CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage of International Application No. PCT/EP2019/060584, filed Apr. 25, 2019, which claims the benefit of European Patent Application No. 18170084.0, filed Apr. 30, 2018, both of which are incorporated herein by reference in their entireties.

The present invention relates to an isolation amplification circuit which comprises an input stage circuitry and a control circuitry stage interconnected through a galvanic isolation barrier. The input stage circuitry comprises a first filter network and a second filter network for supplying first and second output signals in response to the application of first and second electrical input signals. The input stage circuitry comprises a first feedback path configured for applying a first feedback signal to a common node of the first filter network to close a first feedback loop around the first filter network and a second feedback path configured for applying a second feedback signal to a common node of the second filter network to close a second feedback loop around the second filter network.

BACKGROUND OF THE INVENTION

Isolation amplifiers are a form of differential amplifier that allow measurement of small signals in the presence of a high common mode voltage by providing electrical isolation and an electrical/safety barrier, i.e. galvanic isolation. Isolation amplifiers are used in numerous applications and may for example be utilized to protect data acquisition components and systems from harmful common mode voltages. Common mode voltages are electrical potential differences between instrument (earth) ground and signal ground. Isolation amplifiers are for example used in harsh industrial environments, military applications, transportation, medical systems etc.

The ability of a differential amplifier to reject common mode voltage is specified by its so-called Common Mode Rejection Ratio (CMRR). Practical isolation amplifiers have certain limitations on their ability to reject common mode voltage due to various non-ideal characteristics of the differential amplifier and possibly unmatched impedances of input networks connected to respective inputs of the differential amplifier. Isolation amplifiers are for example often used with protective resistors and/or with respective input filter networks coupled in series with the inverting and non-inverting inputs of the differential amplifier. The role of these protective resistors and input filter networks is to remove unwanted frequency components or electrical noise from the inputs of the differential amplifier. Unfortunately, these resistors and input filter networks degrade the inherent CMRR of the differential amplifier and hence degrade the ability of the isolation amplifier to reject common mode noise or common mode voltage such as mains ripple voltage.

Medical isolation amplifiers are one specific type of isolation amplifier. Medical isolation amplifiers must isolate the patient from any possible contact with earth ground or mains/line voltage. In addition, the amount of current that may pass through the patient, i.e. leakage current, if the patient is accidentally connected to earth ground or mains/line voltage must be restricted to extremely small values. Techniques for accomplishing this include magnetic and optical isolation, and these techniques are well known in the art.

US 2014/0336473 A1 discloses isolation amplification circuit for medical applications. The isolation amplification circuit comprises an input stage circuitry and a control circuitry stage inter-connected through an electrical isolation barrier. The input stage circuitry of a single input channel comprises a differential amplifier with a feedback path connected from an output node of the differential amplifier to an internal node of the differential amplifier. The feedback path comprises an active lowpass filter and serves to provide DC-correction around the differential amplifier.

Hence, there remains a need in the art for isolation amplifiers with improved CMRR in particular isolation amplifiers, which can suppress the negative side effects of unmatched impedances of the input resistances or the input filter networks.

SUMMARY OF THE INVENTION

A first aspect of the invention relates to an isolation amplification circuit comprising: an input stage circuitry and a control circuitry stage inter-connected through a galvanic isolation barrier. The input stage circuitry is referenced to a first ground potential and configured to receive at least a first electrical signal and a second electrical signal generated by a signal source. The input stage circuitry comprises:

a first filter network comprising a first input node, a first output node and first common node wherein the first output node is configured to supply a first output signal in response to application of the first electrical signal at the first input node. The input stage circuitry further comprises a second filter network comprising a second input node, a second output node and a second common node, wherein the second output node is configured to supply a second output signal in response to application of the second electrical signal at the second input node. The input stage circuitry additionally comprises a first feedback path configured for lowpass filtering the first output signal and supplying a first lowpass filtered output signal to the first common node; and a second feedback path configured for lowpass filtering the second output signal and supplying a second lowpass filtered output signal to the second common node.

Each of the first filter network and second filter network may comprise at least one of a lowpass filter, highpass filter, bandpass filter and band-reject filter. Each of the first filter network and second filter network may comprise a plurality of interconnected passive components such as resistors, inductors and capacitors to form a transfer function of each of the first and second filter networks.

The skilled person will understand that the input stage circuitry may comprise one or more filter network(s) with respective lowpass filtered feedback paths in addition to the first and second filter networks to provide additional input channel(s) of the isolation amplification circuit.

The signal source may comprise a human or animal body in medically adapted embodiments of the isolation amplification circuit. The human or animal body may supply the first electrical signal as a first electro-physiological signal to the first input node of the first filter network via a first electrode and supply the second electrical signal as a second electro-physiological signal to the second input node of the second filter network via a second electrode. The first and second electrodes may be attached to the skin of the human with a suitable adhesive agent.

The lowpass filtering of the first feedback path may carried out by a first analog lowpass filter and/or by a first digital lowpass filter and the lowpass filtering of the second feedback path may likewise be carried out by a second analog lowpass filter and/or by a second digital lowpass filter. In one embodiment utilizing analog lowpass filters, the first feedback path comprises a first buffer amplifier and a first lowpass filter wherein the first buffer amplifier is connected between the first output node and an input of the first lowpass filter; and the second feedback path comprises a second buffer amplifier and second lowpass filter wherein the second buffer amplifier being connected between the second output node and an input of the second lowpass filter. Each of the first and second buffer amplifiers may comprise an operational amplifier for example coupled in unity gain mode via suitable feedback arrangement.

In one embodiment of the first and second filter networks, the first lowpass filter comprises a first feedback capacitor connected from the first common node to the first ground potential and a first feedback resistor connected between the first common node and an output of the first buffer amplifier; The second lowpass filter may comprises a second feedback capacitor connected from the second common node to the first ground potential and a second feedback resistor connected between the second common node and an output of the second buffer amplifier. Hence, a lowpass corner or cut-off frequency of the first lowpass filter is determined by the resistance and capacitance of the first feedback capacitor and first feedback resistor and a lowpass corner or cut-off frequency of the second lowpass filter is determined by the resistance and capacitance of the second feedback capacitor and second feedback resistor. The cut-off frequency of the first lowpass filter of the first feedback path may lie between 200 Hz and 2 kHz such as between 500 Hz and 1.5 kHz and a cut-off frequency of the second lowpass filter of the second feedback path may lie between 200 Hz and 2 kHz such as between 500 Hz and 1.5 kHz. The capacitance of the first feedback capacitor may be between 10 nF and 100 nF and the resistance of the first feedback resistor may be between 3 kΩ and 30 kΩ. The the second feedback capacitor and the second feedback resistor may be nominally identical to the first feedback capacitor and first feedback resistor, respectively.

According to one embodiment of the input stage circuitry, which incorporates first and second digital lowpass filters, the first feedback path comprises a first analog-to-digital converter, a first digital lowpass filter and a first digital-to-analog converter connected, e.g. in series, between the first output node and the first common node. The second feedback path likewise comprises a second analog-to-digital converter, a second digital lowpass filter and a second digital-to-analog converter connected, e.g. in series, between the second output node and the second common node.

Each of the first and second analog or digital lowpass filters may comprise a first order frequency response characteristic, i.e. a single pole, or a higher order frequency response characteristic. Generally, higher order frequency response characteristics, such as second order or third order, tend to improve the CMRR performance of the isolation amplification circuit albeit at the expense of higher complexity.

At least the first digital lowpass filter and the second digital lowpass filter are preferably integrated on a common or shared digital signal processor (DSP) circuit such as a software programmable DSP or a hard-wired DSP e.g. comprising an ASIC or a suitably configured FPGA. The software programmable DSP may form part of a microprocessor.

The input stage circuitry may form part of a single input channel of the isolation amplification circuit and connected to a single signal source which generates the first and second electrical signals.

The input stage circuitry may comprise a differential amplifier wherein an inverting input of the differential amplifier is electrically coupled to the first output signal of the first filter network and a non-inverting input of the differential amplifier is electrically coupled to the second output signal of the second filter network to produce an amplified output signal. The amplified output signal may be coupled to the control circuitry stage through the galvanic isolation barrier.

The skilled person will understand that corresponding components of the first and second filter networks preferably are nominally identical or matching. For example are passive components of the first filter network nominally identical to the corresponding passive components of the second filter network, e.g. exhibiting nominally identical resistances, capacitances or inductances, as the case may be. The skilled person will understand that such nominally matching components in a practical circuit construction will possess slightly different resistance, capacitance or inductance values due to manufacturing tolerances, drift ageing etc. These component mismatches lead to the previously-discussed discussed problems with impaired CMRR performance in prior art isolation amplification circuits.

The control circuitry stage of the isolation amplification may be connected to a power supply. The power supply may be connectable to a mains line which comprises at least one voltage phase, e.g. 120V/60 Hz, 230V/50 Hz or a local voltage and frequency, and earth ground.

Each of the first and second filter networks may additionally comprise at least one non-linear element(s) such as a transient-voltage-suppression (TVS) diode(s) or similar non-linear voltage limiters to protect each of the first and second filter networks and other circuits and components of the isolation amplification circuit against harmful transient voltages for example defibrillation associated voltage pulses/spikes as discussed in additional detail below with reference to the appended drawing material.

The galvanic isolation barrier may comprise one or more optical isolation element(s) and/or one or more magnetic isolation elements, such as a transformer, discussed in additional detail below with reference to the appended drawing material.

A second aspect of the invention relates to a method of suppressing common mode signals of an isolation amplification circuit, wherein said method comprises steps of:

a) applying a first electrical signal to a first input node of a first filter network of an input stage circuitry of the isolation amplification circuit,

b) applying a second electrical signal to a second input node of a second filter network of the input stage circuitry,

c) generating a first output signal of the first filter network in response to the first electrical signal,

d) generating a second output signal of the second filter network in response to the second electrical signal,

e) lowpass filtering the first output signal to generate a first feedback signal,

f) lowpass filtering the second output signal to generate a second feedback signal,

g) applying the first feedback signal to a common node of the first filter network to close a first feedback loop around the first filter network; and

h) applying the second feedback signal to a common node of the second filter network to close a second feedback loop around the second filter network.

