Rarely Asked Question Page

About The Author
John Ardizzoni ,
High Speed Amplifiers Product Manager

I'm trying to use a potentiometer (pot) in the feedback loop of an op amp circuit, but the adjustment is too sensitive. Do you have any suggestions on how to improve the performance of this circuit?

The first thing I would tell you is to be careful when using pots in feedback loops, especially at higher frequencies. Wire wound pots have lots of inductance and can cause instability. Watch out for carbon pots as they are noisy.

Here are a few things you can try, however. Depending on your situation, you can add a series resistor and use the pot to "fine tune" the value you need. Sized appropriately, the pot sensitivity will be reduced greatly. You can also put the pot in parallel with the feedback resistor. Again, sizing the feedback resistor value is critical as to what contribution the pot will play. The advantage of this option is it allows you to go all the way to zero ohms.

A better option is to use a digitally controlled potentiometer or "digiPOT," which are extremely accurate and avoid typical pot issues such as wear out, vibration, drift, size, mechanical adjustment, and environmental issues, to name a few. Several options are available for the digital control (SPI, I2C, push button, and up/down interface), and the adjustment is accurate and repeatable. A digiPOT can also be used as a rheostat, which is a pot with one of the end terminals tied to the wiper to form a variable resistor. digiPOTs work in both noninverting and inverting op amp configurations.

The bandwidth of these parts covers a wide range as a function of the resistor value. For example, a digiPOT set at 1K ohms has about 5 MHz of bandwidth, whereas a resistor setting of 10K ohms has a bandwidth of 500 kHz. These parts are extremely versatile and offer the designer a new tool for their "tool box." Low power, small footprint, and solid-state reliability make digiPOTs a very attractive alternative to the "old fashioned" option.

Even though they are simple building block devices (as most components are), there is a fair amount of detailed information to be considered. The datasheets for these devices (and all Analog Devices components) contain a wealth of comprehensive information, so a thorough read is recommended to optimize your design experience.


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For More Information

Products
  • AD5272: 1024-Position, 1% Resistor Tolerance Error, Single Channel I2C Interface and 50-TP Memory Digital Rheostat
  • AD5292: Single Channel, 1024-Position, 1% R-Tol, Digital Potentiometer with 20-Times Programmable Memory
  • Digital Potentiometers
Analog Dialogue:
Application Notes:
  • AN-1121: Replacing Mechanical Potentiometers with Digital Potentiometers (pdf)
Reference Circuits:
  • CN0114: Low Cost, High Voltage, Programmable Gain Instrumentation Amplifier Using the AD5292 Digital Potentiometer and the AD8221 In-Amp
  • CN0112: Variable Gain Noninverting Amplifier Using the AD5292 Digital Potentiometer and the OP184 Op Amp

Data Conversion Knowledge Resource

Technical Documentation:
  • About The Author
  • John Ardizzoni
    High Speed Amplifiers Product Manager

John started at Analog Devices in 2002, as an Application Engineer in the High Speed Amplifier group. Prior to joining Analog Devices John worked for IBM, in RFIC applications and at M/A-COM for 20 years. John is also co-author of Analog Devices Rarely Asked Questions (RAQ's) column. John has over 30 years experience in the electronics industry, and has authored numerous articles and design ideas.