Dear Forum Members,

In some of my 8051 based [ P89C668 ] products It is required to calculate RMS value of the incoming signal and accordingly take actions.

The signal is 50Hz having quasi sinusoidal waveform coming from current sensors. As of now I do the following for RMS measurements.

1. Precision rectify the signal using TL054
2. Feed it to a unipolar ADC such as PCF8591.
3. Read 200 samples per cycle from ADC signal freq. 50Hz.
Compute RMS by using squaring, summing, mean and then sqare root.
The sample reading and calculation is done by an interrupt [ I2C interrupt ] when the result is ready that is at end of one cycle. Interrupt sets a bit RMSREADY and my program then reads this RMS from a global variable.

I have seen some of my competetors products using AD637 RMS to DC covnvertor chips. I am also planning to use it and want your kind advices and or suggestions for the same. My queries are,

1. Once a signal cycle is passed can I immediately read the RMS value from these IC's or there is some delay. I remember in an earlier chip from Analog there was a settling time of 50ms+?

2. The RMS value read at the end of cycle does correponds to the immediate prevois cycle or there is a possibility of some trace of earlier?

3. Whats the output of these chips during the cycle ie. while a cycle is in progress.

4. I am looking for 8 bit accuracy at present and 10 bit max. in future. Will it help using AD637 improving measurement accuracy compared my existing software approach of RMS. with 200 samples in one measurement cycle. With Freq of signal 50Hz.

In my application the incoming signal very widely from cycle to cycle and my intrest is measuring the RMS without significant settling time or delay. A delay of 1ms max should not be a problem.

If you have used this kind of chips kindly provide your valuable suggestions/expriences. This chip being very expensive it is not possible to do any trial and error.

Thanks & Regards,
Prahlad Purohit