A high-quality load cell may have a 2-mV/V output-transfer function, meaning that for each volt of excitation you get ±2 mV of the full-scale output signal. With an excitation of 4.096V and a full sensor deflection, the maximum output is ±8.192 mV. In a 12-bit application, half of full-scale might represent 0 to 250 lbs for a bathroom scale. If you want 0.25-lb resolution, you need 1000 points of measurement output. To look at something that is 1/1000th of the full-scale range, you must distinguish a change of 8.192 μV of the sensor output. You can achieve this distinction by keeping the peak-to-peak sensor noise less than 8.192 μV for 99.999% of the time, using a crest factor of 4.4 (Reference 1). With this definition, the least-significant bit at the sensor is 8.192 μV, or 931 nV rms.

The load-cell bridge has an excitation voltage of 4.096V (Figure 1). The INA326 instrumentation amplifier follows the load cell with a gain of 250V/V. The system’s full-scale voltage, 250×±8.192 mV, produces a ±2.048V full-scale signal. The 12-bit ADS7822 digitizes the analog signal.

This 12-bit converter system must have an analog filter. The lowpass OPA333 analog filter’s primary function is to remove the high-frequency signal components at the input of the ADC (Reference 2). Because the load cell in the circuit operates near dc, you limit the bandwidth to 10 Hz. The components in Figure 1 cost less than $6.

Now, look at load-cell measurement with a 24-bit system. You can simply put the load-cell signal through a first-order lowpass filter and into the delta-sigma ADC (Figure 2). The first-order filter in this circuit eliminates high-frequency noise around the converter’s sampling frequency (Reference 3). The sensor provides the resistor for the filter RC pair.

Look at the errors for the 24-bit delta-sigma system in Figure 2. The ADS1232 produces 3.7 μV p-p of noise, with a crest factor of 4.4. This figure is much lower than the sensor’s least-significant bit. Additionally, the full-scale range of the converter is 4.096V, whereas the sensor’s full-scale output range is ±8.192 mV. As you can guess, you will “throw away” most of the output bits of the delta-sigma converter. The components in Figure 2 cost less than $4.

You may find that the 12-bit converter system ends up costing you more money, real estate, and headaches than the alternative 24-bit system.