Ads1232 and ads1234: a full range of front-end solutions for electronic scales

ADS1232 and ADS1234: a full range of front-end solutions for electronic scales

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Source: 21IC China Electronics network Author: Texas Instruments

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Both the ADS1232 and ADS1234 are bridge sensor ADC released by TI ). To better understand these ADC, let's first take a look at the target application: electronic scales. The application scope and quantity of electronic scales are growing. For example, a commercial electronic scale records the price of a commodity by weight. In terms of transportation, electronic scales are used to verify the weight of the transported goods. The computing disk determines the time of filling containers by monitoring the weight of containers in the packaging assembly line, and scientific scales are used to provide precise analysis of the weight during the experiment.

Regardless of the application, the core of all these different types of electronic scales is a high-precision digital process, which converts the weight of the measured object into a digital value that can be displayed or recorded in data. Although there are multiple ways to convert weights to electronic signals, the most common method may be to use a blocking load unit configured as a Whetstone bridge. Figure 1 shows a bridge structure in which the value of a resistor varies depending on the Applied weight. Depending on the different structure of the bridge, the value of more resistors may change when the weight is applied. Either way, you can apply an excitation voltage at the top and bottom of the bridge. At the intermediate node, the output signal is measured in the form of Differential voltage.

 
Figure 1 resistance load unit of huistone Bridge

The challenge when designing an electronic scale is: how to measure the signal generated by the resistance Bridge with high precision, because the signal is usually very small. The load unit is usually determined by the output voltage, which is generated when the maximum rated weight of the load unit is applied to 1 V. The specifications are determined by unit mV/V. For example, a 4mV/V load unit powered by a 5 V voltage has a full-range output voltage of only 20mV. Remember, this is the maximum output voltage. To determine the accuracy required by the digital converter, the full-range voltage of the bridge must be divided by the ideal scale accuracy (which is usually expressed in count ). Assuming that the same 4mV/V load unit is stimulated by a 5 V voltage, the scale requires a 20,000 precision count. This, in turn, requires the digital converter to be able to repeat the measurement of signals (4mV/V) (5 V)/20,000 = 1000nV.

So, let's make a more challenging design! To achieve an excellent electronic scale design, data reading must be extremely stable. That is to say, there cannot be flashing or switching between codes due to noise interference. This requirement puts forward more requirements for the digital converter, which requires more accurate internal accuracy than the value reported by the electronic scale to the user. It is not uncommon to have an internal precision that is 10 times higher than the display value. If it is in the previous load unit instance, the internal precision of NV is required!

Given the existence of extremely small bridging sensor signals that require extremely high-precision measurements, many electronic scaling vendors have used an extremely low noise gain level to zoom in on signals from bridging before digitization. The gain-level bandwidth is usually not a big problem when the weights on many electronic scales change slowly. However, the key is that the gain level can be very stable in both temperature and time changes. Most electronic scales only require regular calibration by the manufacturer or user. Any gain change caused by PGA time or temperature drift will negatively affect the accuracy of the electronic scale. In fact, in some high-end electronic scales, the stability of the gain level in time and temperature changes determines the overall electronic scale specifications. Generally, a high-precision ADC behind the PGA performs digital conversion on the amplified voltage. When the measured signal changes slowly and requires extremely high accuracy, the delta-Σ topology is often used to implement the ADC. Due to the use of the gain level, the ADC becomes very important for the stability of time and temperature, so as not to restrict the overall performance.

In addition, since the bridge excitation voltage can be used as the reference voltage (see figure 2), the ADC should be able to perform a "proportional measurement" measurement. The output signal from the bridge is proportional to the excitation voltage with the attenuation coefficient, which is determined by the weight applied to the load unit. ADC is used to measure the signal of the load unit in a "proportional measurement" manner. That is to say, using the excitation voltage as the reference voltage of the ADC can offset the change of the absolute value of the excitation voltage. However, this approach will in turn reduce the sensitivity and robustness of the electronic scale design.

 
Figure 2 proportional measurement of load units using an ADC

Considering these requirements, TI developed the ADS1232 (dual-channel input) and ADS1234 (four-channel input ), it provides a simple high-performance, low-cost, single-chip bridge sensor output digital solution for Electronic Scale designers. Both ADS1232 and ADS1234 integrate all the key modules in the front end of an electronic scale (see figure 3). The only difference is that they support different numbers of input channels. A Programmable Gain Amplifier (PGA) allows users to select a gain of 1, 2, 64, or 128. The gain of the gain coefficient of 64 and 128 is used only when the bridge is directly connected to the ADS1232/4. Gain levels 1 and 2 allow an optional external gain level between bridging and ADS1232/4. The ADS1232/4 PGA, manufactured using TI's new advanced high-performance, sub-micron mixed-signal CMOS technology, is an innovative solution, it has the characteristics of minimizing low-frequency noise and keeping the minimum offset drift throughout the temperature range. High-Precision board resistors used in PGA provide excellent gain stability throughout the temperature and time range.

 
Figure 3 structure of ADS1232/4

The PGA is followed by a 24-bit Delta-Σ ADC on the board, which allows a 5 V reference voltage to support proportional measurement. The ADC's on-board digital filter provides an optional data rate of 10 sampling/second (SPS) or 80 SPS. When 10 SPS are used, 50Hz and 60Hz line pressure cycle interference can be restrained at the same time, and a higher speed leads to faster updates. This is useful for electronic scales that require rapid response or post-processing algorithms that require high data rates.

Although an external clock source can be used as needed, the high-precision on-board oscillator of the ADS1232/4 can work without an external oscillator or crystal oscillator. All controls of the ADS1232/4 are implemented by some special pins. By eliminating the need to program all registers, this architecture greatly simplifies software development. Finally, you can use a simple read-only interface to easily retrieve the data output of the ADC. Due to the high-density capacity of the TI hybrid signal process, the ADS1232 is suitable for 24-pin ultra-thin and compressed small shape packages (TSSOP), while the ADS1234 uses 28-pin TSSOP.

To better illustrate the performance of ADS1232/4, figure 4 shows an output reading at a 10-second interval with a data rate of 10 (SPS) and a PGA gain coefficient of 128, the reference voltage is a 5 V bridging excitation voltage. The output reading of ADS1232/4 is displayed on the Left axis in LSB, while the output reading is displayed on the right axis in nV. The root mean square (rms) noise is only 17nV, and the peak-to-peak noise is only 110nV. Let's look back at the 4mV/V load unit instance with a 5 V excitation voltage mentioned above. When the ADS1232 is used with this load unit, it will provide an internal precision of over 180,000 counts, no additional components or post-processing of output data are required. The noise of the ADS1232/4 will change in the form of data rate, PGA, and reference voltage function, which is important to note. You can log on to www.ti.com to obtain the product description about ADS1232/4. The noise List displays the performance of different settings.

 
Figure 4 noise performance of ADS1232

As a help for Electronic Scale designers, TI also developed a reference design for electronic scales using ADS1232, that is, ADS1232REF. Figure 5 shows its structure. As the core of the design, the ADS1232 directly digitizes the signal of the Electronic Scale load unit. The MSP430 microcontroller can collect the ADS1232 data and drive the LCD display. It decodes user input from the switch and uses a USB to connect to the same optional PC for communication. Figure 6 highlights several key components of the circuit board. The user connects the server load unit to the indicated connector. The Jumper can bypass the optional RC Filter before the input of the ADS1232. You can switch the reference voltage between an external excitation voltage or a simulated power supply. The power supply is provided by an external DC power supply. In standalone mode, the main control switch controls the entire operation. Then, MSP430 displays the selected data on the LCD. In PC mode, the USB interface allows a PC to control the operation and display the data output on the PC display. For more information about ADS1232REF, visit TI's website to Download user guide.

 
Figure 5 structure of the ADS1232REF electronic scale reference design

 
Figure 6 reference design of the ADS1232REF Electronic Scale

In short, electronic scales are becoming increasingly popular in various applications. The load unit may be 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 provide Single-Chip Solutions, allowing electronic scaling designers to quickly develop a small, low-cost, and high-performance electronic scale. The ADS1232REF reference design allows users to use their own load units to evaluate the performance of the ADS1232, and serves as the basis for the complete electronic scale design.