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AD8655 feeding an inductive load

Category: Hardware
Product Number: AD8655


I want to amplify a signal from a DDS (AD9833) in order to feed an inductive load (27µH, 3 ohms) at 100kHz, amplitude around 3V.

When I use the OPA564 from Texas, everything works well. But this component is not available. So I tried to use the AD8655 but here is what I get :

As you can see, the current is good, but the voltage signal is a bit distorted a the max values, when the current is inversed. I guess it is linked to the back emf.

The 10µF capacitor is here to avoid the DC. Changing this capacitor doesn't change the distorsion. Symetric supply of the opamp and placing the µF capacitor between the DDS and the opamp doesn't change anything. When I increase the frequency, the signal is better. When I decrease it, the signal is worst.

Any idea of the reason of this phenomenon? Any idea how to solve it? Any idea radically different to feed my coil with these properties and without distortion?

Thank you very much!

  • Hello  ,

    I am currently looking on this. Just a few clarifications in your query:

    1. Is your input signal (output of DDS) a sinewave signal with the following parameters: 320 mV peak, 320 mV offset, freq of 100 kHz?

    2. Is your expected output a 100kHz sinewave with an amplitude of 3 V peak-to-peak? (1.5 V peak)

    3. When you increased the frequency, is the capacitor located between the DDS and opamp? or is it located after the opamp just like in the schematic? At what frequency did the signal became better?


  • Hello,

    Thank you for your answer.

    1. The signal from the DDS has a 320mV offset and 320mV in amplitude, so it's a signal oscillating between 0 and 640mV. FQ 100 kHz.

    2. The expected signal for feeding the coil (so after the capacitor that is used to cancel the DC part of the amplified signal from the opamp) is indeed 3Vp-p, around zero.

    3. No, the capacitor is at the output of the opamp, as in the schematic. It becomes to begin better at 500kHz ; when I continue increase the frequency, I think the slewrate is reached because the signal is becoming triangular.

    Thank you in advance

  • Hi,

    I think the AD8655 can't handle such current and voltage combination in this configuration. One problem is, that your opamp is configured to output a voltage from 0V to 3.2V, but at such high current the output can go down to 0V (see figure 10 in the datasheet), so distortion certainly occurs. The datasheet specifies 220mA output current amplitude only if the output voltage amplitude is 0.5V. In your case the amplitude is over 1.5V, so the maximum current could be much less (for the same output power 73mA). So I think for your case the datasheet does not guarantee proper operation. I've played a bit with the simulation, and I have found that in unity gain configuration and output signal centered at midsupply the distortion is minimal:

    Anyway, it may still be out of what is specified, so it would be better to find another opamp with higher specified output current at 3V peak-to-peak output voltage swing.

  • Hello  ,

    I agree with . I also don't think that AD8655 can handle such current and voltage combination. Looking at the part's plot, we can verify that at such high current, the output can go down to 0V, which in turn causes distortion.


  • Ooops, just a simple mistyping I have made: the output can't go down to 0V, as can be seen in the figure Paul has been shared -thanks, Paul!

  • Hello,

    Apologies also on my end, it should be the output can't go down to 0V.  When I have simulated the circuit, I got a load current of approximately 90 mA. Now, using again the plot from figure 10 of the data sheet, the VOL and VOH should be about 0.5 V and 4.2 V respectively. 

    The opamp is configured to output a voltage from 0-3.2 V, but since VOL and VOH should be about 0.5 V and 4.2 V respectively, it's output can't go down to 0 V that's why distortion occurs just like what  said.