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Why are V(rms) measurements frequency dependant?

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Kleinstein:
An amplification smaller than 1 is not per se bad for stability.  Noise gain is still close to 1. The problem is more with the extra capacitance at the inverting input from the switches, that can have a negative effect on stability.

The inverting amplifier is definitely an option - some DMMs use it successfully.

Another possible option not considered so far is having different switchable gain stages in series. It is quite common to use something like a switchable 1:100 divider at the input and than switchable gains like 1:10:100 and maybe another 0.1 from later stages. Having a single switch to choose from more than 3 settings can be tricky with the load capacitance.  If it is planed to be fast I would really look at scope input stages to learn about gain switching - it does not have to be that fast, but 100 kHz is no more DC, especially if at 1 MOhms.
One may also have to include over voltage protection, as it also adds extra capacitance.

There is no real need for super fast OPs. Something like 10 MHz GBW OPs can be sufficient for stages with a gain of 10 or less.

sourcecharge:

--- Quote from: Kleinstein on August 07, 2018, 08:14:00 pm ---An amplification smaller than 1 is not per se bad for stability.  Noise gain is still close to 1. The problem is more with the extra capacitance at the inverting input from the switches, that can have a negative effect on stability.

The inverting amplifier is definitely an option - some DMMs use it successfully.

Another possible option not considered so far is having different switchable gain stages in series. It is quite common to use something like a switchable 1:100 divider at the input and than switchable gains like 1:10:100 and maybe another 0.1 from later stages. Having a single switch to choose from more than 3 settings can be tricky with the load capacitance.  If it is planed to be fast I would really look at scope input stages to learn about gain switching - it does not have to be that fast, but 100 kHz is no more DC, especially if at 1 MOhms.
One may also have to include over voltage protection, as it also adds extra capacitance.

There is no real need for super fast OPs. Something like 10 MHz GBW OPs can be sufficient for stages with a gain of 10 or less.

--- End quote ---

I'm having trouble visualizing your new option, would you give a schematic or further clarify?

The reason why I was thinking of using a inverting amplifier for a attenuator was so that I could use a high impedance.

This would limit the effect on the circuit being measured.

I was thinking 10Mohms would be a standard.

What do you think?

The reason why I was thinking that 300Mhz BW for the op amps was because the two op amps would have to have a Gain of 1000.

I'm guessing that even in a Gain of -1/1000, and if it was stable, then it would still correspond to the 1000 Gain calculation.

So, 300Mhz/1000 = 300khz

Is that right?  That way the BW is 2x more than the actual BW of the limit of 0.1% error of the Trms converter.

One thing that I've noticed about the Trms converter's datasheet is that it looks like the error is the most flat between 100mVrms to 200mVrms input.

Would it be overkill to design the divider networks to work within this range?

I was thinking that over voltage protection is a luxury, just like the auto-ranging.  I think that I would rather have accuracy than safety.  If something blows, its going to be an op amp or the Trms converter, both are relatively inexpensive.  As long as the highly precise resistor networks are not damaged, I don't think I would include it in at least the bare bones version of this thing.

Is there any way that you can think of, conventional or unconventional, to limit the capacitance that the switches would have?

Say for instance as an unconventional solution, what if the "switch" was to plug in the correct resistor in a gold contacted socket?

Just spit balling.

Kleinstein:
The LTC1968 does not have provisions to compensate for background noise (I don't know if other analog solutions have that). So there is some limitation on how small signals can be measured. With a background of some 10 µV, there is not much sense in ranges for less than 1 mV full scale. Measuring voltages like 0.1 mV (or smaller) would be limited by the noise background more than from using the 1 mV range.

So the overall gain does not need to go much beyond 100. Having something like 200 mV full scale seems reasonable as it allows the usual crest factor of about 3-5 V.

For a reliable measurement it needs some indication for overflow, as the RMS reading alone is not enough to detect possible peaks that are too large. This part can be relatively simple - e.g. 2 comparator to check for positive and negative peaks that are too large and some latching / stretching to make small peaks more visible.

Some over-voltage protection is essential, at least against ESD damages.

The inverting amplifier does not solve the noise problem - it's actually even slightly higher noise. The main advantage is that a single stage could be used for many gain / attenuator settings - though with problems at higher frequency accuracy. So it is not that attractive for a good solution, more like for a cheap one.

sourcecharge:

--- Quote from: Kleinstein on August 08, 2018, 05:57:41 am ---The LTC1968 does not have provisions to compensate for background noise (I don't know if other analog solutions have that). So there is some limitation on how small signals can be measured. With a background of some 10 µV, there is not much sense in ranges for less than 1 mV full scale. Measuring voltages like 0.1 mV (or smaller) would be limited by the noise background more than from using the 1 mV range.

So the overall gain does not need to go much beyond 100. Having something like 200 mV full scale seems reasonable as it allows the usual crest factor of about 3-5 V.

For a reliable measurement it needs some indication for overflow, as the RMS reading alone is not enough to detect possible peaks that are too large. This part can be relatively simple - e.g. 2 comparator to check for positive and negative peaks that are too large and some latching / stretching to make small peaks more visible.

Some over-voltage protection is essential, at least against ESD damages.

The inverting amplifier does not solve the noise problem - it's actually even slightly higher noise. The main advantage is that a single stage could be used for many gain / attenuator settings - though with problems at higher frequency accuracy. So it is not that attractive for a good solution, more like for a cheap one.

--- End quote ---

The input to the LTC1968 or an opamp that you recommended?

The input to the LTV1968 has to be within the 50mVrms to 500mVrms from the OPs for it to be within the 0.1% error up to 150khz.

right?

50uVrms could be the lowest input of the 1st range to an opamp with a 1000 gain.

That would give a 50mVrms input to the LTC1968.

If 500uVrms was the lowest, than the gain would only be 100, but we are talking about just another resistor for the opamp to measure down to 70uVrms.

Regarding the LTC1968, If you look at the graph marked, Linearity performance in the datasheet and in the 3 captured graphs in the extra pic, the flattest of the error at zero is between a 100mVrms input to 200mVrms input for the LTC1968.

What I meant was that the number of range resistors could be increased to make the input signal into the LTC1968 between that range, and then instead of using only a buffer on the output, use the same gain/attenuation to output a corresponding range, easily read by the V DC of the meter.

What would you use for the ESD protection, and how would it effect the overall accuracy?

Regarding the noise, are you talking about the op amp output noise, the resistor noise, or the parasitic capacitance?

I was hoping for an opamp that had a high BW, low noise, low offset, and unity gain up to 1000 gain up to 150 - 300khz.

But looking at the max4238/9, the graphs show the gain is not the max gain advertised all the way up to their BW, and the phase shift is a problem too, as the inverting output is 180 degrees from the input, if the phase changes, then there is going to be error.  The phase shift seems to be frequency dependent.

The resistor noise, I was thinking that the opamp would be able to take care of that, because of the virtual ground.

The resistor capacitance is resistor dependent.  One resistor is not going to have the same as another of the same resistance.  But a range of resistors made by the same manufacturer all as the same resistor line, may have a consistent capacitance ratio between the ratio of resistance.

The High impedance input resistor will have the least amount of capacitance in parallel, the higher the resistance = the lower the capacitance, and that capacitance is in series with the second parasitic capacitance of the inverting opamp attenuator.  Therefore the total capacitance has to be less than the lowest capacitance.  This would indicate that the higher the resistance of the 1st resistor, the less the total parasitic capacitance.  Is this right? Am I understanding this correctly?

Or is the virtual ground of the opamp only see the two parasitic capacitances as if they were in parallel?

sourcecharge:
Just did some simulations, and they are in parallel.

it really increases the error quite a bit.

hmmm

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