Design of Σ-Δ ADC Precision Electronic Scale Based on AD7190

Circuit functions and advantages

This circuit is an electronic weigh scale system that uses the AD7190, an ultra low noise, 24-bit Σ-Δ ADC with internal PGA. The AD7190 simplifies the design of the electronic weigh scale because most of the components of the system are included on the chip.

The AD7190 maintains good performance over the full output data rate range from 4.7 Hz to 4.8 kHz, which enables it to be used in weigh scales at higher speeds as well as in low speed weighing systems operating at scales, as shown in Figure 1.

Circuit Description

Since the AD7190 provides an integrated solution to electronic weigh scales, it can be connected directly to the load cell. The only external components required are some filters on the analog inputs and a reference pin capacitor for EMC purposes. The low level signal from the load cell is amplified internally by the AD7190, operating with a gain of 128. The conversion from the AD7190 is then sent to the microcontroller where the digital information is converted to weight and displayed on an LCD.

Figure 2 shows the actual test setup. This enables system performance to be optimized. The load has two sense pins in addition to the excitation, ground and two output connections. Its sense pins are connected to the high and low sides of a Wheatstone bridge. The voltage across the bridge can thus be accurately measured regardless of voltage drops due to wiring resistance. In addition, the AD7190 has differential analog inputs and accepts a differential reference. Connecting the load sense lines to the reference inputs of the AD7190 creates a ratiometric configuration where low supply voltages excite frequency changes. In addition, it eliminates the need for a precision reference. With a 4-wire load, the sense pins do not exist, and the ADC's reference pins are connected to the excitation voltage and ground. With this arrangement, the system is not perfectly proportional because there will be a voltage drop between the excitation and the sense + voltage due to the wire resistance. There will also be a voltage drop due to the low resistance of the wire.

The AD7190 has separate analog and digital power supply pins. The analog section must be powered from the 5 V digital power supply which is the analog power supply and can be any voltage between 2.7 V and 5.25 V independent of the microcontroller using a 3.3 V power supply. Therefore, DVDD is also powered from 3.3 V, which simplifies the interface between the ADC and the microcontroller because no external level shifting is required.

The AD7190 has separate analog and digital power supply pins. The analog section must be powered from the 5 V digital power supply which is the analog power supply and can be any voltage between 2.7 V and 5.25 V independent of the microcontroller using a 3.3 V power supply. Therefore, DVDD is also powered from 3.3 V, which simplifies the interface between the ADC and the microcontroller because no external level shifting is required.

Figure 3 shows the rms noise of the AD7190 for different output data rates when the gain is equal to 128. This plot shows that the rms noise increases as the output data rate increases. However, the device maintains good noise performance over the full output data rate range.

If a 2 kg load cell with a sensitivity of 2 mV/V is used, the full scale signal from the load cell is 10 mV when the excitation voltage is 5 V. The load cell has a mutual offset, tare, associated with it. This tare can have an amplitude that is 50% of the load cell full scale output signal. The sensor also has a gain error that can be up to ± 20% of full scale. Some customers use a DAC to eliminate or null the tare. When 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 part is configured for operation at both poles. The wide analog input range of the AD7190 relative to the load cell full scale signal (10 mV) is beneficial because it ensures that the offset and gain errors of the load cell do not overload the front end of the ADC.

The AD7190 has an rms noise of 8.5 nV when the output data rate is 4.7 Hz. The noise free count value is equal to

The 6.6 factor converts the voltage to a peak-to-peak voltage.

, therefore, equals






The resolution is noise-free equals



In practice, the load cell itself will introduce some noise. In addition, there is some time and temperature drift due to drift of the AD7190 sensor. To determine the accuracy of the entire system, the weigh scale can be connected to a PC via the USB connector. Using LabVIEW software, the performance of the weigh scale system was evaluated. Figure 4 shows the output performance when a one-kilogram load cell was placed and 500 conversions were gathered. The noise of the system was calculated by the software to be 12 nanovolts rms and 88 nanovolts peak-to-peak. This equates to a resolution of 113,600 noise-free counts, or 16.8 noise-free code bits.

Figure 5 shows the performance in terms of weight. The peak-to-peak variation in the output is 0.02 g at 500 yards. Therefore, the weigh scale system achieves an accuracy of 0.02 g.

The plot shows the actual (raw) conversions read back from the load cell when connected to the AD7190. In practice, a digital post filter is used in the weigh scale system. Additional averaging is performed after the filter and will further increase the number of noise-free counts at the expense of a reduction in the data rate.

The AD7190 is a high-end high-precision ADC for weigh scales. The other suitable ADC and AD7191 is the AD7192. The AD7192 is pin-for-pin compatible with the AD7190. However, its rms noise is slightly higher. The AD7192 has an rms noise of 11 nanovolts at an output data rate of 4.7 Hz, while the AD7190 has an rms noise of 8.5 nanovolts at an output data rate. The AD7191 is a pin programmable device. It has four output data rates and four gain settings. Due to its pin programmability and simplified feature set, it is an easy-to-use device. Its rms noise is the same as the rms noise of the AD7192.

For mid-range electronic weigh scales, the AD7799 is a suitable device. At an output data rate of 4.17 Hz, the AD7799 has an rms noise of 27 nanovolts.

Finally, for low-end electronic weigh scales, the AD7798, AD7781, and AD7780 are suitable devices. The AD7798 has the same functional setup as the AD7799. Its rms noise trades for 40 nV at 4.17 Hz. The AD7780 and AD7781 have a differential analog input and are pin programmable, allowing output data rates between 10 Hz and 1760 Hz and gain rates of 1 or 128. The output data rate is 10 Hz at 44 nV rms noise.