| Electronics > Beginners |
| Why are V(rms) measurements frequency dependant? |
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| sourcecharge:
--- Quote from: Kleinstein on August 05, 2018, 04:17:56 pm ---The LTC1968 looks like a good one. I don't see a need for a voltage reference here - the voltage reference would be needed for the measurement of the DC output only, which would be a normal DMM in voltage mode here. The LTC1968 would just need a reasonably regulated supply. For the auto-ranging, there is no absolute need for this here. It could be done by hand too if there are suitable indications. The relevant numbers are the peak voltages - so one should have some extra circuitry to check the peak voltages. As a minimum this would be something like 2 comparators to check the upper limits and than use manual adjustment with try and error (use smallest range that does not indicate error from peak values). The actual gain setting can be quite tricky for higher BW if it needs to be really accurate. This is because the divider would be not just resistors, but also with parallel capacitance that needs adjustment (a little like the compensation at scope probes). Also electronic switches have limited isolation when off. --- End quote --- Well this is what I got so far, it's nothing fancy, but it will work in the ranges for the LTC1968 for 50mVrms to 500mVrms so that it can operate within the 0.1% error tolerance up to 150kHz. It should give the add-on the ability to measure between 50uVrms to 500Vrms, with the output of the meter between 50mV DC to 500mV DC. Any more additional components, and I think I would be increasing the error. Basically, it's got the MAX4239 op amps like the uCurrent, and a resistor network that are switched between outputs. The additional switch to the input of the 1st op amp is needed to limit the voltage on the input leg for higher voltages. I was going to run the MAX4239 along with the LTC1968 with some type of voltage regulator at 5V and a 9V battery, but I'm not sure if the op amps still have the same characteristics when run at 5V vs 3V. I have discounted the op op amp LMV321 and all of the required capacitors that is on the uCurrent in this schematic, but they will be in the final design. I will also try to find 0.01% or better resistor tolerances. I have not included the LTC1968 because B2 doesn't have even 1 rms to DC converter let alone this specific one, but the datasheet shows that one of the inputs to the converter needs to have a series capacitor and a capacitor on the output of the DC conversion. I have included the datasheet for the LTC1968 and a 10V reference that I was going to use for a DC calibration for my bench meters. I also got what I think are the most relevant graphs from the LTC1968 together to show the linearity. Anyone have any thoughts, suggestions, maybe what switch to use, and maybe cheap improvements that don't increase the error? The ranges for this network using the LTC1968 are of the following: Closed Switch(es)- (while all other switches are open) XSW8 and XSW1- Input: 50 uVrms to 500 uVrms Output: 50mV DC to 500 mVDC XSW8 and XSW2- Input: 500 uVrms to 5 mVrms Output: 50mV DC to 500 mVDC XSW8 and XSW3- Input: 5 mVrms to 50 mVrms Output: 50mV DC to 500 mVDC XSW4- Input: 50 mVrms to 500 mVrms Output: 50mV DC to 500 mVDC XSW5- Input: 500 mVrms to 5 Vrms Output: 50mV DC to 500 mVDC XSW6- Input: 5 Vrms to 50 Vrms Output: 50mV DC to 500 mVDC XSW7- Input: 50 Vrms to 500 Vrms Output: 50mV DC to 500 mVDC EDIT: The resistor network shown is not available at digikey, but 20M, 2M, 200k, 20k + 2k + 200 (22200) are available at 0.01% 5ppm/C at about a total of 70 bucks. |
| sourcecharge:
So, just checked the pricing at digikey and the only resistors with a greater tolerance of 0.1 were through hole types, and they were pricey. Total cost of just the resistor network before tax and shipping, 196 bucks. :-- |
| Zero999:
--- Quote from: sourcecharge on August 06, 2018, 11:34:41 am ---So, just checked the pricing at digikey and the only resistors with a greater tolerance of 0.1 were through hole types, and they were pricey. Total cost of just the resistor network before tax and shipping, 196 bucks. :-- --- End quote --- That's because you've used weird values. 9×10x is not a common resistor value, so it will be expensive, especially in 0.1% tolerance or better. You would have more luck if you used standard E24 of E96 values. If you divide all of the precision resistor values in that circuit by 5, then it would give you much more widely available resistor values. |
| David Hess:
--- Quote from: Kalvin on August 05, 2018, 06:21:57 pm ---The RMS can be calculated digitally if you have a fast enough ADC = more energy required compared to analog solution due to ADC and DSP implementation. For a signal with 150 kHz bandwidth, one has to sample at least with 300 kHz - in practice somewhat faster say 500 kilosamples/second. For 1 MHz signal one should probably sample at 3 Ms/s. --- End quote --- The only thing which matters is the sampling bandwidth; the sample rate is irrelevant except for uncertainty. The RMS calculation is just the standard deviation. Reducing the number of samples does not change the standard deviation so operating the analog to digital converter below the Nyquist frequency is completely acceptable. Another way to look at it is that aliasing folds the signal over inside the Nyquist bandwidth but the standard deviation of the entire signal is still there to be measured. This can also be done in the analog domain. Use a sampler to capture the input and feed the sampler's output to a standard analog translinear RMS converter. Now the input bandwidth is limited by the sampler and not the analog RMS converter. The Racal-Dana 9301 and HP3406 sampling RF voltmeters worked this way to make RMS measurements into the GHz range. Some old analog sampling oscilloscopes had a sampling output which could be attached to a low frequency RMS voltmeter to do the same thing up to 10+ GHz and beyond. As far as the original question, most DSOs can do what is needed if their accuracy is acceptable. Just be careful because not all DSOs compute the RMS function correctly. This is likely to be a problem if they make measurements on the processed display record (this destroys the standard deviation) like the often recommended Rigol DS1000Z series. If you want to build something simple without using the sampling and standard deviation method, then I suggest using the AD637 or LTC1967 RMS to DC converter IC. |
| Tomorokoshi:
--- Quote from: sourcecharge on August 05, 2018, 04:03:29 am ---Is there a meter that is already out that can accurately measure VAC across a broad range of frequency up to say 1 Mhz, or even non frequency dependent? --- End quote --- Additionally, the Hewlett Packard 3403C True RMS Voltmeter can be coaxed to an accuracy of 10% at 100 MHz. At 1 MHz it can be good to around 3% depending on the input voltage level. These use a thermopile converter. http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1972-03.pdf http://www.analog.com/media/en/technical-documentation/application-notes/an106f.pdf They show up on ebay for $100 to $200. |
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