Designers sometimes find op amp datasheet specifications confusing because not all performance characteristics have minimum or maximum specifications. Sometimes, you have to use the “Typical Values” from a specification sheet or a typical performance chart. But what exactly does this “typical value” mean? How much does it vary?
It is not easy to answer this question, it depends on the specific specification. Below, we explain one by one the 3 features that are easy to cause doubts:
bandwidth——The gain-bandwidth product (GBW) of an op amp is primarily controlled by the input stage current and the value of the on-chip capacitance. Changes in these two variables can produce GBW changes in the range of ±20%. This may seem like a relatively wide range, but by choosing a large headroom op amp, large-range GBW designs can be made much easier. If necessary, some feedback components can be used to control the closed-loop bandwidth of your application. Note that on the open-loop gain/phase diagram (see Figure 1), this change appears to be very small.
conversion rateAffected by the same variables such as bandwidth, internal current, and capacitance. Typically, it is sufficient to choose an op amp that is 20% faster than the minimum required speed. Perhaps you want more headroom in some critical applications. Most applications don’t push the amplifier near its slew rate limit, so it’s fine to do so.
voltage noise——The broadband or flat-band voltage noise of an amplifier depends primarily on the currents of one or more input stage transistors. High current reduces noise in a square root fashion. So a 20% change in current results in about a 10% change in flat-band noise density (see Figure 2).
Low frequency 1/f noise (also known as flicker noise) is a different matter, with a wider range of variations. The noise amplitude in the 1/f region varies in the range of about 3:1. The difference between the JFET and CMOS fabrication processes may be slightly larger. This noise region determines the peak-to-peak noise level in the low frequency band (usually specified as 0.1 to 10 Hz).
There are some good guidelines, but it’s impossible to specify the exact range of variation in amplifier design and IC process used. However, some data is better than none, and most designs can accommodate these estimated differences well.
The margin appropriate for your application may vary depending on the type of equipment you are designing (and possibly the end product testing you are doing). Margins that are out of specification can affect the target margins you design for. This “engineering judgment” is an important factor in good analog design.
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