Project Analog - November 2008 - (Page 16)

Once you select an appropriate signal-conditioning amplifier, take time to consider the demands on the output of that amplifier. If you are planning for digital signal acquisition, what is the equivalent input capacitance and sampling frequency of the converter? If the signal-conditioning amplifier lacks sufficient bandwidth to settle to a desired accuracy within the sampling time of the converter, an additional higher-bandwidth drive amplifier may be needed. Many board- and system-level designers realize this unfortunate truth in the latter stages of development. Thus, the need for a drive amplifier at the signal output conditioning stage becomes the first culprit for op amp afterthought. rule 2. when optimizing an instrumentation or difference amplifier for accuracy, pay careful attention to the reference pin. Not every design is driven for the highest possible system accuracy, but it is always a good idea to have an understanding of the minimal amount of accuracy your design can tolerate. If you are designing with an instrumentation or difference amplifier, you have probably selected your device because of a combination of gain accuracy, high CMRR, low drift, and low noise. While these are important specifications for overall system accuracy, don’t let your hard work go to waste – op-amp afterthought will happen if you neglect to account for how the reference pin influences the amplifier output voltage. Difference amplifiers are at the heart of many instrumentation amplifier topologies, so for simplicity we will look at the influence of the reference pin on a difference amplifier. The output voltage of a unity gain difference amplifier is as follows: VOUT = (VIN+ - VIN¬) + VREF For single-supply systems, the instrumentation amplifier REF pin is optimally biased to mid-supply to achieve maximum symmetric output swing. The above equation shows that errors on VREF will translate to direct error at the output. It is important to ensure that an accurate voltage is applied to the REF pin. When creating a reference voltage for the difference amp, for reasons of cost and simplicity, many designers use an available supply from which they achieve a desired reference voltage with a cheap resistor divider. While this may seem like an efficient use of available supplies and be an easy enough solution, a case of op amp afterthought is waiting. The difference amplifier REF pin requires a low-impedance source to avoid a divider error with the internal difference scaling resistors. By using a divider that applies a relatively high impedance to the reference pin, an error is introduced at the output of the difference amplifier. Figure 1a shows the basic difference amplifier topology with R = 10 kΩ and a modeled equivalent resistance (RREF) used to represent the impedance introduced by a voltage divider. With the introduction of additional impedance at the REF pin, the output voltage equation now becomes: VOUT = [2VIN+( R+RREF)/(2R+RREF)] – VIN-] Based on the original output voltage equation, it is apparent that the role of RREF can be significant. Figure 1b describes the total error as a percent of ideal output voltage resulting for resistances from 1 Ω to 10 kΩ. Depending on the equivalent impedance seen at the REF pin, an error of up to 50% of the actual voltage output value can result. This situation can be avoided by introducing a buffer (as shown in Fig. 2) to drive the REF pin to a known and stable value. The low closed-loop output impedance of a buffer will maintain the overall accuracy of the difference amplifier. rule 3. understand the role of the adc voltage reference for accurate digital conversions. The issue described in Rule 1 is circumvented by an amplifier with sufficient bandwidth to keep up with the charge demands of the input switching on certain data converters. Contents Viewpoint Selecting the Proper Amplifier Using a Digital Potentiometer to Optimize a Photo Detection Circuit Resistor Networks in Critical Applications Analog news Microchip analog page Sample center microchipDIRECT Reference designs/ app notes Technical training 16 · PROjeCt ANAlOG · VOlUme 2 / NUmbeR 6 http://www.microchip.com http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog 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

Table of Contents Feed for the Digital Edition of Project Analog - September 2008

Project Analog - September 2008 Contents Viewpoint About Project Analog Sponsor Selecting the Proper Amplifier for Strain Gauge Applications An Overview of Analog Sensor Conditioning Circuits Using a Digital Potentiometer to Optimize a Photo Detection Circuit Resistor Networks in Critical Applications Selecting the Right Op Amp Analog News Contact Microchip New Microchip Products Treelink MINDI™ Active Filter Designer Microchip Advanced Parts Selector (MAPS)

Project Analog - September 2008

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