uCurrent for uVolt measurement

[electrolust]

Super noob here!

This is as much for a practical project as it is to learn something along the way.  So even though in practice one might not go about things this way, I think it's valuable for me as a learning experience.

I have built a two stage 100x (10x10) audio amplifier to boost dynamic mic signals.  This seems like a simple circuit and so it should be obviously correct.  Still, I want to verify it on the scope.  Also I want to be able to troubleshoot a non-working device.

For initial testing I'm expecting to start with about a 60uV signal and expect to amplify it to 6mV.  I may need to get this up to about 40mV, I'll have to see as the project develops.  In that case I'll just add another gain stage.

So I figure I will apply a ref signal 1k hz sine wave.  I'm hoping to use my function generator and add a 40dB pad.  I think I can just generate (eg) a 40 mV sine wave and insert 20dB attenuation and verify I see 4mV on the DVM and scope with a direct measurement.  Once I verify how much attenuation I am getting from each pad stage I will generate (say) a 6mV signal and pad it to 60uV as my input signal.  I don't expect to be able to generate that (60uV) signal (too small) which is why I'm measuring the attenuation of a bigger signal for verification then applying that attenuation to a signal in the range that I can reasonably generate.  (Unless you can point me to a cheap enough function generator that can produce microvolt signals directly.)

Now with my 60uV test signal I can at least measure the final output of the amp at 6mV.

But I'd like to measure the first stage output of 600uV, and also I'd like to measure a REAL microphone input signal and see what I'm really starting with.  Clearly, I don't NEED to do that as I can infer the starting level from the final level.  But it would be great to see the mic output level directly.

It looks like I can use the uCurrent device and get about 100x gain on the source signal.  That seems perfect.  I certainly can't afford something like the ADA400A probe.  Is the uCurrent going to work for me?  Any other affordable products out there?  I won't be able to DIY something, just too time consuming with my level of experience and a distraction from my main objective.

[kolonelkadat]

Im no expert on the subject, but I dont think the uCurrent does what you think it does. I dont have the schematic handy but I want to say its just a precision 10k resistor and  a couple of amplifier stages so that you can measure current with a volt meter.

[rs20]

Im no expert on the subject, but I dont think the uCurrent does what you think it does. I dont have the schematic handy but I want to say its just a precision 10k resistor and  a couple of amplifier stages so that you can measure current with a volt meter.

The uCurrent is a precision resistor (3 different ones can be selected), followed by a 100x amplifier (in two stages). Dave said himself that when the 10k resistor is selected, it is just a 100x voltage multiplier (with a 10k resistor shunt to ground).

OP said that he wants to use the uCurrent to give a gain of 100x.

So I think the uCurrent does exactly what the OP thinks it does...

[singapol]

You already have a function generator, it boils down to how good  it is.That is, how small a "clean" signal it can produce and the attenuation available. Next you need a good/accurate multimeter, preferably one that can give true rms voltage reading to verify this output. Take note that the meter will have an ac voltage frequency limit.

[electrolust]

I hadn't considered the frequency limit of my DVM.  I have a Fluke 28 II.  In the manual I don't see the upper limit in a glance (not looking too hard), but there is a switchable LPF @ 1k Hz, which happens to be the frequency I intend to use.  The chart that goes along with the function description shows flat response to 1k and then it starts to roll off.

I also have a Fluke 115, it says it goes to 5kHz.

So I guess either should be ok.

[electrolust]

Just to follow up, this worked great.

I have 40dB of pad on a 1kHz 10mVpp sine wave signal, then through the uCurrent at the nA setting.  The DVM reads back 7.6mV RMS, nearly perfect.

The unattenuated and unboosted 10mV signal reads nearly perfect 7.07mV RMS, as does an attenuated 100mV signal.  So the error would appear to be in the uCurrent.  (It's documented to have some error.)  If I cared to be that precise I could characterize the error a bit better.  For my use it's not so important.