how ad633 works
Hi. I was working on a project the other day and I needed to build a circuit that finds the reciprocal of the input voltage. I tried many designs, but had only limited success. Then I noticed that the AD633 could also divide one voltage by another which was perfect. I built the circuit and it worked perfectly - that is until either input voltage was in the region of 5 volts. At these voltages, I got a sinusoidal or triangular oscillation that was greater than about 0.5 MHz. I checked on the internet to see if I could find any help. Well, I didn't find any help, but I did find this post.
After consulting with a lecturer of mine, we found it was very easy to fix the problem. When oscillations occur, the output is DC offset. This indicates that the DC signal is still there, but there is an additional high frequency signal superimposed on the DC signal. So, what we did to fix the problem is to put a capacitor in the feedback path of the op-amp i.e. I placed a 0.47 uF cap between the ouput (pin 6) and the inverting terminal (pin 2) of the op amp. This fixed the problem immediately and killed the bad frequencies. I'm not too sure what you are using this for but you just need to make sure that the frequency and bandwidth that you are using fall well below the frequency of oscillation. My frequency is at most 10 kHz which is much smaller than 500 kHz (or whatever it was - I don't remember
). If you find that you are cancelling out your signal by putting this cap there, you probably need to try a smaller cap - probably 0.22 uF or 0.1 uF, but there is a possibility that the 0.1 uF will still allow oscillations. Also, you could try connecting another cap between the inverting terminal (pin 2) and ground. To work out the size of this cap:
C = 1/(2 * pi * f * R).
R is the equivalent resistance of the parallel combination of the two 10 k resistors, i.e. R = 5 k for the circuit given in figure 7 of the datasheet and f is your frequency.
I hope this helps and that it is not too late. :wink: