Intelligent signal conditioning chip MAX1460 and its application

      1 Overview

　　MAX1460 is an intelligent signal conditioning chip produced by Maxim Corporation of the United States. It integrates a complete set of 16-bit data acquisition and correction systems on a single chip for the first time, thus proposing a novel signal conditioning concept. MAX1460 can correct the output after analog-to-digital conversion within a certain temperature range through the on-chip 16-bit DSP according to the compensation equation given by the user, and can store the correction coefficient in the on-chip 128-bit E2PROM. The conditioned signal can be output as 12-bit digital output or analog output through the on-chip 12-bit DAC.

    MAX1460 has not only ADC, DSP and DAC, but also programmable gain amplifier, temperature sensor and offset calibration circuit, so it is particularly suitable for signal conditioning of piezoresistive sensors, but also for signal conditioning of accelerometers, strain detection and other resistance sensors. MAX1460 only needs a small number of peripheral components, and can calibrate and compensate the sensor's offset (Offset), full-scale output (FSO), offset temperature coefficient (OffsetTC), full-scale output temperature coefficient (FSOTC) and full-scale output nonlinearity through simple settings and adjustments. The output signal accuracy of MAX1460 after conditioning can reach 0.1%.

      2 Structure and Principle of MAX1460

　　Figure 1 is the internal functional structure block diagram of MAX1460. As can be seen from Figure 1, the chip can be divided into several main parts, such as analog front end, test interface, digital processing module and output module.

       2.1 Analog front end  

　　The analog front end of MAX1460 includes programmable gain amplifier (PGA), coarse offset DAC (CODAC), temperature sensor and 16-bit Σ-Δ ADC. Before the input signal is quantized, it should first be amplified and coarse offset adjusted by PGA and CODAC, so that the signal can be within the dynamic range of ADC. There are 5 bits in the configuration register of the chip that can be used to set PGA (2-bit) and CODAC (3-bit).

　　MAX1460 has a built-in temperature sensing bridge. When MAX1460 is required for compensation, this temperature sensing bridge can be used instead of an external sensor. Therefore, the chip should be placed as close to the sensor as possible so that they are in the same thermal environment. The output signal of the temperature sensing bridge also passes through a 3-bit coarse offset DAC before being quantized by the ADC. The configuration bit of the temperature sensor offset (TSO) should also be set in the configuration register. Choosing the appropriate setting can make the half-bias temperature value close to 0.0 after quantization, so that the correction coefficient is applicable to the maximum temperature range.

　　The 16-bit ADC in the MAX1460 chip can quantize the adjusted input signal and temperature signal and then send them to the DSP register. The dynamic range of the ADC is -VDD to +VDD. When the input of the ADC is in the range of ±85% VDD, the chip can maintain a high accuracy.

      2.2 Test interface  

　　Testing of the MAX1460 can be achieved through the chip's test interface. The interface signals mainly include chip select signal (CS1, CS2), start signal (START), test enable signal (TEST), reset signal (RESET), serial data signal (SDIO, SIO) and conversion end mark signal (EOC). The tester can configure the chip, write the correction coefficient (thereby determining the correction equation) and read the result corrected by DSP through this interface according to the given operation sequence.

       2.3 Digital processing module

　　MAX1460 has a built-in low-power 16-bit DSP, so it can calculate the quantized input signal and temperature signal sent by the analog front end according to the correction equation determined by the user to obtain the corrected output. The built-in correction algorithm uses polynomial fitting, and its expression is: 

        D = Gain (1 + G1T + G2T2) (Signal + Of0 + Of1T + Of2T2) + DOFF

　　Where D is the expected output, G represents the sensitivity of the sensor, G1 and G2 represent the first-order and second-order temperature sensitivity coefficients of the temperature effect, Of0 represents the offset of the sensor, Of1 and Of2 represent the offset coefficients of the first-order and second-order temperature sensors of the temperature effect, S and T are the quantized input signal and temperature signal sent by the analog front end, and DOFF is the output offset. The correction coefficient can be flexibly configured according to the different temperature characteristics of the specific sensor system. Since the correction equation uses a polynomial fitting algorithm and compensates for the high-order components of the temperature effect, it can achieve a correction accuracy of 0.1%. [page]
       2.4 Output module The  

　　corrected digital signal can be directly output from the parallel data port (high 12-bit), or it can be output from the analog output port after the digital-to-analog conversion of the 12-bit DAC on the chip. In addition, there is an operational amplifier on the MAX1460 chip, and the user can use this operational amplifier to form a low-pass filter to filter the output of the DAC.

　　MAX1460 not only has high compensation accuracy, but also has high flexibility. It can adapt to different systems and can be widely used in many fields such as industrial pressure sensors, handheld instruments, smart charging systems and automation systems.

      3 Application in high-precision CNC attenuation system

　　In the radar signal simulation system, the signal amplitude describing the target distance and angle is usually accurately attenuated, and the requirements for the attenuation system are very high (step value 0.1dB, accuracy 0.05dB). In this way, the temperature effect of the core components (analog multiplier and DAC) of the digital control attenuation system must be considered and solved. After demonstration, the author uses MAX1460 to perform temperature compensation on the attenuation system. Figure 2 shows the overall structure of the temperature compensation system. This system can be used to input the same attenuation control word at different temperatures, and the output voltage of the DAC can be changed by adjusting the reference voltage of the DAC to achieve the purpose of correcting the attenuation. Since the conversion speed of MAX1460 is only 15 times/second, the output voltage of the DAC cannot be directly adjusted in the control voltage.

        Figure 3 shows the actual application circuit composition of the conditioning system. Its main control computer communicates with MAX1460 serially through test interface signals such as RESET, TEST, XIN, SDIO, and SDO to write the configuration of the chip and read out the conditioning results. The test program is written in C++ language according to the given read and write timing. Figure 4 shows the read and write timing of the MAX1460 test interface. Since the reference voltage of the DAC is 2V, considering the dynamic range of the MAX1460 analog front end, voltage division should be performed first during design. This example uses the operational amplifier OP07 to achieve this. Its output end is smoothed by the built-in operational amplifier in the chip, and then proportionally amplified by OP07 and finally sent to the DAC.

       4 Conclusion

　　Preliminary experimental results show that using MAX1460 for temperature compensation can greatly improve the accuracy of the attenuation system and can fully meet the system requirements.

       references

1. Low-Power 16-BitSmart ADC, Datasheet, Maxim Co. US, October 1999

2. Li Guangjun. Microcomputer Interface. University of Electronic Science and Technology Press, 1998