ADC for wireless unit

[brennen]

Hey, guys. I'm a first time poster, but a long time reader, so be easy on me . I am currently trying to make a wireless transmitter for my guitar. I want an ADC that is hopefully low-power, but still has good specifications. The ADC I am currently looking at is the Wolfson WM8737, as it is low-power and has a decent S/N ratio. Other than the fact that the ADC be low-power, I also need it to output in DSP mode to work with the uC I'm planning on using. Analog Devices' AD1871 can output in DSP mode, but I can't seem to find the power dissipation in their specs. Any suggestions?

[rfbrw]

I make it 290mW for the AD1871. Unless you plan to build your circuit deadbug style, working with the QFN package the WM8737 comes in may prove difficult, expensive or both.

[brennen]

Thanks for your reply. I'll probably get a schmartboard to prototype with the Wolfson unit if I use it. Do you think that 290 mW will eat a 9V battery pretty fast? Also considering I'll be using a 2.4 GHz wireless transmitter and a 60 MHz ARM processor. Something tells me I'll be draining it pretty fast, even if I put the processor to sleep as much as possible.

[FastEddy]

" ... Stereo 24-bit multi-bit sigma-delta ADCs are used with digital audio output word lengths from 16-32 bits, and sampling rates from 8kHz to 96kHz supported. ..." From your link ( http://www.wolfsonmicro.com/products/WM8737/ ) ... ... Good choice!

There is a page or three devoted completely to "Power Consumption" ... beginning on page 9 of the data sheet: http://www.wolfsonmicro.com/uploads/.../en/WM8737.pdf ... (it looks like the upper limit on power requirements is about 29 milliWatts = 3.3 VDC (with no mic or preamp) and "Stereo / Digital Mono Mix" or "Analog mono mix (with continuous dc monitoring via Right ADC) " ... Your results may vary.)


Hey, 1/30 Watt can't be too hard to deal with, right? ... This seems to be a lot better than the Analog part = longer battery life ...

[brennen]

Well, the Wolfson part overall seems to be the winner. The thing that I do give up is some quality, but I can sacrifice a little quality to avoid having batteries that last 30 minutes . Do you guys have any recommendations on low-voltage, low-power op amps that I could use as an input buffer on this guy? I'll probably be running everything at 3.3V. I am currently using Analog Devices' OP295 in another project, but I don't know if it would really be regarded as audio quality, since I'm only using it as a buffer to a non-audio signal.

[FastEddy]

" ... The thing that I do give up is some quality, but I can sacrifice a little quality to avoid having batteries that last 30 minutes ..."

I don't think you are giving up very much quality ... most recording studio equipment expects to "see" 24bit / 96K stereo digital info and +100 db range / resolution.

[rfbrw]

Originally posted by brennen
Well, the Wolfson part overall seems to be the winner. The thing that I do give up is some quality, but I can sacrifice a little quality to avoid having batteries that last 30 minutes . Do you guys have any recommendations on low-voltage, low-power op amps that I could use as an input buffer on this guy? I'll probably be running everything at 3.3V. I am currently using Analog Devices' OP295 in another project, but I don't know if it would really be regarded as audio quality, since I'm only using it as a buffer to a non-audio signal.

Not sure what sort of levels to expect from a guitar but there is every chance the WM8737's onboard mic preamp will render an external op-amp unnecessary.
As for power, if you opt for rechargeable NiMH AA batteries, 3 will get you 3.6volts at 2500mAh. Add an LDO regulator like the LTC1844 and you are done.

[brennen]

I don't think you are giving up very much quality ... most recording studio equipment expects to "see" 24bit / 96K stereo digital info and +100 db range / resolution.

The thing I was looking at with respect to quality is that the Wolfson part has a S/N ratio of 97 dB, whereas the AD part's is 105 dB. Also, the THD+N figure for the Wolfson part is 86 dB compared to 95 dB for the AD part.

Not sure what sort of levels to expect from a guitar but there is every chance the WM8737's onboard mic preamp will render an external op-amp unnecessary.

I'm pretty sure that a guitar's output is lower than a microphones, but I do know for sure that it needs a very high load impedance at the preamp (>= 1 Mohm). I'll try it out when I get the part to see if it will do the trick or not.

As for power, if you opt for rechargeable NiMH AA batteries, 3 will get you 3.6volts at 2500mAh. Add an LDO regulator like the LTC1844 and you are done.

Probably a good idea, but I already have some Li-ion batteries sitting around, as well as a charger IC. That, coupled with an LDO regulator, as you mentioned, should get me pretty well to where I need to be.

I've read before that most audio guys don't recommend using switching regulators because of the switching noise that can be induced into the rest of the circuit. I know that I could get more life out of a battery with a good switcher, but are there any that are quiet enough to use for this purpose?

[FastEddy]

FE: " "I don't think you are giving up very much quality ... most recording studio equipment expects to "see" 24bit / 96K stereo digital info and +100 db range / resolution. " "

brennen: " The thing I was looking at with respect to quality is that the Wolfson part has a S/N ratio of 97 dB, whereas the AD part's is 105 dB. Also, the THD+N figure for the Wolfson part is 86 dB compared to 95 dB for the AD part. ..."

My point being basically, no matter what the source, guitar or mic, the dynamic range of either the Analog or the Wolfson parts is plenty big enough for studio work. I believe that e. guitars and e. bass [the e. pickups] generally are in the 10 db to 70 db or 80 db range from very soft, almost inaudable to too loud to handle (the threshold of pain is considered to be above 120 db (absolute), dead quiet of a pine forest can approach 2 to 3 db (absolute) .... Examples for comparisons might be the sound pressure levels of acoustic string and wind instruments into the microphone(s) compared to direct connect electric guitars === differences of only a very few (undistorted) db.

The dynamic range of many modern 24 bit / 96K stereo D to A converters (in high quality play back) can approach 130 db ... meaning that as long as Your guitar ADC can deal with and deliver two channels of 24 bit / 96K with a dynamic range of 80 or more db >> into your computer hard drive capture or >> directly into digital studio recording ... the results should be more than satisfactory no matter which chip you use.