an amplifier to fill in the shortfalls in your instrumentation, supporting your full-bridge sensor. For multi axis sensors, such 6 DoF Force Torque sensor, one needs a multi channel load cell amplifier circuit able to process all the mV/V outputs of the channels.

Signal conversion:

The majority full-bridge load cells and force measurement sensors or transducers generate an analog output in the millivolt range (mV/V). Thus, signal processing is traditionally analog. So, if you req PLC or DAQ system requires an amplified analog (i.e.: load cell amplifier 4-20mA, load cell amplifier 0 10v) or a digital output (USB, SPI), the load cell or torque sensor needs a strain gage signal conditioner to convert the mV/V signal to the required signal output.

Some applications require digital output, which will require a signal conditioner with a analog to digital converter (ADC). For those applications, two critical parameters must be taken into consideration when selecting the digital amplifier: noise free resolution and sampling rate. In that regards, FUTEK has a broad range of load cell USB output kit (USB force sensors amplifiers) that provides microprocessed load cell signal converter with internal high resolution (24 bits), capable of offering up to 21 ENOB and up to 19 bits noise free resolution. Our entire USB load cell amplifier line has a ± 0.005% of FSR for both accuracy and non-linearity.

Some applications require an aggressive sampling rate, where a thousand samples per second maximum just won't cut it. FUTEK’s USBs offer excellent sampling rates, ranging between 5 and 15k samples per second. Please note that resolution will differ as your sampling rate increases.

When selecting a force measurement solution, an analog full-bridge sensor, such as a load cell, torque sensor or pressure sensor, is one piece of the puzzle. A complete solution also requires integration with your DAQ, PLC or computer. So all the signal conditioning stages are vital: excitation voltage, noise filtering, signal amplification and output conversion.

For more information, check out this white paper on “How to select load cell amplifier for strain gage based force sensors".