This circuit is an electronic scale system, using an ultra-low noise, low drift, 24-bit Σ-Δ ADC AD7190 with built-in PGA . The device simplifies scale design by placing most of the system building blocks on-chip.

The AD7190 maintains good performance over the full output data rate range of 4.7 Hz to 4.8 kHz and can be used in scale systems operating at lower speeds, as well as in higher speed scale systems such as hopper scales.

The AD7190 provides an integrated scale solution that interfaces directly with load cells. Just use some filters on the analog inputs and configure some external components such as capacitors on the reference pins to meet electromagnetic shielding (EMC) requirements. The low-level signal from the load cell is amplified by the AD7190's built-in PGA. The PGA is programmed to operate at a gain of 128. The conversion results of the AD7190 are sent to the microcontroller, which converts the digital information into weight and displays it on the LCD.

Figure 2 shows the actual test setup. For optimal system performance, this test setup uses a 6-wire load cell. In addition to the excitation, ground and 2 output connections, the 6-wire load cell has 2 sense pins. These sense pins are connected to the high and low sides of the Wheatstone bridge respectively. Therefore, the voltage developed across the bridge can be accurately measured despite the voltage drop caused by the line resistance. Additionally, the AD7190 has differential analog inputs that accept differential reference voltages. The load cell differential SENSE line is connected to the AD7190 reference voltage input to form a ratiometric configuration that is neither affected by low-frequency changes in the power supply excitation voltage nor requires a precision reference voltage source. If a 4-wire load cell is used, there is no sense pin and the ADC reference voltage pin will be connected to the excitation voltage and ground. In this configuration, there will be a voltage drop between the excitation voltage and SENSE+ due to the line resistance, so the system is not fully ratiometric. In addition, there will also be a voltage drop caused by line resistance on the low end.

The AD7190 has separate analog power supply pins and digital power supply pins. The analog part must be powered by a 5 V power supply. The digital power supply is independent of the analog power supply and can be any voltage from 2.7 V to 5.25 V. The microcontroller runs on 3.3 V power supply. Therefore, DVDD is also powered by 3.3 V power supply. This eliminates the need for external level translation, simplifying the interface between the ADC and microcontroller.

Figure 3 shows the rms noise of the AD7190 at different output data rates with a gain equal to 128. This graph shows that the rms noise increases as the output data rate increases. However, the device maintains good noise performance over the entire output data rate range.

If a 2 kg load cell with a sensitivity of 2 mV/V is used, then with an excitation voltage of 5 V, the full-scale signal from the load cell is 10 mV. Load cells have an associated offset voltage, or TARE. The amplitude of this TARE can be up to 50% of the load cell's full-scale output signal. Load cells also have a gain error of up to ±20% of full scale. Some customers utilize DAC to eliminate or offset TARE. If the AD7190 uses a 5 V reference, its analog input range is equal to ±40 mV when the gain is set to 128 and the device is configured for bipolar operation. The AD7190's wide analog input range relative to the load cell's full-scale signal (10 mV) helps ensure that the load cell's offset voltage and gain errors do not overload the ADC front end.

The AD7190 has an rms noise of 8.5 nV at an output data rate of 4.7 Hz. The number of noise-free samples is equal to

The coefficient 6.6 is used to convert the root mean square voltage to the peak to peak voltage.

Therefore, resolution in grams (g) is equal to

Noise-free resolution is equal to

In actual operation, the load cell itself will introduce a certain amount of noise. The drift of the AD7190 will also cause certain time and temperature drift in the load cell. To determine the accuracy of the complete system, the scale can be connected to a PC via a USB connector and LabView software can be used to evaluate the performance of the scale system. Figure 4 shows the output performance measured by placing a 1 kg weight on the load cell and collecting 500 conversions. The software calculated system noise to be 12 nV (rms) and 88 nV (peak-to-peak), which is equivalent to 113,600 noise-free samples or 16.8 bits of noise-free code resolution.

Figure 5 shows the performance in terms of weight. Relative to 500 codes, the peak-to-peak variation of the output is 0.02 grams. Therefore, the accuracy of this electronic scale system reaches 0.02 grams.

Shown above is the actual (raw) conversion result read back from the AD7190 after connecting the load sensor. In actual operation, electronic scale systems will use digital post-filters. Performing additional averaging in the post-filter further increases the number of noise-free samples, but at a reduced data rate.