THE QUASI BALANCED CONDUCTOMETRIC BRIDGE FOR BIOSENSOR SYSTEM WITH BALANCING MODULUS AND PHASE
В. Г. Мельник, О. В. Сліцький, О. Д. Василенко · Sensor Electronics and Microsystem Technologies · 2016
The AC bridge for differential conductometric biosensor system is discussed, which may be installed in a quasi-balanced state with a non-zero output, but balanced for common mode noise on the conductometric transducers even if they are not identical. Such interference may occur due to changes in the background conductivity of the test solution in the measurement process and significantly distort the results. The bridge is built by the compensation scheme with the comparison of currents in the two branches, the first of which includes the working transducer, and the second includes the reference one. The balancing of the bridge is done by the module and the phase difference between the currents in the transducers. It is implemented using the two-phase generator of test voltage on the transducers, which forms the two quasi-sinusoidal voltages from one reference voltage. Phase shift between them is adjustable. The basis of these circuits is the Johnson’s ring counter with DAC on the precision resistors. The difference of the test signal’s phase, which is necessary for balancing of a bridge, may be set with using delay of the synchronizing pulse of a ring counter by a counter with the preset, and the ratio of the amplitudes is regulated by the DAC. In the process of balancing, which is analyzed by using the vector model, is determined the phase angle difference of the conductometric transducers and achieved the equality of voltages on the active components of the impedances of their sequential equivalent RC-circuit. Thereafter, the additional change of the test voltage phase angle on the reference converter on the angle equal to the difference phase angle of the transducers is done for moving the bridge to the quasi-equilibrium state. Correctness and high precision complying with the necessary phase relations in the formation of test signals and the regulation of their phase in the described two-phase generator are confirmed by represented timing diagrams.