What is the use of an instrumentation amplifier?

In modern electronic devices, operational amplifiers are an indispensable component. Particularly in the design of domestically produced op‑amp circuits, selecting appropriate components and optimizing the circuit architecture have become focal points for many engineers. Today, we will delve into the key principles and techniques of designing domestic op‑amp circuits, helping you elevate your circuit‑design expertise. 1. Understand the Basic Characteristics of Operational Amplifiers First and foremost, when designing a domestic op‑amp circuit, it is essential to grasp the amplifier’s fundamental characteristics. The core performance of an op‑amp hinges on parameters such as gain, input impedance, and output impedance. Gain determines the signal‑amplification capability; input impedance affects the source‑load interaction; and output impedance influences the signal‑transmission efficiency. Only by thoroughly understanding these basic traits can we proceed with more effective circuit design. 2. Choose the Right Domestic Operational Amplifier While the market offers a wide array of domestically manufactured op‑amps, not every model is suitable for your specific project. When making your selection, consider several critical factors: bandwidth, power consumption, operating temperature range, and cost. For instance, if your application involves high‑frequency signal processing, bandwidth becomes the primary consideration. Conversely, for projects with stringent low‑power requirements, prioritize low‑power consumption.

 

　　1. What is the function of an instrumentation amplifier?

　　Instrumentation amplifiers are sometimes misunderstood: not all amplifiers used in instrumentation are instrumentation amplifiers, and not all instrumentation amplifiers are limited to instrumentation applications. The most critical function of an instrumentation amplifier is its ability to detect weak signals in the presence of noise. Since noise voltages typically appear as common-mode signals while the desired signal is differential, the high common-mode rejection ratio (CMR) of an instrumentation amplifier enables it to extract and amplify the useful signal while rejecting the noise. Furthermore, in practical applications, signal sources often have output impedances on the order of several kiloohms (kΩ) or even higher, necessitating an amplifier with very high input impedance—typically on the gigohm scale. Instrumentation amplifiers not only meet this requirement but also ensure that the input impedances of both inputs are equal. The operating frequency range of an instrumentation amplifier generally spans from DC to around 1 MHz. At higher frequencies, input capacitance becomes more significant than input resistance; in such cases, differential amplifiers are typically employed to handle high-speed signals. While this approach increases speed, it concurrently reduces input impedance.

　　2. How to Prevent Overvoltage at the Input of an Instrumentation Amplifier

　　Design engineers must use external resistors to prevent excessive drive current from causing overvoltage across the internal ESD‑clamp diodes. The value of the current‑limiting resistor depends on the instrumentation amplifier’s noise level, the supply voltage, and the required overvoltage protection. Recommended values are provided in the instrumentational amplifier’s datasheet.

　　Since employing external current‑limiting resistors increases noise, an alternative approach is to use external high‑current clamping diodes, which can significantly reduce the required resistance value. It should be noted that most standard diodes exhibit substantial leakage currents, which can introduce significant offset errors at the output of an instrumentation amplifier. Moreover, because such leakage currents increase exponentially with temperature, design engineers should avoid using ordinary diodes in high‑impedance signal‑source applications.

　　3. How RFI Filtering Works

　　Long lead wires between the sensor and the instrumentation amplifier are susceptible to radio‑frequency interference (RFI). After rectification, RFI can manifest as DC output offset errors in the instrumentation amplifier. A solution for filtering out RFI before it reaches the instrumentation amplifier is shown in the figure. Components R1a and C1a form a low‑pass filter at the non‑inverting input, while components R1b and C1b constitute a low‑pass filter at the inverting input.

　　It is crucial that the cutoff frequencies of the two low‑pass filters mentioned above be well matched; otherwise, common‑mode signals may be converted into differential signals. C2 should be at least ten times larger than C1, though at high frequencies this requirement can be relaxed somewhat because the two input terminals are effectively shorted. Even so, matching C1a and C1b is essential. These capacitors should be ±5% C0G ceramic film types. The differential‑mode bandwidth of this filter is {1/2πR(2C2+C1)}, while its common‑mode bandwidth is [1/2πR1C1].

 

　　Our company currently offers a range of mature products, including the HJ118 low-power precision instrumentation amplifier, the HJ52301A/B/C high-temperature precision instrumentation amplifiers, and the HJ333 low-drift rail-to-rail precision instrumentation amplifier, among others. Please feel free to contact us!