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  • Making Your Own Difference Amp—sometimes 1% resistors are good enough

    Other Parts Discussed in Post: INA133

    Last week we concluded that the very accurately matched resistors of an integrated difference amplifier were crucial in getting the needed common mode rejection. Review it here. But...

    There is one relatively common case, however, where 1% resistors and a good op amp can make an adequate difference amplifier. When measuring current with a shunt on the “low side” of the load, the common…

  • Difference Amplifiers—the need for well-matched resistors

    Other Parts Discussed in Post: INA133

    In the design of monolithic ICs we routinely exploit the ability to accurately match internal components. Input transistors of op amps, for example, are accurately matched to provide low offset voltage. If we had to make our own op amps with discrete transistors we would have offset voltages of 30mV or more. This ability to accurately match components includes on-chip resistors.

  • Internal Input Bias Current Cancellation

    Other Parts Discussed in Post: OPA209

    Last week we reviewed the use of an input bias current cancellation resistor to balance the source resistance at the two inputs of an op amp. The conclusion was that this practice is often not necessary and may even be detrimental. This discussion builds on the previous blog so you may want to review it, first.

    I ended last week saying that there are certain op amps for which this…

  • Input Bias Current Cancelation Resistors—do you really need them?

    Do you add a resistor to match the DC resistance at the inputs of your op amp circuits?  Check the circuits in figure 1 below. Many of us were instructed to add Rb as “good practice,” making its value equal to the parallel combination of R1 and R2. Let’s look at the reason for this resistor and consider when it’s appropriate and when not.

    The purpose of Rb is to reduce the voltage offset caused…