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The ADS1232 and ADS1234 are bridged sensor analog-to-digital converters (ADCs) from Texas Instruments. To better understand these ADCs, let's first take a look at the target application: electronic scales. The range and number of electronic scales is increasing. For example, commercial electronic scales record the price of a good by weight. In terms of transportation, electronic scales are used to verify the weight of the transported goods. The calculation disk determines the time to fill the container by monitoring the weight of the packaging assembly line container, and the scientific scale is used to provide an accurate analysis of the weight during the experiment.Regardless of the application, the core of all these different types of scales is a high-precision digitization process that converts the weight of the object being measured into a digital value that can be displayed or recorded for data. Although there are many ways to convert weight to electrical signals, the most common method may be to use a resistive load cell configured as a Whetstone bridge. Figure 1 shows a bridge structure in which the value of one of the resistors will vary depending on the weight applied. Depending on the structure of the bridge, more resistor values ​​may change when weight is applied. In any case, an excitation voltage can be applied to the top and bottom of the bridge. At the intermediate node, the output signal is measured in the form of a differential voltage.
Figure 1 Wheatstone bridge resistive load cell
The challenge in designing an electronic scale is how to measure the signal produced by the resistance bridge with high precision because the signal is usually small. The load cell is typically determined by the output voltage that is generated by a 1V excitation voltage when the maximum rated weight of the load cell is applied. Specifications are determined in units of mV/V. For example, a 4mV/V load cell excited by a 5V supply has a full-scale output voltage of only 20mV. Remember that this is the maximum output voltage. To determine the accuracy required by the digitizer, the bridged full-scale voltage must be divided by the ideal scale accuracy (which is usually expressed in counts). Assuming that the same 4mV/V load cell is excited by a 5V voltage, the scale requires a count of 20,000 precision. This, in turn, requires the digitizer to be able to make repeated measurements on signals of (4mV/V) (5V) / 20,000 = 1000nV.
So let's do a more challenging design! In order to obtain an excellent scale design, data reading must be extremely stable. That is to say, there cannot be flickering or switching between codes due to noise interference. This requirement places additional demands on the digitizer, requiring more accurate internal precision than the value reported by the scale to the user. It is not uncommon to have an internal accuracy that is 10 times higher than the displayed value. If it is in the previous load cell instance, it requires an internal accuracy of 100nV!
Given the extremely small bridge sensor signals and the need for extremely accurate measurements, many electronic scale manufacturers have used a very low noise gain stage to amplify signals from the bridge before digitizing. In the case of weight changes on many electronic scales, the gain level bandwidth is usually not a big problem. Nevertheless, the key is that the gain stage is very stable in both temperature and time variations. Most electronic scales require only regular calibration by the manufacturer or user. Any gain change caused by PGA time or temperature drift can have a negative impact on the accuracy of the scale. In fact, in some high-end electronic scale designs, the stability of the gain stage over time and temperature changes determines the specifications of the entire scale. Typically, a precision analog-to-digital converter (ADC) behind the PGA digitally converts the amplified voltage. In the case where the measured signal is slowly changing and requires extremely high precision, the ADC is often implemented using a delta-sigma topology. Due to the use of the gain stage, the stability of the ADC over time and temperature becomes very important to avoid constraining overall performance.
In addition, since the bridge excitation voltage can be used as a reference voltage (see Figure 2), the ADC should be able to perform a “proportional metering†measurement. The output signal from the bridge is proportional to the excitation voltage with the attenuation factor, which is determined by the weight applied to the load cell. The load cell signal is measured by "proportional metering" using the ADC, which means that the excitation voltage is used as the reference voltage of the ADC to offset the change in the absolute value of the excitation voltage. But this in turn will reduce the sensitivity and robustness of the electronic scale design.
Figure 2 Proportional measurement of load cells using an ADC
With these requirements in mind, TI developed the ADS1232 (dual-channel input) and ADS1234 (four-channel input), providing electronic scale designers with a simple, high-performance, low-cost, single-chip bridged sensor output digital solution. Both the ADS1232 and ADS1234 integrate all of the critical modules in one front end of the scale (see Figure 3), the only difference being the number of input channels they support. A programmable gain amplifier (PGA) allows the user to select a gain with a gain factor of 1, 2, 64 or 128. Gains with gain factors of 64 and 128 are used when the bridge is directly connected to the ADS1232/4. The gain of gain factors 1 and 2 allows an optional external gain stage to be used between the bridge and the ADS1232/4. The ADS1232/4 PGA, built on TI's new advanced high-performance, sub-micron mixed-signal CMOS process, is an innovative solution that minimizes low frequency noise and maintains minimal offset drift over temperature. The high precision on-board resistors used in the PGA provide excellent gain stability over temperature and time.
Figure 3 ADS1232/4 structure diagram
The PGA is followed by an on-board 24-bit delta-sigma ADC that allows the use of a 5V reference to support proportional metrology measurements. The on-board digital filter of the ADC provides an optional data rate of 10 samples per second (SPS) or 80 SPS. When using 10 SPS, both 50 Hz and 60 Hz line-voltage cycle interference can be suppressed, while higher speeds result in faster updates. This is useful for electronic scales that require fast response or post-processing algorithms that require high data rates.
Although an external clock source can be used when needed, the ADS1232/4's high precision on-board oscillators do not require an external oscillator or crystal to operate. All control of the ADS1232/4 is implemented by a number of dedicated pins. This architecture greatly simplifies software development by eliminating the need to program all registers. Finally, the ADC's data output can be easily retrieved via a simple read-only interface. Due to the high density of the TI mixed-signal process, the ADS1232 is suitable for the 24-pin ultra-thin compact small outline package (TSSOP), while the ADS1234 is a 28-pin TSSOP.
To better illustrate the performance of the ADS1232/4, Figure 4 shows a 10 second interval output reading with a data rate of 10 (SPS), a PGA gain factor of 128, and a reference voltage of 5V bridge excitation voltage. The left axis shows the output reading of the ADS1232/4 in the least significant bit (LSB), while the right axis shows the output read in nV. The root mean square (rms) noise is only 17nV and the peak-to-peak noise is only 110nV. Going back and looking at the 4mV/V load cell example of the 5V excitation voltage mentioned earlier, the ADS1232 provides over 180,000 counts of internal accuracy when used with the load cell, and requires no additional components or output data. Post processing. It is important to note that the noise of the ADS1232/4 will vary in the form of data rate, PGA and reference voltage functions. You can find a product manual for the ADS1232/4 at , which provides a list of noise showing the performance under different settings.
Figure 4 ADS1232 noise performance
As a help to electronic scale designers, TI has also developed an electronic scale reference design using the ADS1232, the ADS1232REF. Figure 5 shows the structure of the structure. As the core of the design, the ADS1232 directly digitizes the scale load cell signal. The MSP430 microcontroller collects ADS1232 data, drives the LCD display, decodes user input from the switch, and communicates via a USB connection to the same optional PC. Figure 6 highlights several key components of the board. The user connects the load unit to the indicated connector. The jumper bypasses the optional RC filter in front of the ADS1232 input. The reference voltage can be switched between an external excitation voltage or an analog power supply. The power supply is provided by an external DC power supply. In standalone mode, the main control switch controls the entire run. The MSP430 then displays the user-selected data on the LCD. In PC mode, the USB interface allows a PC to be used to control operation and display the data output on the display of the PC. To learn more about ADS1232REF, please visit the TI User's Guide to download the User Guide.
Figure 5 ADS1232REF electronic scale reference design structure
Figure 6 ADS1232REF Scale Reference Design
In summary, electronic scales are becoming more and more popular in a variety of applications. The load cell is perhaps the most common weight sensor, and its output signal is extremely small. It is a huge challenge to accurately measure this small signal. The ADS1232 and ADS1234 offer a single-chip solution that allows scale designers to quickly and easily develop a small, low-cost, high-performance scale. The ADS1232REF reference design allows the user to evaluate the performance of the ADS1232 using his own load unit and as the basis for a complete scale design.
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