Electronics > Beginners

Why are V(rms) measurements frequency dependant?

<< < (5/11) > >>

Kleinstein:
The max4239 is not that suitable to amplify small AC signals. It has a rather limited BW and quite some higher frequency noise. With amplification it's usually better to separate the DC part out, so that no AZ OPs are needed.

The simple divider with 900 K will not work well without compensating caps. Normal mechanical switches may not offer sufficient isolation when off and the later OP stages driven to saturation may give quite some higher frequency components if really fast.
So the proposed circuit is not really good for higher frequencies - maybe up to a few kHz. With AC 0.1% resistors should be good enough - even with adjustment the caps will not be that accurate and stable.

When using the digital method there is no need for that much ADC resolution: for a single RMS reading there will be quite a lot of ADC samples (usually at least in the 10000 range) used and in a kind of oversampling way this will give a resolution for the RMS value that can be quite a bit higher than the ADC resolution. For a simple DIY test something like the STM32F3...  µC with a reasonable fast (a few MSPS) 12 Bit ADC would be a good start.

sourcecharge:
Ya, I wasn't thinking..

digikey has a 20M, 2M, 200k, 20k, 2k, and a 200 good through hole resistors...and those come out to be about 70 dollars (I think).

I didn't want to decrease the impedance too much so I just did the calculation this time with what was available.

These resistors are 0.01% tolerance and it looks like they are meant for this application.

The 0.1% error of the LTC1968 is good only up to 150khz, which should be plenty for what I was hoping for..not sure if it is though...

The max4239 has a BW of 6.5Mhz, and with only a 10x gain, that means the adjusted BW at the last op amp should be

BW(tot) = 6.5Mhz/10 x (2^(1/3)+1)^(1/2) because I am using three with all the same gain.

That gives a BW of 977khz.

The uCurrent has proven to me that these op amps can amplify DC, but you are saying that these can't do small AC signals?

Why?

Regarding the switches, what about isolation relays coupled with a simple switch that turns the relays on and off with 5V?

Not too sure what to do regarding the caps, what are the caps going to help with?  Do you think by using the high resistance values of 20M..etc..will limit that problem?

sourcecharge:

--- Quote from: Hero999 on August 05, 2018, 04:54:16 pm ---I've just quickly read through the data-sheet for the LTC1968. The reference is the common voltage for the AC waveform. The IC measures the difference between the voltage on its inputs. At least one input must be DC coupled to a steady voltage between the supply rails. If it's a single supply application, connect one pin to a potential divider with a bypass capacitor to 0V and the other input to the signal source, via a capacitor. See page 12.
http://www.analog.com/media/en/technical-documentation/data-sheets/1968f.pdf

The output of the LTC1968 is high impedance and needs a buffer amplifier, before going to the DVM. A decent, low offset, high input impedance, low bias current, op-amp is required for the buffer.

--- End quote ---

I was thinking about simply using another MAX4239 in unity gain for the buffer op amp, is that reasonable?

Kleinstein:
The max4239 is not unity gain stable. For the output the unity gain stable version max4238 would be better.

The problem with the max4239 at the input is having quite some noise, especially at the higher frequencies (e.g. the AZ frequency). So it would likely not make much sense to have that much amplification. The BW calculation is also a little off - it should be a little below 650 kHz. So at 150 kHz the loop gain would be somewhere around 5 and thus significant errors could start to appear.

The capacitive problem is that the 20 M resistor will have some parasitic capacitance in parallel. To make the divider work well, there should be a parallel capacitive divider with the same rations. So the smaller resistors would need correspondingly larger caps in parallel.  If there is some parasitic 1 pF at the 20 M , the 200 K should have some 100 pF and so on.  As an additional complication the OPs input and switch will also have some capacitance, that changes with the switch setting. So to make is reasonably work in all settings the capacitance should be large compared to the load capacitance - so one has to add larger caps, including one with the largest resistor. So it may be more like 10 pF, 100 pF, 1 nF , ....
The higher frequency divider would be set by the caps, not the resistors.

Also a 2 M resistor will have quite some noise by it's own, which would limit the use of smaller ranges. There is a good reason, why bench DMMs usually use a 1 M  resistive divider for the AC ranges. Those 20 M dividers are more made for DC.

For the higher frequency isolation relays are not per se better than manual switches.  For good attenuation, one usually uses more than just a single switch and avoids to send the signal to amplifiers that are not needed / used.

Zero999:

--- Quote from: sourcecharge on August 06, 2018, 04:57:31 pm ---Ya, I wasn't thinking..

digikey has a 20M, 2M, 200k, 20k, 2k, and a 200 good through hole resistors...and those come out to be about 70 dollars (I think).

I didn't want to decrease the impedance too much so I just did the calculation this time with what was available.

These resistors are 0.01% tolerance and it looks like they are meant for this application.

The 0.1% error of the LTC1968 is good only up to 150khz, which should be plenty for what I was hoping for..not sure if it is though...

The max4239 has a BW of 6.5Mhz, and with only a 10x gain, that means the adjusted BW at the last op amp should be

BW(tot) = 6.5Mhz/10 x (2^(1/3)+1)^(1/2) because I am using three with all the same gain.

That gives a BW of 977khz.

The uCurrent has proven to me that these op amps can amplify DC, but you are saying that these can't do small AC signals?

Why?

Regarding the switches, what about isolation relays coupled with a simple switch that turns the relays on and off with 5V?

Not too sure what to do regarding the caps, what are the caps going to help with?  Do you think by using the high resistance values of 20M..etc..will limit that problem?

--- End quote ---
Why do you want 0.01% tolerance resistors, when the LTC1968 isn't that good?

The low resistor values makes no difference to the input impedance, when the op-amp is configured as a non-inverting amplifier.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod