GB1298685A - Automatic compensation of baseline drift - Google Patents

Abstract

1298685 Base line drift correction BODENSEEWERK PERKIN-ELMER & CO GmbH 12 Dec 1969 [14 Dec 1968] 60795/69 Heading G4H A circuit for correcting base line drift (used with a gas chromatograph or ionization detector) measures the drift between observed peaks and corrects it by applying a signal of opposite polarity to the input. The input Ue is fed into a Miller integrator 24, the output from which passes through two threshold devices K 1 and K 2 , set to operate if the signal is more positive or more negative than a certain value. From the threshold devices the signal passes to AND gates 34, 36, the other input 38 to the AND gates is controlled by the presence or absence of a peak at the input, so that the AND gates are switched off when a peak signal appears at the input. The AND gates 42, 40 have inputs from clock pulse generator 44 and AND gates 36, 34, the outputs leading to a bidirectional counter 50. If the input between peaks is not zero, clock pulses are counted in the forward or reverse direction by the counter. The output of the counter controls a D to A converter comprising voltage input Vk, transistor switches 52, 54, 56, 58, 60, and precision weighted resistors 62, 64, 66, 68, 70. The output of the D to A converter is fed to the input of the Miller integrator so as to oppose a non-zero signal. Thus a feedback loop is established which in equilibrium maintains the base line, i.e. the input signal between peaks at zero. When a peak appears at the input, the correction signal is determined by the value stored in counter 50 just prior to AND gates 36, 34 switching off. Voltage Vv is used to obtain a reading on the counter and hence operation of the feed back loop when there is zero base line drift. In another embodiment a binary decimal counter is employed and the polarity of the reference signal fed to the D to A converter is switched by a pair of relays according to the input polarity of the base line drift. In a third embodiment the D to A converter is of the potentiometer type.