An introduction to electronic weighing

Precision

Precision of a weigh scale is often quoted in "counts." For example, in a machine designed to weigh up to 10kg at 10,000 count precision, the least significant digit would represent 1g. In reality, precision at this level is difficult to achieve and often not required. If the load cell in such a machine has an output of 10mV at full scale, one count is represented by 1µV. Such low-level signal changes can be difficult to detect, especially in the presence of electrical noise and, indeed, vibration from the surrounding environment, which can render the weight signal unstable to begin with. Long-term stability of the load cell itself (known as creep), temperature effects, and tiny voltages generated between dissimilar metals in the wiring between the load cell and the amplifier can all contribute to errors in measurement and mandate regular calibration of the machine, especially if it is used at a point of sale. In these cases, various regulatory bodies [3] enforce calibration intervals to ensure customers get what they pay for.

Workshop Weighing System

As an example design, I wanted to produce something compact and portable that might prove useful in my workshop. I found a small load cell, freely available on eBay.com or other online sources, with a capacity of 100g (3.5oz) at low cost. The load cell is physically quite small (5x1x0.6cm) and easy to mount, and at such low capacity, it is easy to assemble into a plastic enclosure to produce a practical instrument. The load cell is one of a family of force transducers, so it would be easy to adapt the design to weigh heavier items for different applications.

The electronic design comprises four basic elements: an instrumentation amplifier, an excitation supply, a microcontroller, and a display.

Analog Front End

A number of application-specific integrated circuits are available for weigh scale applications that comprise a differential instrumentation amplifier and a 24-bit sigma-delta analog-to-digital converter (ADC). Some of these integrate other functions, such as an excitation regulator, but in essence, they perform the same task.

One of the most popular of these ADCs for weigh scales is the HX711 [4] from Avia Semiconductor, a Chinese company. It is readily available from Alliexpress.com and eBay.com, both as a bare chip and mounted on a simple printed circuit board (PCB). I assume this chip forms the basis of many of the kitchen scales and the like manufactured in China. I did some initial prototyping with this chip, and it seems very capable. Certainly, if you want to hook up a load cell to an Arduino or a Raspberry Pi, the datasheet provides example drivers in C and assembler, and you can get something up and running in a few minutes.

An alternative part from a more established manufacturer is the ADS1232 [5] from Texas Instruments. This part is available through more traditional distribution, such as mouser.com and digikey.com. The datasheet (and associated application notes) is somewhat more detailed, with an application circuit provided for a weigh scale. The ADS1232 lacks the excitation regulator of the HX711, but that in itself has some limitations, and a separate excitation regulator is not difficult to implement and is more flexible and potentially more stable. Interestingly, both of these chips have a very similar digital interface, suggesting they share a common heritage.

My choice for this design was the somewhat more expensive ADS1232 with a separate excitation regulator implemented with a REF5040 [6] (also from Texas Instruments), a high-stability reference supply chip capable of supplying 4V at 10mA directly to the load cell.

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