Howland current source with automatic gain feedback control

Abstract

There is provided a circuit comprising a mirrored enhanced Howland current source, the mirrored enhanced Howland current source comprising an input and an output; and a feedback loop, the feedback loop comprising: an instrumentation amplifier arranged to measure a current at the output of the mirrored enhanced Howland current source; a comparator arranged to compare an output of the instrumentation amplifier to a reference voltage source; an integrator electrically coupled to the output of the comparator; a multiplier arranged to multiply the output of the integrator with the reference voltage, wherein the output of the multiplier is electrically coupled to the input of the mirrored enhanced Howland current source.

Description

HOWLAND CURRENT SOURCE WITH AUTOMATIC GAIN FEEDBACK CONTROLFIELD OF THE INVENTION

[0001] The present disclosure relates to high bandwidth current sources with low error. In particular, the disclosure relates to adaptive current sources suitable for bioimpedance applications.BACKGROUND

[0002] Bioimpedance is a sensing method used in diagnosis for various medical applications, ranging from body composition assessment to detecting skin cancer. However, the method’s clinical utility and accuracy is hindered by output impedance issues in the current injection circuitry, resulting in limited bandwidth, particularly at low current amplitudes. While the bandwidth over which AC current sources exhibit less than 1% current amplitude error has been conventionally limited to several hundreds of kHz.

[0003] There is therefore a need for a high bandwidth, low error, adaptive current source suitable for bioimpedance applications.SUMMARY OF THE INVENTION

[0004] There is provided a circuit comprising: a mirrored enhanced Howland current source, the mirrored enhanced Howland current source comprising an input and an output; and a feedback loop, the feedback loop comprising: an instrumentation amplifier arranged to measure a current at the output of the mirrored enhanced Howland current source; a comparator arranged to compare an output of the instrumentation amplifier to a reference voltage source; an integrator electrically coupled to the output of the comparator; a multiplier arranged to multiply the output of the integrator with the reference voltage, wherein the output of the multiplier is electrically coupled to the input of the mirrored enhanced Howland current source.

[0005] There is further provided a method of forming a circuit, the method comprising: electrically connecting an input of an instrumentation amplifier to an output of a mirrored enhanced Howland current source; electrically connecting an output of the instrumentation amplifier to a first input of a comparator; electrically connecting a reference voltage source to a second input of the comparator; electrically connecting an output of the comparator to an input of an integrator; electrically connecting an output of the integrator to a first input of a multiplier; electrically connecting the voltage reference source to a second input of themultiplier; electrically connecting an output of the multiplier to an input of the mirrored enhanced Howland current source.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. In the drawings, like reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.

[0007] Figure 1 shows a block diagram of an Adaptive Howland Current Source, in accordance with examples and embodiments of the present disclosure.

[0008] Figure 2 shows a circuit diagram of an implementation of an Adaptive Howland Current Source, according to examples and embodiments of the present disclosure.

[0009] Figure 3 shows a block diagram of a self-correcting current source, according to examples and embodiments of the present disclosure.DETAILED DESCRIPTION

[0010] In the following description, for purposes of explanation, numerous specific details of certain examples are set forth. Reference in the specification to “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the example is included in at least that example, but not necessarily in other examples.

[0011] Bioimpedance techniques have been adopted as a valuable diagnostic tool for the detection of various diseases. One of its most common applications is in the estimation of body composition, where it can provide important indicators of various diseases such as obesity, malnutrition, and muscle wasting by assessing body fat percentage, muscle mass, and cell water content. In addition, bioimpedance has been used to detect venous thrombosis, a condition characterized by the formation of blood clots in the veins, by measuring changes in the electrical impedance of chest, veins, and other parts of the body. It has also been utilized for the non-invasive detection of skin cancer, by detecting changes in tissue conductivity and cell membrane integrity at the tumour site.

[0012] In the diagnosis and monitoring of neuromuscular diseases, electrical impedance myography (EIM) has been used to assess muscle health by measuring the electrical impedance characteristics of muscle groups. Furthermore, bioimpedance has been employed in the detection of other diseases such as kidney disease, heart failure, and diabetes, by measuring changes in body fluid volumes. Overall, bioimpedance has the potential to improvedisease detection and diagnosis by providing a non-invasive, convenient, and cost-effective method for assessing various physiological parameters.

[0013] Despite its numerous advantages, the clinical utility of bioimpedance technology has been limited by instrumentation challenges, which can be categorized into injection side and measurement side instrumentation. One of the major limitations of injection side instrumentation is the limited bandwidth of current sources, particularly at low current amplitudes, due to the degradation of output impedance. This limitation presents a significant challenge towards the widespread implementation of bioimpedance for disease diagnosis, as wider bandwidth of scans allows for deep penetration of tissue components, providing a more complete picture of disease target areas and stages, and thus increasing accuracy.

[0014] A wider bandwidth of scan can greatly improve the resolution and sensitivity of bioimpedance, allowing for more accurate differentiation between different tissue types and structures, such as fat, muscle, and bone during diagnosis. While efforts have been made to improve the limited bandwidth of current bioimpedance instrumentation, most improvements thus far have been limited to fixed frequencies with no flexibility for swept / multi-frequency applications. This limitation has resulted in current state-of-the-art bioimpedance systems having less than 1MHz bandwidth in the microampere amplitude range, as higher frequencies suffer from poor signal-to-noise ratios.

[0015] The present disclosure provides a current source suitable for bioimpedance systems, said current source having an automatic gain-controlled feedback topology. In some implementations, the current source is capable of delivering 100pA to a bandwidth of up to 3MHz, which represents a minimum three-fold improvement over existing implementations and aids the clinical usability of bioimpedance in deep tissue scan and analysis, such as in the diagnosis of skin cancer, neuromuscular disease assessments, and other medical conditions. With this advancement in bandwidth, bioimpedance measurement can provide benefits in mammography, in the diagnosis and detection of breast cancer, skin cancer biopsies, and other vital medical applications.

[0016] Bioelectrical impedance or bioimpedance measurements (BIM) involve the injection of very low alternating electric current into a biological tissue sample and the direct derivation of the sample’s impedance from the measurement of the resulting voltage. This technique has been established across a wide range of biomedical applications ranging from lung disease diagnosis and detection (Electrical impedance tomography), cancerous tissue characterisation and segregation, neuromuscular disease assessment, cardiac outputs monitoring, body composition analysis, and in food quality assessment.

[0017] The accuracy of bioimpedance measurements depends to a great extent on the accuracy of the front-end instrumentation. This can be categorised into two stages, comprising the current injection and voltage measurement circuitry. The measurement instrumentation stage design can vary depending on the bioimpedance topology chosen, i.e. synchronous detection, synchronous sampling, or magnitude and phase. On the other hand, the design of the current injection instrumentation usually involves an AC current source comprising an AC voltage source connected to a transconductance amplifier, also referred to as a voltage-to- current converter. For accurate BIM, the current source needs to be capable of delivering a constant amplitude (typically with less than 1 % variability) AC current over a wide range of loads, typically hundreds of Q to tens of kQ - corresponding to respective injected currents of a few mA down to tens of pA (sub-mA). Desired bandwidths range from 1 kHz to 1 MHz in average, however nonidealities degrade the output impedance of current sources, notably reducing their frequency range thus limiting their applicability.

[0018] With the exception of integrated circuit (ASIC) realisations, the majority of bioimpedance circuitry makes use of the Howland Current Source (HCS), which is a voltage controlled current source (VCCS) topology. Variants of the HCS include fully differential and enhanced accuracy designs. The Mirrored Enhanced Howland Current Source (MEHCS) is widely considered the best performing topology due to its ability to provide a true differential injection through its floating output terminals and also due to its overall simplicity and robustness relative to other designs. Still, like all open-loop current sources, MEHCS also suffers from drawbacks, including mismatches in the feedback resistors; limitations due to the operational amplifiers used; stray capacitances in tracks and cables; and other issues degrading its output impedance and causing more than 1% current amplitude degradation for bandwidths over a few hundreds of kHz.

[0019] To overcome this barrier, the present disclosure provides a MEHCS-based automatic gain control (AGC) design offering a high bandwidth adaptive current source achieving significantly low amplitude error. We term this the “Adaptive Howland Current Source” (AHCS). This design adjusts the gain of the current driver automatically, to maintain the current output's accuracy, regardless of any possible degradation in the output impedance.

[0020] The disclosed current source designs of the present disclosure can be used as an input to any circuit that makes use of a current source, in order to improve the performance of said circuits. For example, the MEHCS stage of the disclosed examples could be replaced by any current source capable of generating an AC current from a voltage input. Similarly, the automatic gain control (AGC) stage of the presently disclosed circuits (e.g. elements 110, 130, 140, 150, 160, 170 of Figure 1) can be applied to any source in order to increase the bandwidth of said source.

[0021] Figure 1 shows a block diagram of an Adaptive Howland Current Source, according to examples and embodiments of the present disclosure. With switches Si (102) on and switches S2 (104, 106) off, the system operates as an open-loop MEHCS 120 with a transconductance gmconverting the input voltage Vinto output current lout. Once the automatic gain control (AGC) stage is on (s1 (102) off; s2 (104, 106) on), the output current lout of the MEHCS 120 is continuously monitored through a transimpedance stage with gain of 1 / gm, at instrumentation amplifier 130. Peak amplitudes of Vsense and Vin are obtained via peak detectors 140 and 150 respectively. The peak amplitude of the transimpedance stage’s 130 output Vsense is then compared with the amplitude of Vm through a comparator 160 whose output assumes a “high” or a “low" state depending on which of the inputs is higher. Being a bistable, the comparator 160 generates a high frequency oscillatory output when its inputs are equal. An integrator 170 translates either settled comparator output to a positive or a negative ramp whilst high frequency oscillatory inputs result to an effectively zero variation DC around the last ramping value. The integrator 170 output controls the gain of a variable gain amplifier (VGA) 110 whose initial gain is unity and its output Vtrack will be increased or decreased when lout decreases or increases respectively. In that way, the MEHCS 120 input voltage amplitude (now Vtrack) increases when the current amplitude drops, thus effectively compensating for the leakage current “lost” through the output impedance as it decays at higher frequencies rather than attempting to increase the output impedance itself at specific frequencies as the aforementioned compensation techniques.

[0022] Figure 2 shows a circuit diagram of an implementation of an Adaptive Howland Current Source, according to examples and embodiments of the present disclosure. In the implementation of the AHCS circuit in Fig.2, the MEHCS stage is comprised of op-amps U9 and U10, and resistors Rhito Rhs, with its input driven by a unity gain single to differential amplifier (U8) in order to increase common mode rejection, reduce DC offsets and thus enhance load capability.

[0023] In an open loop configuration (swi set to Vm), the output current lout, flowing through Rsense and then through the load ZL, is given by: lout=gm Vjn where gmis the transconductance of the MEHCS, given by:Qm=Rh4 / (Rh3*Rh5)

[0024] In some examples, two Rsense resistors are used in order to maintain a balanced output current.

[0025] With the feedback on (swi set to Vtrack), the output current is continuously monitored via sensing resistor Rsense and the sensing instrumentation amplifier U5, with gain Av-Sense, designed so that its output equals the MHCS input voltage by making:Av_sense=1 / (Qm* Rsense)

[0026] The output current becomes lout_track, derived by replacing Vinwith Vtrack in the equation for calculating the output current flowing through Rsense. Vtrack is the output of the AGC stage, the AGC stage being comprised of two precision peak detectors (a first peak detector comprising U1 , U2, Rri, Rr2, Rr3, CM, Cr2, Dri, Dr2; and a second peak detector comprising U3, U4, Rri, Rr2, Rr3, Cri, Cr2, Dr3, Dr4); a comparator (U6); an integrator (Rint, Cmt), and a multiplier (U7), the multiplier operable to multiply the output of the integrator with the reference voltage.

[0027] Vtrack is given by:Vtrack=Vjn (1 +Vjnt) where Vmt is the integrator output.

[0028] The AGC continuously monitors and compares the peak values of both Vinand the Vsense. If the amplitude of lout_track - and consequently the value of Vsense - starts dropping, the comparator output goes high, causing the integrator output to ramp up, effectively increasing Vtrack. Once this feedback brings Vsense to the same value as Vm, the comparator starts oscillating causing Vmt to stabilise to a settled value.

[0029] Optionally, the circuit may comprise a VGA (U7) implemented by means of an AD735 multiplier, with base gain set to unity through a 1 V DC reference. This avoids errors due to the case of multiplying by zero where the output of the integrator is 0V, and thus maintain the stability of the system. The multiplier may be a four-quadrant multiplier. In examples, the comparator U6 may optionally comprise Schmitt trigger.

[0030] For the purposes of bioimpedance measurement, an instrumentation amplifier U11 may be used to measure the current through a load ZL.

[0031] Figure 3 shows a block diagram of a self-correcting current source, according to examples and embodiments of the present disclosure. The self-correcting current source of Figure 3 may be considered a more generic version of the Adaptive Howland Source of Figure 1 . The system may be connected to any transconductance-type AC current source to boost the performance of said current source, significantly increasing the operating frequency bandwidth of the AC current source. The system operates via an automatic gain control (AGC) stage (the AGC stage comprising elements 310, 340, 360, 370) which features a Variable Gain stage 310 that interjects between the voltage Vinthat drives the AC current source (i.e. the transconductance stage 320) which in turn generates the output current lout and thetransconductance stage 320 input. The AGC stage also connects to the output of the transconductance current source so as to monitor the output current lout continuously. The amplitude of the output current lout is determined at an amplitude detection stage 340, and then compared, at a comparison stage 360, to a fixed pre-programmed reference value which allows for an appropriate level adjustment of a signal that controls a variable gain stage (VGA) 310 whose initial gain is unity. The level of the control signal may be adjusted, at a level adjustment stage 370, so as to increase or decrease when lout decreases or increases, respectively. In that way, the amplitude of the Transconductance Stage 320 input voltage increases when the current amplitude drops, thus effectively compensating for the leakage current “lost” through the output impedance as it decays at higher frequencies, rather than attempting to increase the output impedance itself at specific frequencies as in the commonly applied compensation techniques.

[0032] According to examples and embodiments of the present disclosure, there is provided a circuit comprising: a mirrored enhanced Howland current source, the mirrored enhanced Howland current source comprising an input and an output; and a feedback loop, the feedback loop comprising: an instrumentation amplifier arranged to measure a current at the output of the mirrored enhanced Howland current source; a comparator arranged to compare an output of the instrumentation amplifier to a reference voltage source; an integrator electrically coupled to the output of the comparator; a multiplier arranged to multiply the output of the integrator with the reference voltage, wherein the output of the multiplier is electrically coupled to the input of the mirrored enhanced Howland current source.

[0033] Some examples or embodiments further comprise a peak detector electrically coupled between the output of the instrumentation amplifier and a first input of the comparator.

[0034] Some examples or embodiments further comprise a peak detector electrically coupled between the reference voltage source and a second input of the comparator.

[0035] In some examples or embodiments the comparator further comprises a Schmitt trigger.

[0036] In some examples or embodiments the multiplier is a four-quadrant multiplier.

[0037] In some examples or embodiments the multiplier is arranged to apply an offset voltage to the output of the integrator.

[0038] In some examples or embodiments the input of the mirrored enhanced Howland current source comprises a single-to-differential amplifier.

[0039] According to further examples and embodiments of the present disclosure, there is provided a method of forming a circuit, the method comprising: electrically connecting an input of an instrumentation amplifier to an output of a mirrored enhanced Howland current source;electrically connecting an output of the instrumentation amplifier to a first input of a comparator; electrically connecting a reference voltage source to a second input of the comparator; electrically connecting an output of the comparator to an input of an integrator; electrically connecting an output of the integrator to a first input of a multiplier; electrically connecting the voltage reference source to a second input of the multiplier; electrically connecting an output of the multiplier to an input of the mirrored enhanced Howland current source.

[0040] In some examples or embodiments the output of the instrumentation amplifier is connected to the first input of the comparator via a peak detector.

[0041] In some examples or embodiments the reference voltage source is connected to the second input of the comparator via a peak detector.

[0042] In some examples or embodiments the comparator further comprises a Schmitt trigger.

[0043] In some examples or embodiments the multiplier is a four-quadrant multiplier.

[0044] In some examples or embodiments the multiplier is arranged to apply an offset voltage to the output of the integrator.

[0045] Some examples or embodiments further comprise electrically connecting a single-to- differential amplifier between the output of the multiplier and the input of the mirrored enhanced Howland current source.

[0046] According to further examples and embodiments of the present disclosure, there is provided a current injection device for a bioimpedance sensor, comprising: the circuit of any of the examples or embodiments provided herein.

[0047] All of the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be combined in any combination, except combinations where some features are mutually exclusive. Each feature disclosed in this specification, including any accompanying claims, abstract, and drawings, may be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed in one example of a generic series of equivalent or similar features.

[0048] The present teachings are not restricted to the details of any of the foregoing examples. Any novel combination of the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be envisaged. The claims should not be construed to cover merely the foregoing examples, but also any variants which fall within the scope of the claims.

Claims

CLAIMS1. A circuit comprising: a mirrored enhanced Howland current source, the mirrored enhanced Howland current source comprising an input and an output; and a feedback loop, the feedback loop comprising: an instrumentation amplifier arranged to measure a current at the output of the mirrored enhanced Howland current source; a comparator arranged to compare an output of the instrumentation amplifier to a reference voltage source; an integrator electrically coupled to the output of the comparator; a multiplier arranged to multiply the output of the integrator with the reference voltage, wherein the output of the multiplier is electrically coupled to the input of the mirrored enhanced Howland current source.

2. The circuit of claim 1 further comprising a peak detector electrically coupled between the output of the instrumentation amplifier and a first input of the comparator.

3. The circuit of claim 1 further comprising a peak detector electrically coupled between the reference voltage source and a second input of the comparator.

4. The circuit of claim 1 wherein the comparator further comprises a Schmitt trigger.

5. The circuit of claim 1 wherein the multiplier is a four-quadrant multiplier.

6. The circuit of claim 1 wherein the multiplier is arranged to apply an offset voltage to the output of the integrator.

7. The circuit of claim 1 wherein the input of the mirrored enhanced Howland current source comprises a single-to-differential amplifier.

8. A method of forming a circuit, the method comprising: electrically connecting an input of an instrumentation amplifier to an output of a mirrored enhanced Howland current source; electrically connecting an output of the instrumentation amplifier to a first input of a comparator;electrically connecting a reference voltage source to a second input of the comparator; electrically connecting an output of the comparator to an input of an integrator; electrically connecting an output of the integrator to a first input of a multiplier; electrically connecting the voltage reference source to a second input of the multiplier; electrically connecting an output of the multiplier to an input of the mirrored enhanced Howland current source.

9. The method of claim 8 wherein the output of the instrumentation amplifier is connected to the first input of the comparator via a peak detector.

10. The method of claim 8 wherein the reference voltage source is connected to the second input of the comparator via a peak detector.

11. The method of claim 8 wherein the comparator further comprises a Schmitt trigger.

12. The method of claim 8 wherein the multiplier is a four-quadrant multiplier.

13. The method of claim 8 wherein the multiplier is arranged to apply an offset voltage to the output of the integrator.

14. The method of claim 8 further comprising electrically connecting a single-to- differential amplifier between the output of the multiplier and the input of the mirrored enhanced Howland current source.

15. A current injection device for a bioimpedance sensor, comprising: the circuit of any of claims 1 to 7.

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