Circuit arrangement for a gradiometer having superconducting quantum interferometer (SQUID) for measuring low-intensity biomagnetic fields

Claim 1

In a circuit arrangement for a gradiometer having a superconducting quantum interferometer (SQUID) for measuring low-intensity biomagnetic fields, whereby a pick-up current generated by a gradiometer coil in the gradiometer and which is a function of the intensity of the measured magnetic field is supplied to an in-coupling element which additively mixes the pick-up current with a modulation current generated by a first modulation voltage generator and subtractively mixes the pick-up current with a feedback current and inputs a resultant current to the SQUID which thereby provides a modulated output signal to a demodulator as a modulated voltage via a first bandpass filter, the demodulator multiplying the modulated voltage by a first modulation voltage from said first modulation generator which is phase-shifted by 90° to provide a demodulated voltage and supplying the demodulated voltage via an amplifier to a low-pass filter whose output signal is supplied to the in-coupling element as the feedback current, the improvement comprising the modulated output signal of the first bandpass filter being supplied to an analog-to-digital converter for digitization and quantization, a demodulated output signal of the analog-to-digital converter being a measured signal after passing through a digital low-pass filter, the modulated output voltage of the first bandpass filter conducted to the analog-to-digital converter as an input voltage, the output voltage of the analog-to-digital converter being supplied to a digital demodulator that digitally demodulates this output voltage with a digitized, further modulation voltage generated by a further modulation voltage generator operating synchronously with the first modulation voltage generator and the demodulated voltage being supplied to said digital low-pass filter whose output signal is the measured signal.