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Modified Howland current source (OPA561) with high frequency noise problem

Other Parts Discussed in Thread: OPA561

I am currently using OPA561 for configuring a high current source(200mA~500mA range), in a modified Howland circuit setting, and is encountering high frequency noise at the output terminal. I just don't know why it is happening...

The high frequency noise(200kHz-1MHz!!) is concentrated as the current flow changes its sign from (+) to (-) and vice versa, AS I MEASURED VOLTAGE BETWEEN THE IMPEDANCE LOAD R9.


When the VG1 is set high(at 500mA-LEFT), the output sine wave still retains its intended form to acceptable degree, but when the input control VG1 is set low to allow for small current output(below 285mA-RIGHT), the noise signal completely wraps the output signal, rendering it useless for immediate use. (The Oscilloscope, due to impedance matching issues, displays twice the voltage that it should, so keep this in mind please)

Could anybody point out possible reasons for this anomalous behavior in my current source??? I would really appreciate the help.

This is the setting I used.

-Impedence Load is 0.5 ohm(Which bears 500mAp-p current at VG1 1.75V)

-Operational frequency from 50 to 3.5kHz---->SET AT 3kHz in here.

-Supply voltage is +-5Vdc(YEP DC)

-Input AC voltage of 0Vpp to1.75Vpp is set to linearly control output current from 0 to +-500mA peak to peak

Below is the signal predicted from the TINA simulation....which didn't turn out the way I expected.

  • ------------------------PROBLEM SOLVED!!!!!------------------------

    I digged through Texas instruments documents and found the solution in document AN-1515!!!

    Following recommendations for low frequency improved Howland circuit, I attached a 1000pF

    capacitor across R5, and all the clippings were instantaneously minimized.

    Even the noisy one with the VG1=1V shows a clean sine waveform.

    Attached below are output voltage waveform before(LEFT) and after(RIGHT) adding the capacitor.



  • Hello ChoongHo,

    Congratulations on resolving the Howland stability issue on your own!

    Regards, Thomas

    PA - Linear Applications Engineering