How do I need to measure impedance?

The need to measure impedance (the relationship between voltage and current in a circuit) continues to increase. Therefore, ADI has developed a variety of impedance measurement ICs, such as AD5933 and ADuCM350, and these products have been widely recognized in the market. However, these devices cannot meet the requirements of all applications, and designers still face the challenge of using standard components to design this measurement capability. Some of them may feel a bit at a loss when facing these choices and challenges.

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Let's start with the basics and see what modern ICs can do. Although most people consider impedance from the perspective of the ratio of voltage to current, from the perspective of the circuit, it can be boiled down to the relationship between two voltage signals and a known impedance and an unknown impedance. For example, to apply current through an unknown resistance RU, we can place this resistance in a circuit with a known voltage vi2 and a second known resistance R, which will form a voltage divider with an output voltage of vo, which can be targeted at RU Solve:

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In order to obtain a ratio measurement, vo should not be too small relative to vi, nor should it be nearly equal. When operating with AC signals, this very simple method is suitable for any impedance, but as the frequency increases, measurement errors and circuit parasitics are prone to occur.

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Another typical example is to place known and unknown circuit components in a Wheatstone bridge and adjust the variable components to make the output signal zero. At the equilibrium point (when the signal is zero), the known value of the bridge components can be used to calculate the unknown impedance. The result of this method is very high, but it requires manual operation of large and expensive variable capacitors, inductors and resistors, so it is not practical in many applications.

Compared with the classic method, the improved method includes the automation of the bridge and the use of resistive elements. This can be achieved by inserting a control element instead of a zero detector to drive one arm of the bridge. This method is called "auto-balanced bridge" and can be implemented with a simple operational amplifier. Because this method keeps the zero almost constant, it reduces the CMRR requirement for measuring the voltage on the unknown impedance. Although the operational amplifier is simple, it needs to maintain a high gain over the entire frequency range, and its output should be able to handle the current output by the power supply. Some options are available for measuring impedance at frequencies up to 10MHz or above, such as the LTC6268, ADA4817-1, LTC6252, and ADA4625-1. High-speed instrumentation amplifiers such as AD8250, AD8251, AD8429, or AD8421 can use the differential method to detect unknown voltages, thereby avoiding parasitic effects and reducing measurement errors caused by the zero error of the operational amplifier.

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The next challenge is to find the amplitude and phase relationship between the signals from the known and unknown impedances. The 18-bit high-speed ADC (such as AD4003 or LTC2387-18) allows designers to digitize these waveforms to extract their relationship in the digital domain. Compared with performing the same operation in the analog domain, this approach has several advantages: the results obtained are greater, the PCB area is smaller, and the system is more reliable. Using DDS chips such as AD9834 to form the measurement front end can greatly simplify the process of generating excitation signals.