Impedance Measurement of Sensors
Sensors come in many forms to measure a variety of mechanical parameters (such as strain and position), environmental parameters(temperature, pressure and humidity) and samples (such as materials, tissue and organisms). Changes in the sensed environment lead to a corresponding variation in the sensor's impedance. This impedance variation, which comprises inductance, capacitance and resistance or the complex impedance itself, is measured to a high precision and accuracy on a short timescale appropriate for the sensor. As impedance often changes with frequency, a sensor's impedance spectrum provides an additional fingerprint for the measured sample or environment. The vast array of sensors currently available includes chemical sensors and biosensors. Chemical sensors are typically capacitive: the presence of an analyte in the dielectric causes a change in the capacitance. Impedimetric biosensors are used to measure tissue or organisms in a liquid medium (static or flowing), and in most cases are non-destructive and label-free.
Developing and optimizing a sensor requires a clear understanding of how changes in the sensed environment translate into its impedance. It is important to identify the sensor's optimum working frequency (or frequencies), that is, the frequency at which the sensor has the highest sensitivity (see Figure 1). To find this optimum working point, a common approach is to sweep the frequency while measuring the impedance response and display it on a Bode or Nyquist plot (see Figure 2). Similarly, sweeping the amplitude of the probing voltage makes it possible to find the optimum working voltage. The LabOne control software included with the MFIA Impedance Anayzer features a Sweeper module enabling both frequency and voltage sweeps.
The speed of the sensor's response is another parameter that needs to be characterized over the full frequency range. This can be achieved by inducing a step change in the external sensed environment, or by applying an offset voltage step to the sensor as a proxy for an environmental change.
With the DAQ module of LabOne, measuring the impedance variation triggered by this step change to determine the response time of the sensor becomes straightforward (see Figure 3).
A time-domain measurement at the optimum working frequency and voltage identifies the highest achievable sensing resolution. An ever-decreasing small disturbance (such as an environmental change induced by a rectangular voltage pulse) is applied to the sensor and the resulting impedance change measured. This impedance measurement can be synchronized to the disturbance pulse to allow for averaging of the transients. This technique can also be used to measure the dynamic range of the sensor, comparing the smallest possible resolved signal with the largest. For dynamic range characterization, the MFIA supports both proxy voltage inputs and, when the PID Controller option is installed, closed-loop systems.
The Benefits of Choosing Zurich Instruments
- Optimize the operating frequency and probing voltage of your sensor with a single tool: the LabOne Sweeper allows you to display Bode or Nyquist plots and readily compare them to reference profiles.
- You can track impedance changes that are small and that take place on a fast timescale.
- Zurich Instruments' low-noise analog front-end electronics, with multi-stage inputs and auto-ranging, complement the lock-in detection capability by enabling operation at the 'sweet spot' where maximal signal-to-noise ratio meets minimum measurement time.
- Easy integration of impedance measurements into a larger setup is ensured thanks to the five APIs for the LabOne instrument control software.