Robust capacitive measurement system

摘要

A capacitive sensing circuit is disclosed, wherein the mixer is directly connected to the sense electrode. The AC transimpedance amplifier in front of the mixer in prior art is removed and replaced by a differential DC transimpedance amplifier respectively integrator.;The mixer DC offset voltage or current together with the large amplification factor required after the mixer now would result in an inacceptable DC offset at the output of the signal chain. In order to eliminate the effect of the mixer offset, the amplifying stages after the mixer are AC coupled to the mixer output and one of the signals entering the mixer is phase modulated or amplitude modulated with a known low frequency signal. An additional mixer after the AC coupled amplifying stages is driven with the same low frequency modulating signal, resulting in the wanted DC output signal responsive to the capacitance to be measured.

主权项

1. A capacitive detection system, comprising:
an antenna electrode; a first AC signal generator configured to generate a first AC voltage signal, a second AC signal generator configured to generate a second AC voltage signal, said second AC voltage having a lower frequency than said first AC signal, a first mixer for mixing said first AC voltage signal and said second AC voltage signal and for generating a modulated AC voltage signal, wherein said first mixer is operatively coupled to said antenna electrode to apply said modulated AC voltage signal to said antenna electrode, a control and evaluation unit operatively coupled to said antenna electrode, said control and evaluation unit comprising a current measurement circuit configured to measure current signals, said current signals comprising amplitude and/or phase of a current flowing in said antenna electrode, said control and evaluation unit being configured to determine a capacitance to be measured based upon said measured current signals, and to output an output signal indicative of said determined capacitance; wherein said current measurement circuit comprises a differential transimpedance amplifier circuit comprising a common mode voltage setting input, an inverting input, a non-inverting input and an output, wherein said inverting input and said non-inverting input of said transimpedance amplifier circuit are operatively connected to said antenna electrode by means of a multiplexer in such a way that said inverting input and said non-inverting input are alternately supplied with said current flowing in said antenna electrode, said multiplexer being operatively coupled to and controlled by said first AC signal generator.