Getting The Most Out Of Your
Instrumentation Amplifier Design
Many industrial and medical applications use Instrumentation Amplifiers (INAs) to condition small signals in the presence of large common-mode voltages and DC potentials. Standard INAs using a unity- gain difference Amplifier in the output stage, however, CAN limit the input common- mode range significantly. Thus, common- mode signals induced by adjacent equipment, as well as large differential DC potentials from differently located signal sources, CAN increase the input voltage of the INA, causing its input stage to saturate. Satura- tion causes the INA output voltage, although of wrong value, to appear normal to the following processing circuitry. This could lead to disastrous effects with unpredict- able consequences. This article reviews some principles of the classic three-op-amp INA and provides design hints that extend the input common- mode range to avoid saturation while pre- serving overall gain at maximum value. The article also discusses the removal of large differential DC voltages through active filtering, avoiding passive RC Filters at the INA input that otherwise would lower its common-mode rejection ratio (CMRR). INA128 with OPA2132 providing low-pass Filter and external gain stage
Single-supply applications Portable ECG equipment INA326OPA2335ADS8321REF3125 requiring single-supply operation can use the high-precision Analog front end in Figure 5 Both types of Amplifiers the instrumentation amplifier INA326 and the dual Precision AmplifierOPA2335 operate from a single 5-V supply and apply autozeroing techniques, keeping the initial offset and offset drift over temperature and time near zero. The input gain of the INA326 is set to 5 via G1 = 2R2/RG = 2(200 kΩ/80 kΩ). The 750-pF capacitor parallel to R2 cancels resistor noise. The 3-dB frequency of the integra- tor A4 is set to 0.05 Hz, while the output stage around A3 provides a gain of G2 = 1 MΩ/5 kΩ = 200. The precision voltage reference, REF3125 provides low-noise biasing of the 2.5-V bias voltage to the Amplifiers and the 16-bit, 100-kSPS, SAR-ADC ADS8321. To further reject 50/60-Hz noise, the input common- mode voltage is fed back via the Amplifiers A1 and A2 to the right leg of the patient. This approach requires only a few microamps of current to significantly improve the common-mode rejection and to ensure compliance with the UL544 standard.