Development of electric scheme for 4-channel low noise rail-to-rail operational amplifier aRD824 based on AD824 prototype

Analog Devices chip AD824 is a low-power single polarity Rail to Rail in exit 4-channel operational amplifier with n-channel FET transistors at the input. Its characteristics allow to use them in various high ohmic detectors for photonics and medicine. The task was to construct an operational amplifier aRD824 that has similar performance characteristics of AD824. For this, AD824 chip and its electric scheme were analyzed, and some of its modules were produced and tested. It appeared that the production line of the producer “Integral” could not produce well FET transistors and thin layer resistors that are used in AD824, which resulted in a high percentage of damaged chips. For this, the modification of electric schemes for various modules of AD824 was proposed. Thin film resistors were proposed to be substituted by ion implantation or diffusion resistors. The input stage module got additional source repeaters. The second stage module was modified to be more symmetric. The output stage module obtained additional resistors and capacitors to achieve a frequency compensation. One FET transistor in the current reference module was substituted by other elements. The performance of modified electric schemes of modules was tested in PSpice software. Simulations of the full electric scheme for aRD824 were made and showed that it demonstrates similar characteristics as AD824 in frequency-gain and signal response tests. The targeted characteristics for aRD824 chip include low voltage noise < 4 μV for 0.1Hz to 10 Hz input, low input bias current < 15 pA, and offset voltage < 0.5 mV.