This circuit is an electronic scale system built using AD7192 . The AD7192 is an ultra-low noise, low drift 24-bit Σ-Δ converter with a built-in PGA. The device simplifies scale design by placing most of the system building blocks on-chip. The device operates over the full output data rate range of 4.7 Hz to 4.8 kHz and maintains good performance, allowing it to be used in scale systems operating at lower speeds, as well as in higher speed scale systems such as hopper scales.

The AD7192 provides an integrated scale solution that interfaces directly with load cells. Simply use some filters on the analog inputs and configure some external components such as capacitors on the reference voltage pin to meet electromagnetic shielding (EMC) requirements. The low-level signal from the load cell is amplified by the AD7192's built-in PGA. The PGA is programmed to operate at a gain of 128. The conversion result of AD7192 is 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 ends of the Wheatstone bridge, so the voltage developed across the bridge can be accurately measured. Additionally, the AD7192 has differential analog inputs that accept differential reference voltages. The load cell differential SENSE line is connected to the AD7192 reference voltage input to form a ratiometric configuration that is not affected by low-frequency changes in the power supply excitation voltage. 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 AD7192 has separate analog and digital power supplies. 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.

There are several ways to power this scale system, such as from mains power or from a battery (as shown in Figure 1). A 5 V low noise regulator is used to ensure low noise power to the AD7192 and load cell. A low noise voltage regulator ADP3303 (5 V) is used to generate the 5 V supply. The filter network shown in the dotted box is used to ensure that the system obtains low noise AVDD. Additionally, noise reduction capacitors are provided at the regulator output as recommended in the ADP3303 (5 V) data sheet. To optimize electromagnetic shielding performance, the regulator output is filtered before powering the AD7192 and load cell. The 3.3 V digital supply can be generated using the ADP3303 (3.3 V) regulator. Because any noise on the power supply or ground plane will introduce noise into the system and degrade circuit performance, a low-noise regulator must be used to generate all power to the AD7192 and load cell.

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 AD7192 is used with 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 AD7192'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 error do not overload the ADC front end.

The AD7192 has an rms noise of 11 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.

Resolution expressed in grams (g) is equal to

The noise-free code resolution is equal to

In actual operation, the load cell itself will introduce a certain amount of noise. The drift of the AD7192 will also cause the load cell to drift over time and temperature. 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 measured output performance by placing a 1 kg weight on the load cell and collecting 500 conversions. System noise calculated by the software is 14 nV and 98 nV peak-to-peak, which is equivalent to 102,000 noise-free samples or 16.6 bits of noise-free code resolution.

Figure 4 shows the performance in terms of weight. Relative to 500 codes, the change in output is 0.02 grams. Therefore, the accuracy of this electronic scale system reaches 0.02 grams.

The figure above shows the actual conversion result read back from the AD7192 after connecting the load cell. 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.