we are designing a true rms circuit using the AD8436 (see attached pdf).
The Circuits uses a wide input range of AD8436 to measure input voltages from 0-250V RMS with different input frequencies An external opmamp circuit adjusts the values to the one specified in AD8436's datasheet. The Circuits works but it does not deliver the right values.
The output of the TRMS device always delivers a much to low value. When I remove the 3.3uF capacitor of the low pass filter I can see that the peak value of signal corresponds to the expected RMS value. But as soon as I activate the filter again the value drops to much. The datasheet states 0.5% error but I reach 2%-3.6% depending on the input voltage. The higher the input voltage is the higher the error.
We've already checked the whole circuit and tried to use the sallen key- filter but the result stays the same. I also tried to exchange capacity values without success. Simulating the circuit using the pspice model does not show the effect.
But when I look through the datasheet I recognized the sentence: "Fully specified for 300mV RMS input". Does that mean that every picture is only valid for that option ...? What if the input (as in our case) rises upt to 1.77V RMS? In other datasheets like the AD636 or the RMS- to DC- Application guide I can find a diagram showing the error in correlation of the input voltage. Does the AD8436 not have a correlation like this? When I refer to the effects I've seen in our circuit I would say yes but I can't find any information on this.
Could anyone explain these effects?
What is wrong in our circuit or what do I have to take in to account to get a reseult with higher accuray?
Any help or suggest is very welcomed