elsalty
Newbie level 5
I was looking at the top of page 14 of 20 on the AD736 data sheet
It states that the 8k scaling resistor along with Cc determine the -3dB low frequency roll-off (FL). Then it proceeds to state that at this FL, the amplitude error will be -3dB (or 30%). This makes perfect sense to me. But then it goes on to state that to reduce the error, to 0.5% of the reading, choose a value of Cc that sets FL at 1/10 of the lowest frequency to be measured. This does not make sense to me.
So if I am trying to measure a 1Vrms 200Hz sinusoid signal and set R = 8k and Cc = 100nF, then FL = 200Hz. This is the -3dB frequency, so the output signal would be 0.707 V instead of the correct 1V. So this is ~30% low as expected. But if I do what the data sheet suggests to reduce the error to 0.5% of the reading, I would select Cc = 1uF which would make FL = 20 Hz (i.e. 1/10 of the lowest frequency to be measured). But for a 1Vrms 200Hz sinusoid, that would result in an output of around 0.10 V (-20dB) which is around a 90% error.
If I did the inverse of what they suggest and set FL = 2000, then the output would be 0.995, or 0.5% error for a 1Vrms 200Hz sinusoid. Is this an error in the data sheet or am I just interpreting what they are saying incorrectly? Thanks for any help.
My apologies. I was missing the obvious until I typed this all in. Then I realized I was thinking it was discussing high frequency roll-off, but it is the low frequency roll off.
It states that the 8k scaling resistor along with Cc determine the -3dB low frequency roll-off (FL). Then it proceeds to state that at this FL, the amplitude error will be -3dB (or 30%). This makes perfect sense to me. But then it goes on to state that to reduce the error, to 0.5% of the reading, choose a value of Cc that sets FL at 1/10 of the lowest frequency to be measured. This does not make sense to me.
So if I am trying to measure a 1Vrms 200Hz sinusoid signal and set R = 8k and Cc = 100nF, then FL = 200Hz. This is the -3dB frequency, so the output signal would be 0.707 V instead of the correct 1V. So this is ~30% low as expected. But if I do what the data sheet suggests to reduce the error to 0.5% of the reading, I would select Cc = 1uF which would make FL = 20 Hz (i.e. 1/10 of the lowest frequency to be measured). But for a 1Vrms 200Hz sinusoid, that would result in an output of around 0.10 V (-20dB) which is around a 90% error.
If I did the inverse of what they suggest and set FL = 2000, then the output would be 0.995, or 0.5% error for a 1Vrms 200Hz sinusoid. Is this an error in the data sheet or am I just interpreting what they are saying incorrectly? Thanks for any help.
--- Updated ---
My apologies. I was missing the obvious until I typed this all in. Then I realized I was thinking it was discussing high frequency roll-off, but it is the low frequency roll off.
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