Which oscilloscope/pre-amplifier can measure microvolt impulses?

Thread Starter

Oscilloscope

Joined Jan 16, 2012
3
I need some advice on oscilloscopes/pre-amplifiers for measuring small impulses (microvolt).

I need to measure single impulses, typically of 10 microvolts in amplitude and a few milliseconds in length.

What oscilloscope should I buy? Would I need a pre-amplifier? If so, are there any inexpensive pre-amplifiers available?

Most PC (USB) oscilloscopes (in the price range of €150-500) I have seen have input ranges down to 10 millivolt/Div and a resolution of 8 bit (256 levels). As I understand from this guide, this means the lowest voltage that can be measured with such oscilloscope is 10 millivolt / 256 = 39 microvolt.

Is this true or do oscilloscopes and pre-amplifiers have a minimum threshold, below which they cannot measure/amplify a voltage? I mean, can I measure anything the oscilloscope can display?

Probably I would need to be able to measure with a sensitivity of something like 0,5 microvolt or at least 1 microvolt.

Which model (oscilloscope and pre-amplifier) can I buy that is not too expensive?

I figure minimum quality if I buy an oscilloscope would be 2 (or 5) millivolt/Div and 12 bit (gives 0,5 microvolt sensitivity). But I haven’t found any model like this!

Or I could buy a preamplifier that amplifies the signal at least 100 times (20dB) (i.e. 0,39 microvolts is amplified to 39 microvolts).

The only "oscilloscope amplifier" I have found is this one, which can amplify 1000 times but also cost about 1000 bucks. :(

Can I use any amplifier, something simple like this one or this one (meant as a guitar amplifier, I guess)?

My idea is to use a second channel to measure the noise and just substract that from my real impulse (standard for the kind of measurement I want to do). Would this still work if I use two different pre-amplifiers (one pre-amplifier for each channel)?

I would be very grateful if you could verify if my understand is correct (e.g. there are no minimum thresholds other than resolution) and tips on specific models. Should I go for a pre-amplifier or find a better oscilloscope?

Thanks!
 

MrChips

Joined Oct 2, 2009
20,866
Most oscilloscopes will measure down to 10mV and give only 8-bit resolution (DSO).
You will need to amplify your signal by at least 1000. Maybe 10,000 to get a decent reading.
What is the width of the pulse?
 

jimkeith

Joined Oct 26, 2011
540
In my opinion, a USB scope with a preamp would be good for your requirement. Now there are some good and some poor USB scopes--I have used both--the poor ones are basically unfriendly in the triggering or in the PC software. Unfortunately, I cannot recommend a make or model from memory.

None would come close to the sensitivity you need.

Since you do not need DC coupling and your bandwidth is low, an audio preamp is the way to go. However 10uV sensitivity puts you down into the noise range. For instance, a microphone has typically 1mV output and a good S/N ratio would be 60db--that puts the noise at about 1uV. I believe that something like this will do OK. Since they are so cheap, a little experimentation would be inexpensive.

One thing not mentioned is input impedance--these audio amps may not have high input impedance like the 1M typical scope input--10M with a x10 probe.

I definitely would not go with an expensive scope.
 

Thread Starter

Oscilloscope

Joined Jan 16, 2012
3
Most oscilloscopes will measure down to 10mV and give only 8-bit resolution (DSO).
You will need to amplify your signal by at least 1000. Maybe 10,000 to get a decent reading.
What is the width of the pulse?
The pulses are about 2 milliseconds long. Could be some variance of course, but we are talking milliseconds, for sure.

Amplifying by 10.000 times should work in the same amplifier (40 dB) that I mentioned above.

The problem might be noise.... if noise is 1uV, as someone mentioned, I might be in trouble... but I know it can be done, as others are doing it.
 

Thread Starter

Oscilloscope

Joined Jan 16, 2012
3
In my opinion, a USB scope with a preamp would be good for your requirement. Now there are some good and some poor USB scopes--I have used both--the poor ones are basically unfriendly in the triggering or in the PC software. Unfortunately, I cannot recommend a make or model from memory.

None would come close to the sensitivity you need.
I found some that are 16bit... PicoScope 4262 (10mV, 16bit gives sensitivity to 0,2 uV) and Metrix MTX1052-PC (0,25mV, 9bit, sensitivity of 0,5uV)...

They are a bit over my budget both at about EUR900... but I haven't looked at that many models yet. Maybe I find something cheaper


Since you do not need DC coupling and your bandwidth is low, an audio preamp is the way to go. However 10uV sensitivity puts you down into the noise range. For instance, a microphone has typically 1mV output and a good S/N ratio would be 60db--that puts the noise at about 1uV. I believe that something like this will do OK. Since they are so cheap, a little experimentation would be inexpensive.
60dB would give a factor of 1 000 000, wouldn't it? that would be nano volt.
But I think it is quite resonable with noise in uV...

I think this is good advice (considering my budget, lol).

The audio preamps I've seen cost about $10-20...

Will I be able to subtract noise if I use two different preamplifiers though? I suspect they might not give the exact same amplification...

One thing not mentioned is input impedance--these audio amps may not have high input impedance like the 1M typical scope input--10M with a x10 probe.

I definitely would not go with an expensive scope.
Will this be a problem? what can I do about it?


I guess a budget USB oscilloskop and pre-amplifiers is a good start. If that doesn't work, I switch to a 16 bit one. The agilent that was mentioned were to pricey for me, seemed very good though.
 

MrChips

Joined Oct 2, 2009
20,866
Forget about the number of bits and pre-amplifiers. First you have to say what you are measuring, where is the signal coming from, what is your detector?
 

jimkeith

Joined Oct 26, 2011
540
60dB would give a factor of 1 000 000, wouldn't it? that would be nano volt.
You are getting power mixed up with voltage
60db is a voltage ratio of 1000, but a power ratio of 1,000,000

Tell us what type of circuit are you scoping out?
This will help determine what probe impedance is necessary.
 

Adjuster

Joined Dec 26, 2010
2,148
It is not really possible to truly "subtract" noise, although it is possible for a pulse detection system to set its sensitivity relative to the mean noise level. I think this would normally be done relative relative to the noise in the measuring channel itself.

What is this for, some kind of ionisation detector?
 

crutschow

Joined Mar 14, 2008
24,709
The minimum threshold for noise is determined by the circuit impedance and the required bandwidth. So what is the source of your signal and what is its impedance?

We need to know that to provide a good answer to your question.
 
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