From Altimeter To Variometer

EE JULY/AUGUST 1988 06 FROM ALTIMETER TO VARIOMETER The altimeter published some 18 months ago 0) can be adapted to func- tion as a variometer by the following cir- cuit. The difficulty in the design of the circuit is, of course, that it has to work with very small input voltages. It is based on the fact that differentiating the absolute height gives as result the rate of change of altitude. In the diagram, IC: is the differentiator that operates with a time constant, RIC:, of 1 s. Since this type of differentiator in- verts, it is followed by an inverter. If the amplification is arranged at 60 (P) set to 60 k), the eventual read-out shows the rate of change of altitude in m/min, assuming, of course, that the altimeter has been calibrated as prescribed in Ref. 1. Because of the very low levels of signal input, the choice of components is critical. For instance, C: must be an MKT, not an electrolytic, type. The dif- ferentiator is a CMOS opamp that not only has a very high input im...