One way to cope with harsh environments is to remotely place electronic devices, but this technology increases costs, sacrifices reliability, and typically reduces system accuracy. Therefore, the demand for electronic circuits designed for high-temperature operation above 200 ℃ is constantly increasing. Silicon carbide and gallium arsenide can work at high temperatures, but these processes are not cost-effective. So far, there are not many cost-effective differential amplifiers specifically designed for high-temperature operation.

    The design concept of this article provides a low-cost and high-performance alternative solution. Connect two fast, low noise, and high-performance instrument amplifiers AD8229 to construct a high-temperature differential amplifier. The AD8229 is manufactured using advanced Insulated Silicon (SOI) technology, which is also used to provide precision pressure sensors for major multinational airlines, turbine engines, and petrochemical product suppliers. Circuits manufactured using SOI technology have high precision performance, high reliability, better dielectric compatibility, and extended high-temperature working range. The instrument amplifier AD8229 is designed to operate in extreme high temperature environments using an 8-pin side soldered ceramic dual in line package (SBDIP). Under high temperature conditions, the dielectric isolation process can minimize leakage current to the greatest extent, and the design architecture can compensate for low base emitter voltage at high temperatures.

    ADC typically uses a single power supply of 1.8V to 5V. If a small signal needs to be processed in the presence of a large common mode voltage, an instrument amplifier can be placed in front of the ADC to amplify the signal and suppress the common mode voltage, so that the ADC input is not saturated. Figure 1 shows a fully differential amplifier with a system gain of 2.

    This amplifier is used in conjunction with single ended or differential inputs to provide low distortion differential output and drive high-precision ADCs. This complete high-temperature solution has amplification and adjustment output, which can greatly improve the performance and work efficiency of the system in harsh high-temperature environments.

    Amplifier A serves as a follower and amplifier B as an inverter, forming a gain modulated differential signal between OUTP and OUTN. When the gain resistor is not used, the system defaults to G=2. If the gain is required to be greater than 2, a matching gain setting resistor can be added at both ends of the RG.

    The transfer function of this circuit is:

    VOUT = 2 × G × (VIN+ - VIN-) + VREF

    Among them:

    G = 1 + 6 kO/ RG

 Figure 1. Extreme high temperature differential amplifier

    The gain accuracy depends on the absolute tolerance of RG. The temperature coefficient mismatch between the external gain resistor and the internal thin film resistor will increase the gain drift of the instrument amplifier. When the gain resistor is not used, the gain error and gain drift are kept to a minimum. The ability to set different gains provides users with design flexibility. The system gain G uses multiple standard resistance values, as shown in Table 1. Please note that it requires two gain setting resistors to set the system gain, and these resistors must be able to operate at high temperatures.