Sensor Output Transfer Function Tips

A high-quality load cell might have a 2-mV/V output transfer capability, which means you get ±2 mV of full-scale output signal for every volt of excitation voltage. With 4.096V excitation and full sensor deflection, the maximum output is ±8.192 mV. In a 12-bit application, half of full scale might represent 0 to 250 pounds on a scale. If you want 0.25-pound accuracy, you need 1000 output measurement points. To study something at 1/1000 of full scale, you must be able to discern a sensor output change of 8.192 μV. You can achieve this accuracy by keeping the peak-to-peak sensor noise below 8.192 μV 99.999% of the time, using a crest factor of 4.4 (see Reference 1). With this accuracy, the least significant bit of the sensor is 8.192 μV, or 931 nV (rms).

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

This 12-bit converter system must have an analog filter. The main function of the low-pass analog filter (OPA333) is to remove high-frequency signal components at the input of the A/D converter (see Reference 2). Since the load cell in the circuit operates at nearly DC, you want to limit the bandwidth to 10 Hz.

Now, let’s look at a load cell measurement approach for a 24-bit system. In Figure 2, we can simply pass the load cell signal through a first-order low-pass filter before entering the delta-sigma analog-to-digital converter. The first-order filter in this circuit removes high-frequency noise around the converter’s sampling frequency (see Reference 3). The sensor provides resistance to the filter R|C pair.

Looking at the errors of the 24-bit delta-sigma system in Figure 2, note that the ADS1232 generates 3.7 μV pp of noise, which has a crest factor of 4.4. This is well below the least significant bit of the sensor. Also, the full-scale range of the delta-sigma converter is 4.096V, but the full-scale output range of the sensor is ±8.192 mV. As you might guess, you are “throwing away” most of the output bits of the delta-sigma converter.

You may find that a 12-bit converter system ends up costing you more money, taking up more board space, and requiring more effort than a 24-bit system.