How much noise floor and other things matter in oscilloscope usability

[David Hess]

Meh, ringing on step responses isn't usually Gibbs unless something has been designed or set wrong.

It can be a mixture of both.

You can have ringing (and even pre-ringing) on an analog scope.

But you can't have Gibbs.   

I have wondered about that.  Preshoot in analog oscilloscopes comes from the delay line implemented as either a lumped-element transmission line or counter braided differential coaxial line.  Both have a sharp cutoff frequency and faster propagation of high frequencies than low frequencies, but that sharp cutoff reminds me of the truncated Fourier coefficients which lead to the Gibb's phenomena.

With 1mv/div It gives about 160mV RMS and 1,2mV peak to peak. Whatching other scopes they seem to have about 4 time less noise.
Am I wrong?

It is possible but as I pointed out, noise is not the only consideration.  My preferred oscilloscope has about 120 microvolts RMS input noise because it comes with high input common mode range and differential inputs which are more useful, similar to the AM502 mentioned below.

Trying to do a recap: at this point It seem to me that a "low noise" oscilloscope is important when working with FFT analysis, power supply ripple, audio signal, and high impedance circuits?

Like I wrote earlier, except for FFTs where noise is a direct limitation, it is not very important.  Oscilloscopes are noisy because of the compromises they have to make.

When making measurements, other sources of error usually overwhelm noise.  Ground loops with single ended probes and the probe ground lead will pick up all kinds of noise in excess of an oscilloscope's front end noise.  This suggests that money is better spent on better probes than an oscilloscope with the lowest possible noise which cannot be taken advantage of anyway.

Power supply ripple is separate from power supply noise, and a DSO can be triggered and use averaging to remove the noise and keep the ripple.  Measuring power supply noise on the other hand will often require a low noise preamplifier which is not difficult to build.

In audio applications, an oscilloscope has so much distortion that only gross measurements will be accurate, so noise is not relevant.

If you are interested in general purposes low noise measurements within a 1 MHz bandwidth on any oscilloscope, then you might find a Tektronix AM502 differential amplifier to be useful.  They are easy to repair because unique parts can be sourced from the Tektronix 5A22 and 7A22, and they work well with 1x oscilloscope probes.  Using one does mean acquiring a TM500 series power supply mainframe though.

In regard of the mso5000 with its high sample rate of 8GSa/s I am not sure if in the balance It is an advantage due to its apparently noisy front end.
As an ignorant, at the beginning I thought: the Rigol is better because It has better specs, so I bought it. Could you suggest me a different model if you think It could be better?

Noise specifications are generally lacking so it is difficult to make a recommendation based on them.

Something to consider about the 70 MHz Rigol MSO5072/MSO5074 is that it is bandwidth upgradable to 350 MHz via firmware meaning that it has the higher noise 350 MHz front end whether the bandwidth is limited to 70 MHz or not.  So it must have a higher noise than the example I gave of 100 MHz oscilloscopes.

What is going on here is that for a given device technology, higher bandwidth yields higher noise density.  So for instance a JFET front end which supports a bandwidth of 100 MHz could have a noise density of 3.5 nV/Sqrt(Hz), while a JFET front end which supports 350 MHz would have a noise density several times higher, whether the bandwidth is limited or not.  For the same bandwidth, a lower bandwidth oscilloscope can have lower noise than a higher bandwidth oscilloscope.

I suspect that is a major part of what is going on with the relatively high noise of the MSO5072/MSO5074.  Its noise should be compared to other 350 MHz instruments even though it is limited to 70 MHz.

[bdunham7]

It is possible but as I pointed out, noise is not the only consideration.  My preferred oscilloscope has about 120 microvolts RMS input noise because it comes with high input common mode range and differential inputs which are more useful, similar to the AM502 mentioned below.

I suspect that is a major part of what is going on with the relatively high noise of the MSO5072/MSO5074.  Its noise should be compared to other 350 MHz instruments even though it is limited to 70 MHz.

I also would put up with a bit of noise to get differential inputs.  The isolated scopes that I have are also a bit noisier and I don't complain.

As for the Rigols, I really think the issue is how they manage the lowest ranges by using a digital expansion of a higher range.  Other scopes would have an additional 10X analog gain.  The scope I used for comparison is 200MHz+, the more comparable SDS2000X+ models are 500MHz+ and have slightly better noise levels than mine.  Comparing 300-500MHz class scopes with the old Tek 2465B that I have--which gets very close to what you said is the ideal noise level-- the better Siglent is maybe 50% noisier (hard to compare because the Tek doesn't have 1mV or 500uV/div settings), the cheap Siglent is 2X as noisy and the Rigol is about 8X.  It's like going from a fine-tip ball point pen to a Sharpie and then to a highlighter or magic marker.   Of course that is not using  ERES or 10-bit or any other DSO tricks. 

[G0HZU]

My background is RF so I generally don't use a scope very often but when I do it is nice to look at small signals.

I have a really old HP Infinium digital scope with 500MHz bandwidth and it has the noise performance I would expect. At full 500MHz bandwidth and with the input set to 50R termination and 1mV/div it shows about 100uV rms noise when the Vrms measurement is enabled. When fed with noise as the signal under test it can typically measure wideband or narrowband noise signals with acceptable results down to about 200uVrms. This scope is fairly limited in terms of features (and memory depth) compared to modern scopes but it does at least have the noise performance I'd expect from a scope like this.

[knudch]

Random Noise is one thing..

If you @Fiorenzo would try on your Rigol a thing like this:
(done 50ohm termination)

Discrete "false signals is something else

[G0HZU]

Here's my old HP Infinium scope set to 1mV/div with the 30MHz bandwidth limit enabled.

The Rigol scope was showing about 1mVpkpk  on a 20MHz bandwidth setting which seems really noisy in comparison to my HP scope from the 1990s. 

My advice to Fiorenzo is to consider something else if the poor noise performance of the Rigol is bothering you. That level of noise would put me off buying a Rigol scope although I doubt I would ever buy one anyway.

[Fungus]

Here's my old HP Infinium scope set to 1mV/div with the 30MHz bandwidth limit enabled.

The Rigol scope was showing about 1mVpkpk  on a 20MHz bandwidth setting which seems really noisy in comparison to my HP scope from the 1990s. 

What's the bandwidth/sample rate of that? The Rigol has 350MHz pathways and is sampling at 8GHz which inherently produces noise, no way around it.

Does your HP have waveform averaging mode?

At least it looks like your HP can zoom out.

[David Hess]

I have a really old HP Infinium digital scope with 500MHz bandwidth and it has the noise performance I would expect. At full 500MHz bandwidth and with the input set to 50R termination and 1mV/div it shows about 100uV rms noise when the Vrms measurement is enabled. When fed with noise as the signal under test it can typically measure wideband or narrowband noise signals with acceptable results down to about 200uVrms. This scope is fairly limited in terms of features (and memory depth) compared to modern scopes but it does at least have the noise performance I'd expect from a scope like this.

When a low impedance input is used, higher bandwidth oscilloscopes bypass the high impedance buffer and without that, the input noise can be much lower.  The high impedance buffer has to be bypassed because at some point it will not have enough bandwidth.

An example of this in old oscilloscopes is the venerable Tektronix 485 which was built at a time when the fastest high impedance buffers were 250 to 300 MHz.  In 50 ohm mode, instead of inserting a 50 ohm feedthrough termination before the high impedance buffer, a coaxial relay directs the signal around the high impedance buffer.

Later oscilloscopes managed high input impedance up to 500 MHz and I think some now manage 1 GHz.

[G0HZU]

What's the bandwidth/sample rate of that? The Rigol has 350MHz pathways and is sampling at 8GHz which inherently produces noise, no way around it.
Does your HP have waveform averaging mode?

I'm not quite sure what info you are asking for but this is a really old HP scope using very dated technology. The basic specs are 2GSa/s and 500MHz bandwidth across 4 channels. Yes it has an averaging mode but it isn't turned on. The 30MHz bandwidth limit is turned on for the screenshot in my previous post.

[G0HZU]

When a low impedance input is used, higher bandwidth oscilloscopes bypass the high impedance buffer and without that, the input noise can be much lower.  The high impedance buffer has to be bypassed because at some point it will not have enough bandwidth.

An example of this in old oscilloscopes is the venerable Tektronix 485 which was built at a time when the fastest high impedance buffers were 250 to 300 MHz.  In 50 ohm mode, instead of inserting a 50 ohm feedthrough termination before the high impedance buffer, a coaxial relay directs the signal around the high impedance buffer.

Later oscilloscopes managed high input impedance up to 500 MHz and I think some now manage 1 GHz.

If it helps, I can switch the scope to 1Meg input and attach a 50R load and it looks pretty much the same. There might be a tiny bit more noise but any change is barely perceptible.

[oz2cpu]

some scopes are just badly designed, full of switchmode own noise..
the good old Rigol 1054 that we almost all owned ..
here i posted a few pictures, look and cry

https://webx.dk/rigol/

[Fungus]

If it helps, I can switch the scope to 1Meg input and attach a 50R load and it looks pretty much the same. There might be a tiny bit more noise but any change is barely perceptible.

There's no point. Everybody here knows (and freely admits) that lower noise oscilloscopes exist. They make for pretty screenshots and youtube videos.

The questions is: How much advantage does it give you in real life?

For digital signals? None at all. For that you need bandwidth and high sample rates which is where the MSO5000 shines.

For periodic signals in the mV range? I suspect the answer is "not much if you use averaging", hence me asking if anybody can post a picture of a Rigol MSO5000 showing power supply ripple with 1x probe, 20MHz limiter and waveform averaging. (Prove me wrong!)



For non-periodic signals in the mV range? You'd have an advantage there but they're few and far between and you're probably better off looking for them in FFT mode than trying to trigger on them and view them as a trace.

nb. For any signal in the mV range you can add a signal amplifier. They sell 30dB amplifiers on aliexpress for not much money.

[Kleinstein]

The x1 probe is not always an option: it is slow (e.g. 5-10 MHz BW) and quite some load (e.g. 100 pF range).
With a x1 proble the probe will limit the BW, but it still makes sense to enable the 20 MHz limit to reduce amplifier noise, as there is essentially no signal > 20 MHz anyway.

With a non periodic signal the FFT is not an option. It would not give much (if any) useful information.
Averaging only works well if one has a good signal to trigger from - so if there is only a small / noisy signal this will not help.
Noise in the trigger signal can smoothen out the signal with averaging.

With the rigol scope still at hand, one could measure the noise, to see if it is really much worse, or just looking higher noise with higher sampling rate.
A point to compare would be with a short (input to GND), a time scale to get a comparable sampling rate (e.g. 1 Gs/s and thus BW limited by the sampling rate to 250 MHz) with only 1 channel active and than a high gain (e.g. 1 mV/div or 5 mV/div before the probe setting).

Some digital signals like LVDS are not that large: 400 mV at the input would be 40 mV after a 10:1 probe and thus may want 5 mV/div sensitivity.

[Kleinstein]

Here's my old HP Infinium scope set to 1mV/div with the 30MHz bandwidth limit enabled.

The Rigol scope was showing about 1mVpkpk  on a 20MHz bandwidth setting which seems really noisy in comparison to my HP scope from the 1990s. 

What's the bandwidth/sample rate of that? The Rigol has 350MHz pathways and is sampling at 8GHz which inherently produces noise, no way around it.

Does your HP have waveform averaging mode?

At least it looks like your HP can zoom out.

The picture show 10 Ms/s. So there is anditional BW limit (~ 5 MHz) there. To do a fair comparison one would have the switch the faster scope also a slower hirizontal rate to get the lower sampling rate.

[Fungus]

Averaging only works well if one has a good signal to trigger from - so if there is only a small / noisy signal this will not help.

If the ripple is too small to trigger from then the power supply is probably OK and there's nothing to worry about.

[oz2cpu]

my siglent, looks like same signal as yours fungus
clearly alot less noise, but this is only a 2GS scope :-)

[Kleinstein]

Averaging only works well if one has a good signal to trigger from - so if there is only a small / noisy signal this will not help.

If the ripple is too small to trigger from then the power supply is probably OK and there's nothing to worry about.

It is not just triggering, just a stable triggger to see fast parts too.
Often there is a stable trigger available, but one may have to find it. An extra trigger and than looking at the ripply would even separate contricbutions to the ripple if there are multiple asyncronous parts (e.g. mains and a SMPS).

It really depends on how much ripple you still care. Sometime 0.1 mV could be too much.

[oz2cpu]

1mV div, here is where you really see the internal noise :-)

[G0HZU]

The picture show 10 Ms/s. So there is anditional BW limit (~ 5 MHz) there. To do a fair comparison one would have the switch the faster scope also a slower hirizontal rate to get the lower sampling rate.

I'm not sure there will be a 5MHz bandwidth limit. With the 30MHz limiter enabled I think the bandwidth limit for signals is still 30MHz on this scope even at low sample rates. One would have to be wary of aliasing but the signal being viewed here is noise.

[Fungus]

It is not just triggering, just a stable triggger to see fast parts too.
Often there is a stable trigger available, but one may have to find it.

Yep. No arguments there. That's why I was wondering if OP can actually do it in practice.

He already posted a screenshot of his ripple here so let's see what the 'scope is capable of. It looks like a triggerable signal to me but I don't have an MSO5000 to play with..

[nctnico]

If it helps, I can switch the scope to 1Meg input and attach a 50R load and it looks pretty much the same. There might be a tiny bit more noise but any change is barely perceptible.

There's no point. Everybody here knows (and freely admits) that lower noise oscilloscopes exist. They make for pretty screenshots and youtube videos.

The questions is: How much advantage does it give you in real life?

For digital signals? None at all. For that you need bandwidth and high sample rates which is where the MSO5000 shines.

For periodic signals in the mV range?

You keep taking the wrong turn when only focussing on small signals. Your screenshot clearly shows a wide band of noise where any detail on any signal level is lost. The noise is easely 40% of a division. With 8 divisions and 8 bits you have 32 LSB per division but with 40% noise you might as well have a 5 bit ADC and still get the same result. Needing to use averaging or high-res mode is just a crutch when the noise comes from the DSO itself.

There are so many lower noise alternatives out there from GW Instek, MicSig and (if you have the budget) R&S as well that it makes no sense to buy a noisy Rigol at all.

[Fungus]

There are so many lower noise alternatives out there from GW Instek, MicSig and (if you have the budget) R&S as well that it makes no sense to buy a noisy Rigol at all.

Except for the Pesky Fact that OP says he mainly does digital stuff and nobody else makes a 4 channel, 350Mhz, 8GSample/sec 'scope for only $1000.

To me it seems like you're arguing that a car with leather seats will allow people to carry more shopping.

[Performa01]

I think it’s time to do some myth-busting – and providing some hard evidence instead of presenting just wild guesses.


Myth #1: “Noise is only important when using x1 probes. It is irrelevant when using the much more common x10 probes, because the high source impedance will exceed the noise of the DSO anyway.”

Nothing could be further from the truth. The only relevant effect is the attenuation of the probe, which requires an appropriate vertical gain on the DSO to compensate for it. And of course this won’t matter much at the low sensitivities, i.e. the noise will be about the same with a x1 probe at 10 V/div, a x10 probe at 1 V/div and a x100 probe at 100 mV/div. But if you happen to work with signals much lower than 80 Vpp, you will notice that 1 mV/div with a x100 probe will be noisier than 100 mV/div with a x1 probe and that in a scenario like this, a proper low noise frontend will be much more pleasant to work with. You sure don’t want the excessive noise of an e.g. Rigol MSO5000 when working with x100 probes (e.g. in vintage tube gear).

The output impedance of a x10 probe is dominated by an output capacitance of about 100 pF, therefore the noise bandwidth is only about 1.6 kHz. So except for very low frequencies <100 kHz, we won’t see any significant difference in a properly designed general purpose DSO frontend, whether the scope input is left open, terminated by 1 M or 50 ohms or shorted to ground.

For low frequencies, things are a lot more complex than just a FET buffer, because of the split path design of all contemporary wideband frontend designs. The practical consequence is, that general purpose (wideband) oscilloscopes generally aren’t well suited for low frequency tasks below about 10 kHz regardless of the probes used. There are specialized instruments for this.

Look at the first two screenshots attached. They show the noise spectrum up to 1 GHz of the Siglent SDS2354X (570 MHz bandwidth). First with the input left open in high impedance mode, then the input internally terminated by 50 ohms. There are minimal changes of the spurious signals (because of the different contributions of voltage- and current effects), but the noise changes by less than 1 dB within the 570 MHz bandwidth of the scope frontend.

SDS2354X Plus_FFT_Noise_1M_ BW570M_8bit
SDS2354X Plus_FFT_Noise_50_BW570M_8bit

Btw: please notice, that up to 1 GHz there are few spurious signals and no spur is exceeding -120 dBV, which is equivalent to 1 µVrms. This is another important aspect, because strong spurs near the signal frequency can be at least as annoying as excessive noise.


Myth #2: “Frontends with higher bandwidths are always noisy, even when bandwidth limited.”

In any modern DSO, the bandwidth limit is an integral function of the PGA – and it sits at its output. So all the input noise gets filtered before the ADC. Of course this is only a first order RC-filter, because other than some popular believe, there is also no such thing as an effective AA-filter (Anti Aliazing filter) in a serious DSO. The most important property of any DSO frontend that is not a toy is constant group delay, and this rules out any “effective” AA-filter.

But even with a humble first order filter, the effect of noise reduction is quite obvious.

Next screenshot shows the noise floor at 50 ohms input termination again, but this time with 200 MHz input bandwidth limit. Btw, Siglent scopes show all relevant information on the screen, so screenshots should be pretty much self-explanatory.

SDS2354X Plus_FFT_Noise_50_BW200M_8bit

Compare this with the previous screenshot. At 110 MHz, we already have a difference of 0.8 dB. At 340 MHz it is nearly 6 dB and 7 dB at 560 MHz. That is a difference, isn’t it?

The next screenshot demonstrates what happens if the common 20 MHz bandwidth limiter is activated.

SDS2354X Plus_FFT_Noise_50_BW20M_8bit

At 20 MHz, noise is 2.7 dB down, we get -10.9 dB at 110 MHz, -16 dB at 340 MHz and -17.6 dB at 560 MHz.

The SDS2000 series has an excellent software enhanced 10 bit mode, which limits the bandwidth to 100 MHz and lowers the noise floor even more. See the next screenshot.

SDS2354X Plus_FFT_Noise_50_BW100M_10bit

With this setting, the noise floor fell below the -150 dBV above 200 MHz, so the reference level of the spectrum alanysis had to be adjusted accordingly. Little change up to 110 MHz, but -18.7 dB at 340 MHz and -26.8 dB at 560 MHz make this an excellent low noise mode in applications where 100 MHz bandwidth is sufficient.

Of course we can use the 20 MHz bandwidth limiter here as well, as the next screenshot demonstrates.

SDS2354X Plus_FFT_Noise_50_BW20M_10bit

At frequencies above 20 MHz, noise drops dramatically: About -13.5 dB at 110 MHz, -28.8 dB at 340 MHz and -31.6 dB at 560 MHz.

So this proves that it has nothing to do with the genuine bandwidth of the frontend or any other signal paths. I can easily demonstrate that e.g. a modern 2 GHz DSO like the SDS6204 behaves no different in this regard. The following screenshot shows a noise plot of the 2 GHz scope that can be compared to the very first screenshot in this posting.

SDS6204_FFT_Noise_1M_BW2G_D1G

This is in high impedance mode with open input. Once again, the noise with internal 50 ohms termination is very similar. More interesting is the comparison of this 2 GHz scope with the SDS2354X Plus. Even though the high bandwidth scope produces more spurs (but only very few of them slightly exceed 1 µVrms), the noise is comparable or mostly even better than on its 500 MHz counterpart – within the operating bandwidth of the latter, that is. Of course, at 840 MHz the SDS2304X Plus is already in the stopband of the frontend and noise drops significantly, whereas the SDS6000 has not even reached half its bandwidth, so its noise at that frequency has to be in the same ballpark as the other measurements before.

What these screenshots also reveal, is that the sample rate does not affect (excessive) frontend noise. If anything, higher sample rate helps to reduce noise. The 5 GSa/s, 2 GHz scope produces less noise than its 2 GSa/s, 500 MHz counterpart.

There is the ADC noise itself, which is the granular noise determined solely by the ADC resolution, not the sample rate. A higher sample rate will spread out the noise energy over a wider bandwidth, but the total energy will remain the same. So if only a limited part of that bandwidth is observed (i.e. FFT zoom feature), only a part of the noise is visible, hence will appear lower than the total noise actually is.

High sample rates also do not accentuate high frontend noise.

If the sample rate is excessive with regard to the bandwidth of the frontend, it will just produce redundant data and pointlessly eat up sample memory with little to no effect on the result.

If, on the other hand, the bandwidth of the frontend exceeds half the sample rate, the noise portion above Nyquist will be aliased back into the Nyquist bandwidth, hence we’ll still get all the frontend noise, even with inadequate low sample rates.

[G0HZU]

If, on the other hand, the bandwidth of the frontend exceeds half the sample rate, the noise portion above Nyquist will be aliased back into the Nyquist bandwidth, hence we’ll still get all the frontend noise, even with inadequate low sample rates.

Yes, that's what I find with my old HP scope here when the 30MHz bandwidth limit is selected. It still measures the noise level fine even at very low sample rates. It doesn't matter at all because the signal being measured is noise. The same applies if I deliberately feed wideband noise to the scope front end and use a very low sample rate to try and measure the Vrms of the noise. As long as the bandwidth of the external noise signal is less than the 30MHz bandwidth of the scope it should measure the Vrms quite well even at 100kSa/s.

[bdunham7]

Myth #2: “Frontends with higher bandwidths are always noisy, even when bandwidth limited.”

I think the statement, at least the one I'm thinking of, was that the noise density was higher for higher-BW capable amplifiers.  The noise will still be a function of the noise density and the actual bandwidth, so limiting BW will  still reduce noise as expected.

In any modern DSO, the bandwidth limit is an integral function of the PGA – and it sits at its output. So all the input noise gets filtered before the ADC. Of course this is only a first order RC-filter, because other than some popular believe, there is also no such thing as an effective AA-filter (Anti Aliazing filter) in a serious DSO. The most important property of any DSO frontend that is not a toy is constant group delay, and this rules out any “effective” AA-filter.

I believe that many very-high-BW scopes now have more than a first-order roll off and thus a flatter frequency response but poorer step response. IIRC either Tek or HPAK or both had/have models where you specify which way you want it as a factory option.  I don't think this is specifically for anti-aliasing, but it will have that effect.  I can't be very specific because stuff like that doesn't get into my hands much--I have to read about it here on EEVBlog.

[Performa01]

Myth #2: “Frontends with higher bandwidths are always noisy, even when bandwidth limited.”

I think the statement, at least the one I'm thinking of, was that the noise density was higher for higher-BW capable amplifiers.  The noise will still be a function of the noise density and the actual bandwidth, so limiting BW will  still reduce noise as expected.

Well, the FFT plots in my screenshots show nothing but the noise density. Of course, the total noise is actually higher for the 2 GHz instrument at full bandwidth than what it could ever be on the SDS2kX Plus.

I strongly suggest that, other than in the seventies of the last century, for modern semiconductors the noise density remains fairly constant over frequency. In my screenshots it can be seen that it gets rather lower at higher frequencies and four times the system bandwidth doesn’t mean higher noise density at all.

In any modern DSO, the bandwidth limit is an integral function of the PGA – and it sits at its output. So all the input noise gets filtered before the ADC. Of course this is only a first order RC-filter, because other than some popular believe, there is also no such thing as an effective AA-filter (Anti Aliazing filter) in a serious DSO. The most important property of any DSO frontend that is not a toy is constant group delay, and this rules out any “effective” AA-filter.

I believe that many very-high-BW scopes now have more than a first-order roll off and thus a flatter frequency response but poorer step response. IIRC either Tek or HPAK or both had/have models where you specify which way you want it as a factory option.  I don't think this is specifically for anti-aliasing, but it will have that effect.  I can't be very specific because stuff like that doesn't get into my hands much--I have to read about it here on EEVBlog.

Yes, the genuine bandwidth of most scopes is not first order gaussian – unless they are artificially bandwidth limited. This is also why we don’t get ideal pulse response characteristics and vendors have to specify some overshoot.

There can be all sorts of filters in high end scopes – mainly for pulse response equalization, but you can have higher order AA-filters as well. But this is limited to filters with Gaussian and Bessel characteristics, where the transition from the passband to the stopband is very smooth, so you still need substantial oversampling in order to get a useful attenuation to fight aliasing. Nothing gained for todays top models within a series, where the bandwidth is not at least five times lower than the sample rate.

Furthermore, higher order filters can get tricky with regard to their sensitivity to component tolerances (and their temperature stability) and they might need alignment in the first place. All this increases effort and costs and puts the risk of different frequency/phase response in different DSO channels.

This is probably why standard PGAs for “normal off the shelf” instruments have the bandwidth limiter integrated and designers have decided that a first order RC-filter is the best compromise.