I was trying to simulate AD736 rms to DC converter for 200mV input for fastest settling time. I have attached the circuit below. I might have made mistakes in selecting the Cav and Cf capacitors.
When the input was a 1k sine wave the circuit was working properly. But when i increased the input frequency to 4k , the output started oscillating between +- Vcc. At first it seemed the output was settling to the correct rms value but then it started oscillating. I increased the input frequency to 32k, the oscillation started happening even earlier.
I am not able to understand why this is happening. Is it due to the capacitor values?
The requirements of my circuit is to find the rms value of a 32.786 kHz sine wave as fast as possible.
i have used ADIsimPE SIMetrix
the red curve is for 1kHz wave ,green is for 4kHz wave
thanks in advance
Thank you for your question. I would try the following:
1. Place a 10uF capacitor between CC and ground or between VIN and the signal source. Do not connect both to ground or else the circuit will never settle. It is like connecting both inputs of an op-amp to ground.
2. Place 10uF by-pass capacitors on the supply pins. A SPICE model never actually simulates a real component, especially an integrated circuit. A Spice model is a solution to a network large or small, but this is a mathematical construct, not real parts. You do get a solution very fast of course, but it's just a solution to a very large equation in most instances. If you want to learn how the part works get in touch with your local ADI contact and ask for an eval board. They'll probably give you one, anyway they're supposed to. .
3. I like to add units to plot axes, it helps you if you have to lay the plot aside and you have to come back to it later.