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Constant current source about 500 uA with frequency range from 50 kHz to 200 kHz

Other Parts Discussed in Thread: TL081, OPA827, TINA-TI

Hi everybody, 

I would like to make a current source with current output 500 uA with frequency range from 50 kHz to 200 kHz. I would like to use this source for bio-electrical analysis. So, the R_LOAD would be probably from 1 Kohm to  100 Kohm I hope.  I have tried to design the improved Howland current source with TL081 amplifier, but when I use only 10 Kohm or more than 50 Khz the output current declines.  

I attached my design current source. 

Is there anybody to help me? 

Thank you very much

  • Hi Majo,

    I have found that when using the Improved Howland Current Source in ac applications that the operational amplifier needs substantially higher bandwidth than the maximum frequency and adequate slew rate to support the required output swing. The required output swing changes with the load resistance.

    When I do some TINA-TI simulations of the circuit at 200 kHz the current error I find that the TL081 is runs low on bandwidth and the errors become large. I tried a couple of different, wider bandwidth amplifiers, and found the OPA827 provided very good results. You can find the TINA-TI file and an image of the results in the zipped file.

    Otherwise, the circuit does appear to be set up correctly. Do note that precise resistor matching is required for best circuit performance.

    Regards, Thomas

    PA - Linear Applications Engineering

    TL081_ac_CS_01.zip
  • Thank you very much for your respond. 

    I will try to order OPA827 and I will see what happen, but I have a next problem. When I put more than 5 kohm load the current go down too. Is it some possibilities how to keep this current on 500 uA? Do I have to change my schema or do I have to use some other connection - not the Howland but something else? 

    Best regards 

    Majo

  • Hi Majo,

    That is odd; you should be able to maintain the 500 uA current through a 5 k load. The voltage developed across the 5 k ohm resistor would be 2. 5V, which is within the output compliance range of the circuit. There is plenty of room between 2.5 V and the maximum output swing of the TL081 amplifier. Does the problem happen at both 50 kHz and 200 kHz, or just one frequency?

    Just make sure your circuit is set up correctly. The Improved Howland Current Source should do what you need. You may find the OPA827 takes care of your problems. You should be able to order some free samples of the OPA827 from its web page.

    http://www.ti.com/product/opa827

     

    Regards, Thomas

    PA - Linear Applications Engineering