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For automotive applications precision circuits are a must and one should avoid using passivecomponents such as resistors, capacitors, potentiometers etc. with poor tolerances as they canhinder even the best application of op-amps and data converters.There are two types of tolerances known as Purchase and Drift tolerance. As their namedictates, passive tolerances are at which resistors are purchased such as 0.1%, 1%, 2%, 5%,10% etc. whereas drift tolerances are caused by the external influences such as temperature,humidity, pressure, dust etc. during the assembly phase hence increasing the overall tolerancelevel. Therefore, for tighter tolerances the stable conditions are created and materials withoutdrift are used.For example, if we use a 16 bit data converter which means 1 LSB corresponds to only15 ppm(parts per million) which can easily be accumulated by poor passive components. Asin our case of Howland current source resistor matching is plays a vital role as unmatchedresistor values deviate the output impedance i.e. if R10 is too low, then output impedance will bepositive. If R2 is also too low then output impedance is low but positive. Similarly unmatchedresistances caused by tolerance can degrade its performance.Therefore, passive components with tighter tolerances play a very important role in stabilityand reliability of the application.Similarly as resistors have tolerance values or rating, capacitors are also expressed in (%)percentage or (pF) picofarads. These values define the extent to which the value of capacitormay change. The common tolerance values available in the market are 5%,10%,20% etc. butthese values can go as low as (1%).For example, 100pF capacitor with tolerance (10%) dictates that the value can varybetween 90pF or 110pF.Two other types of capacitors we will be using are feedback and decoupling capacitors. Asseen in the figure 3.3, feedback resistor R2 has a capacitor across it which compensates for theparasitic capacitances at the inverting terminal of the op-amp. Therefore, it is always a goodidea to use a compensating feedback capacitor in the range of 3pF to 10pF.On the other hand, decoupling capacitors or bypass capacitors are used decouple powersupply signals from another by suppressing high frequency noise. They take tiny voltagesignals out from the voltage supply hence protecting the other sensitive circuit elements. Theyare called bypass capacitors because high frequency signals bypass the IC through them toground.34

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