Variable-gain differential amplifier operates at extremely high temperatures
Nowadays, electronic systems must operate at ever increasing temperatures, with turbine engines, oil field equipment, and a variety of other present and next-generation control applications requiring devices that function at temperatures greater than 200°C. Unfortunately, the availability of integrated circuits for high-temperature operation is very limited, especially for temperatures of 200°C and beyond.
One method of dealing with the hostile environment is to locate the electronics remotely, but this technique adds cost, compromises reliability, and generally reduces system accuracy. Therefore, the demand for electronic circuits designed specifically for high-temperature operation exceeding 200°C continues to grow. Silicon carbide and gallium arsenide can operate at high temperatures, but these processes are not cost effective. To date, not many affordable differential amplifiers are specifically targeted for high-temperature operation.
This design idea offers an alternative solution that features lower cost and higher performance. Two AD8229 fast, low-noise, high-performance instrumentation amplifiers are connected to create a high-temperature differential amplifier. The AD8229 is manufactured on an advanced SOI (silicon-on-insulator) process, the same process used to supply precision pressure transducers to major multinational aircraft, turbine engine, and petrochemical suppliers. Circuits fabricated on the SOI process provide precision performance, high reliability, improved media compatibility, and extended high-temperature operation. Offered in an 8-lead side-brazed ceramic dual-in-line package (SBDIP), the AD8229 instrumentation amplifier has been carefully designed for extreme high-temperature operation. The dielectrically isolated process minimizes leakage currents at high temperatures, and the design architecture compensates for low base-emitter voltages at high temperatures.
ADCs typically run on single 1.8-V to 5-V supplies. To process a small signal in the presence of large common-mode voltages, an instrumentation amplifier ahead of the ADC amplifies the signal while rejecting the common-mode to keep it from saturating the ADC inputs.
Figure 1 shows a fully differential amplifier with a system gain of 2. Used with single-ended or differential inputs, it provides a low-distortion differential output that can drive high-precision ADCs. This complete high-temperature solution provides an amplified and scaled output that can greatly improve the performance and operational efficiencies of systems operating in hostile high temperature environments.
Amplifier A serves as a follower and amplifier B as an inverter, creating a gained differential signal between OUTP and OUTN. The system defaults to G = 2 when no gain resistor is used. If a gain larger than 2 is desired, add matched gain-setting resistors across the RG terminals.
The transfer function of this circuit is
VOUT = 2 × G × (VIN+ − VIN−) + VREF
where:
G = 1 + 6 kΩ/ RG
Gain accuracy is determined by the absolute tolerance of RG. The temperature coefficient of the external gain resistor increases the gain drift of the instrumentation amplifier. Gain error and gain drift are kept to a minimum when the gain resistor is not used.
Programmable gain affords the user design flexibility. The system gain, G, using several standard resistance values, is shown Table 1. Note that the gain-setting resistors must be rated for high-temperature operation.
Table 1. Gain achieved using 1% standard resistor values
The ability of this differential amplifier to reject a common-mode voltage is determined by the ratio match of internal laser-trimmed resistors; the closer the match, the higher the common-mode rejection (CMR). With discrete amplifiers and 0.1% external resistors, CMR is limited to 60 dB, but this circuit can achieve CMR better than 80 dB. Thanks to superbeta processing, this circuit also offers 10× lower input bias current than discrete implementations.
The common-mode rejection of instrumentation amplifiers typically falls off at 200 Hz, so common-mode filters are often used to compensate for this shortcoming. This circuit can reject common mode over a greater frequency range, 80 dB CMRR minimum up to 5 kHz, reducing the need for filtering.
With single-supply ADCs and other components, the only circuit requiring dual supplies was the interface at the front end. When used with a dual power supply, this circuit can measure small signals while rejecting large negative common-mode voltages. The output can be level shifted by applying a positive voltage to the REF pins. With a single-supply ADC operating on 5 V, for example, a 2.5-V source on the REF pin sets the output to midsupply. This source, which can be the reference from the ADC, should be low impedance to avoid degrading the CMR and low drift to maintain accuracy over temperature.
The low offset and high CMR over frequency of this circuit make it an excellent candidate for bridge measurements. The bridge can be connected directly to the inputs of the amplifier. It can also be used for voltage-output pressure transducers. Since it delivers low noise and low drift, it also serves diagnostic applications.
Figure 2 shows how the outputs maintain their phase balance, even at 225°C.
The mismatch of two outputs is measured at node “VoCM” which is set by the potential of the external source VCM. Figure 3 shows the common mode of the output.
The bottom trace, VoCM, shows the amplifiers’ mismatch, 10mV/div, at 225°C
Figure 4 demonstrates the gain error and the low nonlinearity of the system with the gain of 2 at 225°C. The maximum nonlinearity is about 300 ppm.
About the authors
Chau Tran [chau.tran@analog.com] joined Analog Devices in 1984, where he works in the Instrumentation Amplifier Products (IAP) Group in Wilmington, MA. In 1990, he graduated with an MSEE degree from Tufts University. Chau holds more than 10 patents and has authored more than 10 technical articles.
Marco Ablao [marco.ablao@analog.com] is a Test Product Engineer at Analog Devices in the Philippines. Since joining ADI in 2010, he has worked on Integrated Amplifier products and continuously learning on other products such as Op-Amps and References. He graduated with a BSECE degree in Mapua Institute of Technology, Philippines.
Sherwin Gatchalian [sherwin.gatchalian@analog.com] joined Analog Devices in 2010, as a Test/Product Engineer for Precision Analog to Digital Converters – Router Protector Sensor (PAD-RPS) group in Gen. Trias Philippines. Prior joining ADI, he worked as a circuit design engineer for Radio Frequency Group at Sanyo Semiconductor Manufacturing Philippines for more than 2 years. Earned a BSECE degree in 2007 from Saint Louis University Baguio City, Philippines.
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