Using the op amp and analog switch capabilities of this CMOS technology, Intersil introduced in early 1979 a new approach to the low offset voltage requirement, the Commutating Auto-Zero or C-AZ amp,shown in Figure 7. However, although these devices have solved many traditional op amp problems.input offset voltage and low frequency noise voltage were not among them. More recently, CMOS technology has moved into the more traditional building blocks of analog circuits, so that now CMOS versions of the standard bipolar op amps,regulators, and timers are available, with comparable or better specifications, lower power dissipation, and close to competitive pricing (Figures 4-6). Most spectacular, perhaps, has been its rapid dominance of the A/D and D/A converter market (Figures 2 and 3).Today very few converter systems are being designed that don't use CMOS devices specifically intended for this purpose, and in most cases they provide virtually the whole function. In the last few years, a new technology, in the shape of CMOS, has entered the analog field, and has led to the introduction of a range of products previously only dreamed of. In addition, the requisite screening of parts is expensive, even with currently available levels of automation. Although this is over an order of magnitude better than a good grade ofμA741 or LM101A,there is still much room for, but little realistic hope of, substantial further improvement in this direction. Careful die layout and circuit balance, in many cases combined with internal offset null trimming,bring initial offset voltages under 100μV, and temperature drifts below 0.5μV/☌. Therefore, considerable effort has been expended to improve the offset and drift characteristics of standard op amp devices, and some very good results have been achieved with several bipolar input devices, such as the OP-05 and OP-07. The only monolithic chopper-stabilized devices previously available are probably best described as disappointing and expensive. The devices are typically bulky and expensive, and the two-path approach frequently used (Figure 1) tends to adversely affect settling times the high-speed path and the low-speed path will settle to different points unless the pole-zero pairs are extremely well matched.
The answers to this concern have been many and varied.Several modules use chopper stabilization to provide very low offset voltages, although most of these do not provide differential inputs and they also have problems with input frequencies near the chopping rate (see Intermodulation Effects). If variation with common-mode voltage, power supply voltage, etc., is covered, it is generally only in a “typical curve” buried in the middle of the data sheet.
Contrast this with the treatment afforded one other important (error-causing) input parameter, input bias current,which usually gets just a specified value under one set of conditions, a variation over temperature, and a term relating to its matching between the two inputs.
#OP AMP OFFSET NULL ADJUSTMENT SERIES#
Also consider the extensive series of specifications devoted to its variability with temperature, time, common-mode voltage (CMRR), power supply (PSRR), output voltage (A-VOL), and sometimes even down to variation of temperature drift with offset null correction. This is indicated by the fact that almost all important op amps from theμA741and LM101 on have offered offset null adjustment pins, special screening to low offset voltage values, and/or internal V-OS trimming (laser or Zener-zap). Historically, the biggest single problem with the application of op amps has been the input offset voltage.