MSP430 Applicationin the medical equipment industry (5)
source: icbuy million core time: 2012-04-05-
Recovery of
ADC amplifying circuit and
As the electrode potential is very high, the actual can reach 500mV, so by AC coupling to eliminate the influence of electrode potential is very important. The design of a feedback configuration ADC recovery amplifier can eliminate the DC bias. When the output voltage of INA326 increases, feedback integrator will exert a negative voltage equal to INA326 reference end, through such linear summation, cancellation can be achieved by electrode biasing. DC restorer is the role of the DC coupling amplifier originally into AC coupled amplifier, with the elimination of DC voltage inherent mode, the output stage can amplify AC ECG signal without saturation.
driven-right-leg circuit
50Hz interference is the most important measure of ECG interference, high common mode rejection ratio on the preamplifier is not sufficient to completely inhibit, good driven-right-leg circuit can improve the anti-interference ability of the circuit, and the body and amplifying circuit to form a closed loop, to ensure the normal work of ECG amplifier circuit.
Since different endpoints of the human body, left arm and the right arm of the ECG signal at different levels, and shall be INA326 amplifier noise, in order to better inhibition of 50Hz, the common-mode voltage is obtained by INA326 is used for inverting common-mode signal, and the 2.5V reference voltage comparison by operational amplifier OPA2335 drive to human right, only a small amplifier can be a small amount of the common mode effectively improve. This method can not only suppress 50Hz interference can also according to the ECG signal adjusting human body level, convenient sampling ADC. Driven-right-leg circuit as shown in Figure 3, where COM is the common-mode output signal amplifier INA326, while in the former also adds a voltage follower for isolation.
notch circuit
Although the preamplifier circuit has strong inhibitory effect on the common mode interference, but some frequency interference is to enter the circuit for differential signal mode, and the frequency is within the frequency band of ECG signal, plus the electrode and the input loop instability and other factors, the ECG former circuit output still exist in the power interference of strong, so often to notch. The design of a 8 order Butterworth 50Hz notch filter based on LTC1068-50 Integrated Switched Capacitor Filter Technology Co. Ltd., the United States of America, by testing the notch depth of up to 50dB, can be attenuated by about 100 times, the ideal effect.
