What it is the max Current I can input into the mbed analog input pins, it says 15pf?? that's really low right?

I can't find a specific value on the site, but I've not searched the datasheets. I wouldn't suggest going above the 40mA ouput of each pin though, though I don't know if the mbed board has an internal current limiting component to prevent damage to the chip.

I also don't quite understand what you're asking. Ohm's Law (V = IR) tells us that current is equal to the voltage divided by resistance. If we presume the resistance of the mbed to be a constant, I don't forsee how (in normal circumstances) you could get a large current going into the mbed board (especially mbed would normally be the device supplying current, rather than drawing it, and definately not via the input pins) For that you'd need a high voltage, and the input pins only go up to 3.3V, though they are tolerant up to 5V. Any more and you'll risk damage to the mbed.

Also, current is measured in Amps, symbol A. Capacitance is measured in Farads (F) or picoFarads (pF). Pin capacitance is a different issue.

Hope this helped a little until someone with more knowledge and experience provides a better answer!

Jordan

The figure 15 pf is Input capacitance, not impedance. The input resistance should be > 10 K. Possibly > 100K. So nearly no current.

Ceri

How do I measure picoFarads, with a oscillscope or would a multimeter read that low, I wanted to connect a mic with a 1.27 ma current so I was very puzzled as to how I was supposed to get it down in the picoFarads, but I feel better now that I know I need to get it down to just 10k or 100k. It is fascinating how sound is capture and recorded and then played back almost unbelievable.

Also I don't understand why in the best equiment people use wima caps and smd resistors, I don't think they make Wima caps as low as 15 pf do they.

the thing that is almost unbelievable is that it all happens from the creation of electricty in volts and currents, recording sound and playing sound.

Have a look at http://www.zen22142.zen.co.uk/Prac/ecm.htm

There is a circuit for a microphone, The 1.23mA is the current through the microphone, not into the MBED.

it only just over 1000ohms the mic, I need to connect a 15 pf from p19 to GND before I connect the microphone right? I just went and got a 10pf got home and it has no polarity it is Nonpolarized, and I have not learn about Nonpolarized capacitors yet. So I don't know if it safe to just connect to p19 and GND

I plan on using op134 but the mountain is starting to ~~~me off

Hi Philips

Take a look at this video: http://www.youtube.com/watch?v=TQB1VlLBgJE

Regards,

Hello,

I must use 200 picofarad on analog output, can anybody tell me this, do I need to put a picofarad on the GND as well, I see they use smd picofarad in souncards.. So i use 200 picofarad and then err 200 UF wima cap or what should the wima cap be and hows is that calculated.

I cannot figure out your schematic (maybe you should draw one here and use "Insert images"), but generally a small cap between output and gnd is for cutting high frequencies from D/A conversion, and a high value (200uF) on output is for cut the DC component out of sound signal. Usually is calculated taking into account the input impedance of the amplifier, the output impedance of D/A converter and the lowest frequency of the sound signal.

Regards.

Don't have cash at moment, so I am not drawing schematic yet need a lot of things yet. But every sound card I look at has small Picofarad SMD's on them at the end right next to the micro controller connections, in fact anything that is most of the really good sound devices with micro controller I have seen always has a Picofarad. I am more interested on why the Picofarads and how do you know to use a 200UF WIMA cap after this.

it says here in LPC1768 manual, my guess is because micro controller are really sensitive so then need to be in the Picofarad any higher and we would get a lot of errors and wrong things happening. I am not student or anything like that so that is why you are getting question like these:)

I'm not sure on what to look at in your big red picture. :-o

CL - load capacitance = 200pF

RL - load resistance = 1kOhm

means that you need some amplifier having at least RL/CL input impedance.

That 200uF capacitor you are concerned about should be put on output of the power amplifier to connect a speaker in good condition.

This should be a good starting point: http://www.hobby-hour.com/electronics/lm386-power-audio.php

Hope that helped.

Regards.

Here is sound card I have had loads of fun with:) I just assumed that these are smd Picofarads and resistors, so unless Picofarads SMD look the same as other type of SMD's. But as you can see their is a lot of them around the IC chip.

Most of my designs are what I have learned from around this DAC, You see it has the Wima caps on and I still don't fully understand the design.

And this DAC

the back of above :)

Do you know of a chart, the mbed outputs in Picofarads right does it go Picofarad nanofarad then ultra farad I need chart pico is smallest right?

Can anybody tell me how many Picofarad the mbed analog out aout output on their scope my scope is old so it does not give the Picofarad Statistics .

it's 22pf if anybody wants to know:)

Hi Phillips,

You seem to be in a spin about capacitors!

You have probably already worked this out but the smallest caps are Pico-Farad (pF), then nano (nF), micro (uF) then milli (mF). Each one is 1000 times as bigger than the last. It is common for smaller caps (<1uF) to be ceramic caps (non-polarised).

I'm not an audio engineer, but I guess the reason the output capacitance in important is because when used with a resistor they form filters that will block certain frequencies, if these are <20kHz they will effect the quality of the sound.

Does this help?

yes but you must use pico caps first or else you lose audio.. like if I use a 2.2uf cap I lost audio but if I use 200 pico cap I do not.. now but still get crackling but fidelity is really cool.

must try 22pf you must use 22pf then maybe nano I don't yet to be continue........

One of the reasons for pico farrad capacitors is to stop the ringing on the rising & falling edges, This can have a massive effect on digital circuits. Causing multiple edges to be miss read.

As the voltage swings from low to high, overshoot occurs. If a very small capacitor is used, then the overshoot, it ringing can be dramatically reduced.

I had a project with CAN BUS running at 250Khz, that would only work with my oscilloscope connected. ...... because their is 22pf between probe and ground.

Hope this is usefull, If I can find it tomorrow, I have some screen shots Which might clear things up.

Ceri.

Yes it does and my friends say analog is even more sensitve when it comes to audio.

I'm not sure what are you talking about, mostly because you don't know what you are talking about.

ceri is saying that a CAN bus running at 250kHz(!) needed a 22pF capacitor to work properly and you are replying that audio is more sensitive.

How sensitive?

Audio (audible) frequencies are between 20Hz and 20kHz (at most). Some HiFi systems may have bandwidth beyond these limits in order to have lower distortions, blabla... but they are not going to work with 250kHz.

Martin Smith noticed well:

What is your real concern? Transitory intermodulation distortion (TIM)? See

Good luck.

Acutally ceri is saying that he put his scope probe on the mbed and the probe was a 22pf probe and a pieace of the mbed DIE became a capacitor. SO he is saying the 22pf reading is a false reading and is acutally just the probe which acted as a 22pf capacitor.

What I do know is that I am getting interference and you recommended a 200uf capacitor which made me lose a whole hell of a lot of fidelity.. and my 220pf wima sounds amazing. But I think the interference is from the usb cable or I don't maybe I just need to get some BLACK GATE N series electrolytic capacitor.

And what I was saying is my friend who I am sure knows what he is talking about says that digital is not as sensitive as analog when is comes to audio and capacitors.

But at the moment I lack the equiment I need.

The mbed outputs in Picofarads and you say a 200uf which is 200000000 picofarads so it will take how long for that 200uf capacitor to store some Energy, probably why their is audio loss right:)

Looks like it was the power supply the usb cable, got a cable, a usb cable with a ferrite bead problem made better. It only has one ferrite bead at the moment probably need a couple.

If you look on your scope on aout you will see the interfence when connect by a usb cable laptop must be connect to mains power supply cause that is where the interfencing is coming from the mains power supply electrically grid, if you run off laptop battery you will not see it as much.

I know of the ultravox, nothing else for cleaning dirty usb power supply

http://diyparadise.com/shop/index.php?main_page=product_info&products_id=124

I see that is a digital isolator then, maybe not so good for dirty usb power not sure.