Substitutes for the LTC1150 and LT6020 Op Amps
Chapter 7 introduces departures from the ideal operational amplifiers discussed in Chapter 6. In particular, our previous claim that the op amp’s inputs draw no current and that the output of the op amp does whatever is necessary to make the voltage difference between the two inputs zero is not quite true. A small amount of “bias current” flows either in or out of the + and – inputs (the magnitude and direction depends on the type of transistors used in the differential input stage) and the op amp output tries to make the difference between the inputs equal to a small “input offset voltage” (this is a result of the inevitable tiny mismatch between the differential input transistors). For modern op amps like the “jellybean” LF411 and LM358, the bias current and offset voltage are small enough that they can often be ignored. However, there are applications where you need to select an op amp with better specifications to make your circuit work. The LT6020 and LTC1150 devices are used in Lab 7 because they have much smaller input offset voltage specifications than the jellybean parts. Unlike the other substitute pages for hard-to-find parts, these two items are still readily available but are expensive. Here are some alternatives at lower cost.
Substitutes for the LT6020 op amp
The LT6020 op amp used in Lab 7L.4.3.1 and 12L.5.1 can and should be replaced with the new MAX74810ARMZ which is both better and cheaper ($4.16 vs $9.27) than the LT6020. Like the LT6020, the MAX74810 is a dual op amp and has the same pinout as the LM358. And like the LT6020, is is only available in a 8-pin MSOP package, which is quite small, and must be soldered into a DIP adapter. (This adapter is designed for an SOIC-8 on one side and the MSOP-8 on the other. While the SOIC package is relatively easy to manually solder by hand if you are careful, the MSOP is much more difficult – search online for videos showing how to do it so you don’t short out the pins.) Because the MAX74810 is cheaper, has better specifications, and uses the same DIP adapter as the LT6020, there is no longer a reason to buy the latter.
Substitutes for the LTC1150 op amp
Lab 7L.1 has you measure the bias current and offset voltage of the LF411 op amp, then asks you compare it to a more expensive auto-zeroing (“chopper-stabilized”) LTC1150 op amp. (See AoE §5.11 for an explanation of chopper stabilization.) The LT1150 is also used in the following Lab 7 experiment to build a drive-motor positioning sensor circuit. The LTC1150 has the same pinout as the LF411 and a typical offset voltage of 0.5uV (compared to 800uV for the LF411). Unfortunately, this orders of magnitude improvement comes at a cost, $12.52 in quantity 1.
You could save about half that by using the LT1012D op amp ($6.73 at Digikey). While not as impressive as the LTC1150, the typical input offset voltage of the LT1012 is 12uV, still much better than the LF411, uses the same pinout, and comes in an 8-pin DIP . If you are willing to solder an SOIC-8 part onto a DIP adapter, the ADA4638-1ARZ op amp ($5.66 at Digikey) offers the same offset voltage as the LTC1150 (0.5uV typical) and uses the same pinout at half the price. (We have not yet tested it but we will let you know when we do.)
Finally, you could use the MAX74810ARMZ recommended as a substitute for the LT6020 discussed above. The MAX 74810 has similar specifications to the LTC1150, claiming 0.7uV typical offset voltage, but is only $4.16 in single quantity. Its pinout is different (it is a dual op amp with the pinout of the LM358) so you would have to modify your LF411 breadboard configuration to measure its bias current and offset voltage. It is in the tiny MSOP-8 package and requires a DIP adapter board. However, not only is it much less expensive than the LTC1050, but you would then only need to buy one low-offset op amp to do all the experiments in Chapters 7 and 12.