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Designing high-temperature differential amplifier

Posted: 20 Feb 2013 ?? ?Print Version ?Bookmark and Share

Keywords:ADC? inverter? thinfilm resistors? Silicon carbide?

Nowadays, electronic systems should operate at ever increasing temperatures. Turbine engines, oil field equipment, and a variety of other present and next-generation control applications require devices that function at temperatures greater than 200C. Unfortunately, the availability of integrated circuits for high-temperature operation is very limited, especially for temperatures of 200C 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 200C 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.

Figure 1: Extreme high temperature differential amplifier.

This design idea offers an alternative solution that features low cost and high 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 silicon-on-insulator (SOI) 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 is designed for extreme high-temperature operation. The dielectrically isolated process minimises 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 k/ RG

Gain accuracy is determined by the absolute tolerance of RG. The mismatch of the temperature coefficient of the external gain resistors and the internal thinfilm resistors increases the gain drift of the instrumentation amplifiers. Gain error and gain drift are kept to a minimum when the gain resistor is not used. The ability to set different gains affords the user design flexibility. The system gain, G, using several standard resistance values, is shown the table. Note that it requires two gain-setting resistors to set the gain of the system and the resistors must be rated for high-temperature operation.

Table: Gain achieved using 1% standard resistor values.


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