In response to the uptrend in applications demanding impedance measurements –the relationship between the voltage and current flowing in a circuit, ADI has developed impedance measurement ICs like the AD5933 and ADuCM350, which have found broad market acceptance. However, these parts don’t cover the requirements for every application, and designers are still faced with the challenges of designing this measurement capability with standard components. To some, the options and challenges can be a bit overwhelming.
Let’s take a look at what can be done with modern ICs by starting with the basics. Even though most people think about impedance in terms of a voltage-to-current ratio, in circuit terms, it comes down to two voltage signals and the relationship between a known and an unknown impedance. For example, to apply a current through an unknown resistor RU, we can place this resistor in a circuit with a known a voltage vi and a second known resistor R. This forms a voltage divider with an output voltage vo which can be solved for RU:
To get an accurate ratio measurement, vo shouldn’t be too small relative to vi nor it should be almost equal. This deceivingly simple method works for any impedance when operated with an ac signal, but is prone to measurement errors and circuit parasitics as frequency is increased.
Another classic example consists in placing the known and unknown circuit elements in a Wheatstone bridge and nulling the output signal by adjusting variable elements. At the balance point (where the signal is nulled), the unknown impedance can be calculated using the known bridge element values. This method yields very accurate results, but requires manual manipulation of bulky and expensive variable capacitors, inductors and resistors, making it impractical in many applications.
An improvement over the classic approach consists of automating the bridge and using resistive elements. This is possible by inserting a control element in place of the null detector to drive one leg of the bridge. This method, known as the “auto-balancing bridge”, can be implemented with a simple Op Amp. Because it maintains the null point almost at a constant, it reduces the CMRR requirement for the measurement of the voltage across the unknown impedance. Though simple, the Op Amp needs to maintain high gain over frequency and its output should handle the current coming from the source. A few choices like the LTC6268, ADA4817-1, LTC6252 and ADA4625-1 can be used in impedance measurements at frequencies up to 10MHz or more. High-speed instrumentation amplifiers like the AD8250, AD8251, AD8429 or AD8421 can differentially sense the unknown voltage, avoiding parasitics and mitigating the errors induced by the null errors of the Op Amp.
The next challenge is to find the magnitude and phase relationship between the signals coming from the known and the unknown impedances. High-speed, 18-bit precision ADCs like the AD4003 or LTC2387-18 enable designers to digitize the waveforms to extract their relationship in the digital domain. This has several advantages over doing the same in the analog domain, yielding more accurate results, smaller PCB area, and more robust systems. Lastly, generating an excitation signal can be greatly simplified by using DDS chips like the AD9834 to complete the measurement front-end.
Have you run into similar challenges when designing impedance measurement systems? Are you working through a project with some of these challenges? Let us know!