Amplifiers with DC Offset Cancellation

Abstract

The invention discloses an amplifier eliminating direct current offsets. The amplifier eliminating the direct current offsets comprises a pre-amplifier circuit, a direct current extracting circuit and an isolating circuit, wherein the direct current extracting circuit is used for extracting direct current component signals output by the pre-amplifier circuit, and the isolating circuit is used for generating an electrical current which is identical in size and opposite in direction with direct current offset currents generated by direct current offset voltages carried by two signals to be measured in the pre-amplifier circuit and feeding back and inputting the electrical current to the pre-amplifier circuit according to the received direct current component signals of the direct current extracting circuit. The amplifier sends signals output by a three-operational-amplifier output end to the direct current extracting circuit, extracts the direct current component signals output by the pre-amplifier circuit, and converts direct current components to analogue signals and outputs the analogue signals to the isolating circuit, the isolating circuit converts input voltages to constant flow sources and feeds back to an input end of the pre-amplifier circuit in an inverse mode, and therefore dynamic elimination of the direct current offsets is achieved.

Description

technical field [0001] The invention belongs to the technical field of circuit design and relates to the design of an amplifying circuit, in particular to the design of a three-operational preamplifier capable of dynamically eliminating the DC offset voltage of the operational amplifier. Background technique [0002] In the acquisition of electrophysiological signals, it is usually interfered by various noises around, especially the interference of common-mode signals. Under the same process conditions, the way to further improve the common-mode rejection ratio is to increase the amplification of the differential-mode voltage of the preamplifier multiple, but the presence of DC offset voltage in the electrophysiological signal limits the differential-mode voltage amplification of the preamplifier, so it is very difficult to further improve the common-mode rejection ratio of the preamplifier without eliminating the DC bias voltage of. [0003] Enrique M et al. proposed an AC...

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Problems solved by technology

[0004] But this scheme has the following problems: 1. The use of the integral circuit is inappropriate. The capacitor Ci in the integrated circuit used has no resistance to form a loop with it, which is easy to cause the amplifier to be saturated. At this time, the maximum DC signal is output, not only no Removing the DC bias voltage will instead generate a new DC voltage offset, which may cause the saturation of the entire operational amplifier circuit in severe cases; 2. The cut-off frequency of the integrating circuit cannot be made very low due to hardware limitations, so the signal it extracts includes DC signals and certain components of AC signals, when these signals are superimposed on the input of the three operational amplifier circuits, these low-frequency AC signals will be superimposed on the useful source signal, because these low-frequency signals are contained in the useful signal frequency band It is difficult to be filtered out within the range, so a new source of interference is introduced, which seriously affects the accuracy of the signal; 3. The selected optocoupler device is a nonlinear device, and there is no current and offset generated by the optocoupler device. In the case of corresponding correction of the current generated by the voltage, the current generated by the optocoupler device may be inconsistent with the current generated by the offset voltage at the input end, so that the DC offset voltage cannot be eliminated or even a new voltage offset may be introduced

Because of the introduction of capacitors in this solution, due to the low matching degree of the capacitors, it is easy to convert the common-mode signal into a differential-mode interference signal, thereby introducing new interference, and the capacitance of the capacitor required to filter out the DC voltage through the capacitor is very large. Big, difficult to implement

[0006] On the issue of eliminating the DC bias voltage, CN102185812A eliminates the offset voltage by extracting the DC signal and then subtracting it from the original signal. Although this solution is relatively simple, in the three-operation differential preamplifier circuit, because the circuit is a differential input, If this scheme is adopted, the symmetry of the circuit will be destroyed, and the common mode rejection ratio of the circuit cannot be improved.

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