Analysis of 50 HZ interference in the biopotential amplifier circuit and circuit design

Jun JI; Mengsun YU

ABSTRACT

BACKGROUND:

In some clinical applications or special work conditions,extremely intonse 50 Hz AC intefference exists.As the amplifier's output reaches or is close to saturation,it cannot be suppressed by general anti-interference circuits or software algorithms.Themfore,the hardware circuit must be impreved to suppress this interference,thus obtajning the desired signals.

OBJECTIVE:

To introduce a biopotential amplifier,which can effectively suppress the impact of 50 Hz AC interference on the weak biomedical signals.

DESIGN:

Create a circuit model based on the advanced theoretical analysis,then make software emulation and finally design the actual circuit implementation.SETTINGSthe 305 Hospital of Chinese PLAInstitute of Aviation Medicine,Air Force of CIIinese PLA.MATERIALSThe experimentaIinstruments and analytical software were provided by the Institute of Aviation Medicine,Air Force of Chinese PLAthe high-voltage electrical field environment was provided by the 305 Hospital of Chinese PLA.

METHODS:

The first step was theoretical analysis at the Institute of Aviation Medicine,Air Force of Chinese PLA in 2005,then the 50 Hz anti-reference model was created.After that,the following three anti-reference measures were put forward①Enhancing the common-mode input impedance of the overall circuit to remove the interference caused by the imbalance electrode impedance;②reducing the equivalent common mode input impedance of the instrument amplifier in the circuit by using a current source controlled by the comlnou mode voltage;③using the driven-right-leg circuit to reduce the common mode voltage.Moreover,emulation analysis was made for the above circuit model by using the circuit emulation software TINA PRO.Through ceaseless improvement of the circuit model,the correct emulation results were obtained.In 2006,the Circuit model helped us realize the electrocardiogram(ECG)amplifier that could suppress the 50 Hz AC intefference effectively in intense interference environments.And that amplifier was put into practical use in the high-voltage electrical field environment produced by the high-voltage electrical field treatment equipment in the 305 Hospital of PLA.The result was satisfyingthe 50 Hz AC interference was suppressed effectively and the high-quality ECG signals were obtained.MlAIN OUTCOME

MEASURES:

Common-Mode Rejection Ratio(CMRR)and the frequency spectrum of signals obtained in intense intefference environment,which are used to assess the ability against the 50 Hz AC interference.

RESULTS:

A 100 ù resistor or a 200 kù resistor were connected in series in the input loop so as to produce the imbalance electrode impedance status.When the imbalalice electrode impedance was 100 ù,the amplifier output was 1.69 Mv,and CMRR was 125.8 Db.When the imbalance electrode impedance was 200 k ù,the amplifier output was 1.78 Mv,and CMRR was 125.3 Db.Two voltage sources(20 Hz,amplitude of peak to peak value 50 ì V,opposite phases)were used to produce differential mode signals.The amplification gain was 1000 times.In the input loop of the circtilt mentioned above,a 200 kù resistor was connected in series to create the inabalance electrode impedance status.The amplifter output the peak-to-peak value 50 Mv.The fast Fourier transform(FFT)frequency spectrum of the output wave contained no 50 Hz spectrum line.

CONCLUSION:

Through the emulation experiment and the real application.we can all find that the amplifier has good signal amplification performance and anti-interference capabillty in intensive 50 Hz AC interference environments and severe imbalance electrode impedance Status.