Project Analog - February 2008 - (Page 6) R1 A VW + C1 VOUT Pot1 W B R2 VIN R3 B Pot2 A R4 fig 1 invErting amPlifiEr with offsEt and gain trimming In this circuit (figure 1), there is no interaction between the offset trimming and the gain trimming, but the input signal (Vin) is loaded by the resistance of R3 plus Pot2’s Rbw value. In the following circuit, a resistor ladder is used to create a voltage window where Pot1 is used to trim the desired offset for the non-inverting amplifier. A second potentiometer is used in a rheostat mode to control the gain of the amplifier. The step resistance of Pot2 relative to resistor R3 determines if the gain trimming is a fine adjustment or a course adjustment. Capacitor C1 is for compensation of the op amp and to inhibit the output from oscillating. In this circuit (figure 2), there is an interaction between the offset trimming and the gain trimming. To minimize this interaction, Pot2 should be small compared to resistor R3 and Pot1 should be small relative to the sum of R1 and R2. But the input signal (Vin) is not loaded. In the following circuit (figure 3), a resistor ladder is used to create a voltage window where Pot1 is used to trim the desired offset for the band pass filter. This resistor ladder setting also works with capacitor C2 to set the high pass filter frequency. A second potentiometer (Pot2) is used in a rheostat mode along with R3 and R4 to control the gain of the amplifier. The step resistance of Pot2 relative to resistors R3 and R4 determines if the gain trimming is a fine adjustment or a course adjustment. Capacitor C1 along with Pot2, R3 and R4 is used to set the low pass filter. Capacitor C1 is also used for compensation of the Op Amp and to inhibit the output from oscillating. If capacitor C1 is not present, then the circuit is a high pass filter, while if capacitor C2 is not present then the circuit is a low pass filter. The following circuit (figure 4) with the use of an RC filter (Potx and Cx) will filter at the selected frequency. That frequency is determined by the rheostat value (Rbw) of the Contents Viewpoint Digital potentiometer application circuits Smart ADC architecture Layout techniques for high accuracy and resolution ADCs Analog news W R1 VIN A Pot1 W B R2 B Pot2 VW C1 A R3 + VOUT fig 2 non-invErting amPlifiEr with offsEt and gain trimming W R1 A VIN Pot1 W R2 R3 C2 + C1 B Pot2 A R4 VOUT Microchip analog page Mixed-signal overview Sample center microchipDIRECT Reference designs/ app notes Technical training B fig 3 band Pass filtEr with offsEt and gain trimming W 6 · ProjeCT ANALog · feb 08 http://www.microchip.com http://www.microchip.com/analog http://www.microchip.com/analog http://www.microchip.com/mixedsignal http://sample.microchip.com/Default.aspx?testCookies=true http://www.microchipdirect.com/catalogselection.aspx?returnURL=default.aspx http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1423
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