application note 1 of 4 www.xicor.com july, 2002 AN50 op amp gain and offset trim using xicor digitally controlled potentiotiometers (xdcps) by applications staff many system con?urations require multiple trans- ducers to be normalized for gain or offset, prior to further a to d conversion or signal processing. the xicor x9241a quad xdcps can provide an elegant and ef? cient solution for performing these tasks under microprocessor control. the con?uration in figure 1shows two dual op amps being controlled by a single quad xdcp. the gain range in this con?uration can be trimmed between -0.5 to -2.0 over 64 steps. increasing r5 and r6 (the feedback deter- mining network in u2a for example) to 100k ? will change the gain trim range to between -0.91 to -1.1. the inverting con?uration shown is particularly suitable as this allows input signals to vary from ?v without violating the digital pots terminal voltage speci?ations of +5v/-3v. an offset trimming con?uration is shown in figure 2 for the same pair of dual op amps. this con?uration yields an offset trimming range of ?00mv over 64 steps. the offset range can be increased or decreased by changing the value of r13 (in ampli?r u2a for example). changing r13 to 1m ? will decrease the offset range to ?0mv if ?er resolution, but less range is desired. this con?uration also ensures that the x9241a is operating within the speci?d terminal voltage range of less than +5v/-3v. potpourri ampli?r figure 3 illustrates a microprocessor controlled circuit that can provide adjustable gain from -1 to in?ity with two of the pots of the quad x9241aw xdcp. the other two pots can be ganged together using the cascade mode for extended offset range. the purpose of this circuit is not to solve a particular analog signal processing problem, but rather to demonstrate the analog control ?xibility that can be attained with the x9241a. with r2 set to 0 and r1 set to 63, the overall gain from input to output will be -1. gain values in the range 0 to -1 can be set over 64 steps by incrementing the value of r1. setting r1 back to 0 and setting r2 to 32 results in an overall gain of +1. increasing r2 to 63 yields a gain of plus in?ity. this is essentially a positive comparator with the threshold level being set by the cascade combi- nation of r0 and r3. national semiconductor s lmc6482 is a nice ampli?r for this application because of its rail-to-rail input and output range.
2 of 4 AN50 application note www.xicor.com jul y , 2002 figure 1. x9241a controlled gain adjust
3 of 4 AN50 application note www.xicor.com july, 2002 figure 2. x9241a controlled offset adjust
4 of 4 AN50 application note www.xicor.com jul y , 2002 figure 3. extended range gain adjust
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