on to the microelectronic signal processing element or indirectly via a bonding post there are various ways in which the appropriate current to voltage and impedance conversion may be effected using for example a dual differential transistor pair or an operational amplifier as will readily be appreciated by those skilled in the art the technique and circuit elements described hereinafter are chosen as illustrative examples only and not intended to define or impose limitations on the way in which signal processing is carried out rapid progress in microelectronic technology will undoubtedly generate more sophisticated semiconductor elements suitable for this application and we do not exclude any of these from the scope of the present invention for descriptive purposes we have chosen to illustrate the principle of operation and mode of fabrication of i c a s devices employing an operational amplifier 3 in current to voltage conversion mode fig1 as the signal processing element as will be readily appreciated by those skilled in the art such that the input current i from current source 1 feedback resistor r 2 and output voltage e o exhibit the relationship e o i u00d7 r however the currents generated by amperometric sensing electrodes are small typically several nanoamperes for po 2 150 mmhg which necessitates the.