AN864| Application Note
Maxim > App Notes > AMPLIFIER AND COMPARATOR CIRCUITS
DIGITAL POTENTIOMETERS
Keywords: EPOT Applications: Gain Adjustment in Op-Amp Circuits
Nov 28, 2001
APPLICATION NOTE 864
EPOT Applications: Gain Adjustment in Op-Amp Circuits
Variable gain amplifiers often use a mechanical potentiometer to set the gain. An example is a volume control dial. However, when the analog signal path is digitally controlled, a digital potentiometer might be used to control gain. This article discusses the use of the digital potentiometer to form digitally controlled gain or filter blocks.
General
Throughout this discussion of digital potentiometers, the term EPOT is used rather than digital pot, EEPOT or E POT. The term EPOT describes the Maxim series of volatile digital potentiometers, while the other terms describe a variety of non-volatile and volatile devices manufactured on one of several different processes. The specifics of how these different processes affect digital potentiometer performance are outside the scope of this article. While the equations given below can be applied to all digital potentiometer circuits, the special considerations relating to voltage ratings, current ratings are intended specifically for Maxim's EPOT devices. Even though the designer needs to adhere to the standard constraints associated with semiconductor devices, (i. e., absolute maximum ratings) there are a couple of characteristics specific to EPOTs that the engineer should keep in mind when using them in op-amp circuits. One of the first things to remember is that the tolerance of the end-to-end or absolute re
sistance of digital potentiometers is usually specified for a maximum of 25% to 30%. This is mainly due to process variations and a technique for handling this is discussed throughout this article. Additional things to consider are the characteristics of the EPOT's "wiper". The "wiper" in an EPOT is effectively a string of CMOS switches and will exhibit the typical characteristics. Important specifications for the wiper include its resistance, the variation of its resistance, its maximum current rating and its temperature coefficient. For example, the wiper resistance of the MAX5400 is specified for a typical value of 250 !& but with a maximum value of 800 !&. The absolute maximum current rating of the wiper (as well as the rest of resistor in the EPOT) is 1mA. Care should be taken not to exceed this current rating as excessive current can damage the device. Lastly, the temperature coefficient for the wiper is typically about 300ppm/ C. One final EPOT consideration is how it will behave near either extrem
e of the adjustment range. For example, when an EPOT is set to one of its higher or lower settings the ESD structures, such as wiper characteristics, can begin to drastically affect performance. A general rule of thumb is to try and keep the EPOT set to between 10% and 90% of full scale. In general, the principles described here apply to all of the EPOTs in the Maxim line. The 256 tap EPOT has been chosen for illustration purposes. Internally, an EPOT is viewed as a string of resistors with CMOS switches "tapping" off that string. In linear EPOTs the spacing along the resistor string is even, in logarithmic taper EPOTs, the spacing would be logarithmic. Therefore, an EPOT with a given number (m) of taps has one fewer resistors (m-1). For the purposes of discussion in this article, the variable "n" refers to the absolute setting of the EPOT (i. e., 0 to 31 or 0 to 255 and so on) and "N" refers to the ratio of the setting of the EPOT to its maximum value (i. e., 0 to 1). For example, in a linear taper EPOT wit
h 256 taps a setting of n=89 would give N=0.3490, which is 89/255. "Re" refers to the total end-to-end resistance of the EPOT, thus an EPOT can be thought of as 2 resistors with values N Re and (1-N) Re. Furthermore, gain equations are shown in this article as a function of the variable "N" as G(N). The total available range of adjustment is shown as G(0)<=G(N)<=G(1) or as G(1)<=G(N) <=G(0), as applies.
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