Measuring circuit and a method for determining a characteristic of the impedance of a complex impedance element for facilitating characterization of the impedance thereof

Abstract

A single chip integrated circuit measuring circuit (1) for determining a characteristic of the impedance of an external complex impedance circuit (2) for facilitating characterization of the impedance of the complex impedance circuit (2) comprises a signal generating circuit (7) for generating a variable frequency stimulus signal for applying to the complex impedance circuit (2). A first receiving circuit (10) receives a response signal from the complex impedance circuit (2) in response to the stimulus signal and conditions the response signal. A first analog-to-digital converter (68) converts the conditioned response signal to a first digital output signal, which is read from the first analog-to-digital converter (68) through a first digital output port (14). The response signal from the complex impedance circuit (2) is a current signal, and a current to voltage converter circuit (64) converts the response signal to a voltage signal. A first RMS to DC level converting circuit (70) converts the AC voltage of the response signal to a DC voltage level, and a fourth multiplexer (67) selectively applies the voltage response signal or the DC voltage level signal to the first analog-to-digital converter (68), depending on whether it is desired that the first digital output signal should be indicative of the phase shift or amplitude change in the response signal relative to the stimulus signal. A second receiving circuit (20) receives the stimulus signal, and similarly converts the stimulus signal to a second digital output signal for facilitating comparison of the response signal with the stimulus signal.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a measuring circuit for determining a characteristic of the impedance of a complex impedance element for facilitating characterization of the impedance thereof, and in particular, though not limited to such a measuring circuit for determining a characteristic of the impedance of an impedance based sensor for in turn facilitating measuring the change in impedance of such a sensor as a result of an external effect. The invention also relates to a method for determining a characteristic of the impedance of a complex impedance element for facilitating characterization of the impedance thereof, and in particular, though not limited to such a method for determining a characteristic of the impedance of an impedance based sensor for in turn facilitating measuring the change in impedance of such a sensor as a result of an external effect. BACKGROUND TO THE INVENTION [0002] Impedance based sensors are well known, and are commonly ...

AI Technical Summary

Benefits of technology

[0043] The advantages of the invention are many. By virtue of the fact that the measuring circuit is implemented on a single chip, errors in the first output signal outputted by the measuring circuit are minimised. This is due to the fact that there is little or no accumulation of signal errors between the signal generating circuit and the first receiving circuit. Additionally, by virtue of the fact that the measuring circuit is implemented on a single chip, the respective components of the signal generating circuit and the first receiving circuit can be relatively accurately matched. Furthermore, when the measuring circuit is provided with a second receiving circuit, the components in the second receiving circuit can likewise be relatively accurately matched with the components in the signal generating circuit and the first receiving circuit. Thus, mismatch of the respective circuits of the measuring circuit due to ratiometric operation of the measuring circuit are minimised. Additionally, any drift in the measuring circuit can be minimised. Matching of the components of the signal generating circuit, and the first and second receiving circuits is further facilitated when the measuring circuit is implemented as an integrated circuit.

[0044] The measuring circuit according to the invention, in particular, facilitates the phase shift and amplitude change in a response signal from a complex impedance element, which is responsive to a stimulus signal applied to the complex impedance element to be readily and easily determined. Additionally, the frequency response of the complex impedance element can also be readily easily determined.

[0045] Additionally, the provision of the measuring circuit on a single chip facilitates synchronous timing of the entire measuring circuit. Furthermore, R-C timing components such as glitching and high frequency interference which arise where discrete elements are used are substantially eliminated in the measuring circuit according to the invention. Such R-C timing components in prior art circuits may result from track resistance and associated capacitance and inductance of the tracks of a printed circuit board on which the discrete elements are mounted. Such R-C components can be particularly problematical at relatively high frequencies, and result in phase errors as well as other errors. Accordingly, by providing the measuring circuit according to the invention as an integrated circuit on a single chip, such R-C timing components are effectively eliminated.

[0046] Furthermore, by providing the measuring circuit on a single chip, the board area requirement of a printed circuit board on which the measuring circuit is provided is significantly less than the board area requirement of prior art arrangements where discrete components are mounted on a printed circuit board.

[0047] By implementing the signal generating circuit as a direct digital synthesis frequency signal generator, the performance of the measuring circuit according to the invention is significantly enhanced. Additionally, the measuring circuit is suitable for determining the frequency response of a wide range of complex impedance elements, for example, a wide range of complex impedance sensors, and complex impedance circuits, and in particular, the provision of the direct digital synthesis frequency signal generator allows the measuring circuit to generate stimulus signals of a wide breadth of frequency ranges with range of frequency resolution. Additionally, the implementation of the signal generating circuit as a direct digital synthesis frequency signal generator permits the frequency range of the stimulus signal to be readily easily digitally selected, and the resolution of the frequency steps through which the stimulus signal is to be swept can likewise be readily easily digitally selected. The provision of the adder in the frequency generator for adding a phase offset digital word to the count from the phase accumulator provides the further advantage that the phase of the stimulus signal can be readily easily offset by a desired selectable amount.

[0048] By appropriately selecting the bit size of the counter of the phase accumulator, and the bit size of the digital-to-analog converter of the signal generator, relatively high frequency resolution can be achieved.

Problems solved by technology

However, the impedance of many such sensors is complex, where the sensor includes a significant amount of reactance from capacitive or inductive elements.

Thirdly, a separate signal analysing circuit is required for analysing response signals from the sensor in response to the stimulus signal.

This results in many problems, in particular, each such circuit, namely, the signal generator, the signal conditioning circuits and the signal analysing circuit, each introduce errors into the respective signals.

Matching of the components of the various separate circuits for minimising mismatch errors is also problematical.

Furthermore, the errors introduced into the signals by each separate circuit are cumulative.

Another problem with such arrangements is that the signal generator must be set up to provide the stimulus signal continuously at specific frequencies, and additionally, must be capable of being accurately swept through an appropriate range of frequencies, depending on the variable and the liquid which is being measured.

By virtue of the fact that separate circuits must be used in determining the impedance characteristic or characteristics of such impedance circuits and transmission lines, similar problems arise in the measuring of the impedance characteristic as already described with reference to impedance based sensors.

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