Square Rooting Circuit with AD633 Not Working

[Chance92]

Update 1: I have attached the .asc file for the simulation.

Thanks for clicking my post. I am trying to building a circuit that can do this:

output = (input1²+input2²)^0.5

After some googling, I decided to use AD633 to do this. I basically copied the squaring and square rooting examples from the datasheet and added a summing circuit:

Here are the examples from the datasheet:


However, the simulation results are very strange:

It can be seen that the output from the summing section is correct (V(sum)), but the output from the square rooting section is wrong, and it is stuck at -8.01V. The datasheet says: 'The 1N4148 diode is required to prevent latchup, which can occur in such applications if the input were to change polarity, even momentarily.' Seems to me this is probably a 'latchup', as you can see from the results that the voltage before the diode is -15V. It seems the diode tried to prevent the 'latchup' but failed. What I don't understand is the input to this section has always been negative, why would this happen? By the way, I have simulated the square rooting section by its own and it works fine.
I produced this circuit on a breadboard and the results were similar to the simulations. The squaring+summing and the square rooting works fine by themselves, but they clearly don't like each other when connected together. I have tried switching to different op-amps and/or diodes. Some of them works better than others, but this 'latchup' can still happen sometime, especially when the input signal is very low.
What am I doing wrong?
Thanks for your time.
I have attached the datasheet for AD633 as well.

[Chance92]

Please could someone kindly tell me how I can attached inline pictures?

[RoGeorge]

The inline pics is a plugin that doesn't work any more, and will probably not working in the future either, there are some software compatibility problems.

To inline pictures, you post them at the end, then view your post, click on the pics to see them big, then right click to find the link to the big displayed pic, then edit your post and put the link between img tags (there is a button in the editor called 'Insert Image').

[Chance92]

The inline pics is a plugin that doesn't work any more, and will probably not working in the future either, there are some software compatibility problems.

To inline pictures, you post them at the end, then view your post, click on the pics to see them big, then right click to find the link to the big displayed pic, then edit your post and put the link between img tags (there is a button in the editor called 'Insert Image').

Thank you very much! That's one problem solved.

[ledtester]

Can you post the .asc file?

[Chance92]

Can you post the .asc file?

Thanks. I have attached the .asc file in the original post as advised.

[ledtester]

First, have you checked that your model works for the "Figure 13" circuit?

I have included the .sub for the AD633 which I found here (as a .circ file):

https://www.analog.com/en/products/ad633.html#product-evaluationkit

I've also included the auto-generated .asy file and a schematic of the Figure 13 circuit.

The .txt suffixes were added so I could include them as attachments.

File placement: my .sub file is in the lib/sub directory and the .asy file is in the lib/sym/AutoGenerated directory


Secondly, don't know if this matters, but in Figure 15 there is a cap parallel to the diode. And have you checked just the Figure 15 circuit by itself?

[Chance92]

First, have you checked that your model works for the "Figure 13" circuit?

I have included the .sub for the AD633 which I found here (as a .circ file):

https://www.analog.com/en/products/ad633.html#product-evaluationkit

I've also included the auto-generated .asy file and a schematic of the Figure 13 circuit.

The .txt suffixes were added so I could include them as attachments.

File placement: my .sub file is in the lib/sub directory and the .asy file is in the lib/sym/AutoGenerated directory


Secondly, don't know if this matters, but in Figure 15 there is a cap parallel to the diode. And have you checked just the Figure 15 circuit by itself?

Thanks for sharing these. I managed to run your circuit successfully. The results seems correct:

I have isolated the squaring + summing and they seem to work fine:

While simulating the square rooting circuit (the one in Figure 15.), I have found something interesting. DC test shows the circuit works fine with DC inputs, as long as they are smaller than -6mV. If the input is larger than this (say -4mV), the output will be locked to -8V from the start. This is probably what happened in the original simulation. Further tests show that once this happens, it will required an input <-4.76V to push the output to the positive side. After that the circuit can operate normally, but with a big ripple. By the way I have added the missing cap:

I think this has also been observed during the bread board experiment.