9.2.3.2 Detailed Design Procedure

As shown in Figure 84, the bridge excitation voltage is simultaneously used as the reference voltage for the ADC to implement a ratiometric bridge measurement. With this configuration, any drift in excitation voltage also shows up on the reference voltage, consequently canceling out drift error. Either the dedicated reference inputs can be used, or the analog supply can be used as the reference if the supply is used to excite the bridge.

The PGA offers gains up to 128, which helps amplify the small differential bridge output signal to make optimal use of the ADC full-scale range. Using a symmetrical bridge with the excitation voltage equal to the supply voltage of the device ensures that the output signal of the bridge meets the absolute input voltage requirement of the PGA.

Using a 3-mV/V load cell with a 5-V excitation yields a maximum differential voltage at the ADC inputs of V_INMAX = 15 mV at maximum load. Equation 29 then calculates the maximum gain that can be used.

Accordingly Gain = 128 is used in this example.

A first-order differential and common-mode RC filter (R_F1, R_F2, C_DIF1, C_CM1, and C_CM2) is placed on the ADC inputs. The reference has an additional capacitor C_DIF2 to limit reference noise. Care must be taken to maintain a limited amount of filtering or the measurement is no longer ratiometric.

To find the repeatability of the readings, perform the following calculation. The load cell produces an output voltage of 15 mV at the maximum load of 1 kg. At a Gain = 128 and DR = 20 SPS the ADS122U04 offers a noise-free resolution of 0.46 µV_PP. Equation 30 then calculates the repeatability.

Table 29 shows the register settings for this design.