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Old 5th October 2001, 03:50 AM   #1
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Ok, this is a very broad question, but what is the I/V stage of a DAC and why do I need one? Can i feed the output of the DAC directly into a preamp?
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Old 5th October 2001, 04:21 AM   #2
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Another stupid question, how do i obtain an evaluation board from a big company like TI or Crystal? After looking at these, i realized they are exactly what i need. Do you have to be a company to get one, or can i just buy one? hwo do i have to bribe to get one? j/k...
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Old 5th October 2001, 11:30 AM   #3
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Default Demo Boards

Most companies will sell a demo board (or even give them away if cheap enough). Check the websites. I know TI and AD will both do it. Also check the sites for "classes" on thier products - TI just came through town demo'ing new OPAMPS from them and Burr-Brown. Lots of good info, no samples.
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Old 5th October 2001, 03:01 PM   #4
paulb is offline paulb  Canada
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Surprised nobody else answered this yet...
I think the output from most DACs is a current that has to be converted to a voltage for your preamp. A resistor will do this, but you have to buffer it so it doesn't get loaded down by the preamp.
There should be info on this in app notes from Burr-Brown (TI) or others.
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Old 6th October 2001, 04:42 AM   #5
hifiZen is offline hifiZen  Canada
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Some companies are very averse to dealing with individuals, others don't care, and still others don't bother to really check if what you put into their "online registration" is true. So, it's often a good idea to pretend your an employee at some random fictional company. However, if they start asking for all kinds of cray info and want you to pay for stuff with POs and the like, you're kinda stuck. Usually, TI will send out free samples without much info ...I once got 10, yes 10!, OPA-627BP opamps from them! ... i felt a little guilty after that one... Crystal rarely does samples (especially on parts that hobbyists like), and they make you jump through a few hoops to get them. I've not tried actually ordering parts from either of these guys, but i would expect the relative difficulty to be similar. For Crystal parts, expect to go through a distrib. like NuHorizons.

I have found that Analog Devices is the easiest to deal with. You can do everything over the web, just by registering with a phony business title and your real name. They have a good samples program, and you can order most of their eval kits and dev tools online with a credit card. I bought my DSP processors direct this way, since their distributors didn't have them in stock. Always fast and easy to deal with.
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Old 6th October 2001, 05:24 AM   #6
hifiZen is offline hifiZen  Canada
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Now, as for DAC I-V conversion...

Generally, a current cannot be used to drive another piece of audio equipment. So when a dac spits out current, we need to convert this to a voltage signal. In any case, most DACs aren't designed to directly drive any kind of a load. DACs usually have sensitive internal circuitry which could be upset by any kind of non-specified impedence connected to the output.

Burr-Brown R2R multibit dacs for instance have the current output you describe, and in order for the chip's current sources to operate correctly and consistently, the output should be held at ground potential. This is one reason I shy away from using a resistor for I-V conversion, since output voltage introduces non-linearity into the DAC. That is, the size of an LSB step changes as the output amplitude increases.

Most often, the current output is sent into a virtual ground node at the inverting input of an opamp. A resistor from opamp output to negative input generates the output voltage from the current, since the current flowing into the opamp input should ideally be zero.

Obviously, a high-grade opamp is required to extract the best performance for this rather delicate task... often favoured are the OPA627 and AD811. These are both low-noise, fast opamps, though the OPA627 should give a lower overall noise, depending on the value of the feedback resistor used (not that it matters much... either will give excellent performance). The AD811 is also a different type of opamp, though in practice, it is used the same anyway. A low-pass filter is a Good Thing(tm) to use after this stage, to remove high frequency components from the signal and prevent that trash from getting into your preamp and poweramp circuitry. Finally, an output buffer of some sort is necessary to drive the interconnect and preamp nicely.
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Old 6th October 2001, 12:01 PM   #7
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I think that you may be interested by Erno borbely ' s article abuot that soubject.Try at http://www.BORBELYAUDIO.com
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Old 19th October 2001, 08:07 AM   #8
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Hi,

You can purchase eval. boards from TI's or Cirrus' distributors. I believe you can even purchase directly from the TI's website if you reside in the US. I purchased the CBD 8420 upsampling and CBD 4397 DAC boards through Cirrus' distributor. Both are priced at $150 each.

I am making several changes to the 4397 board including using battery power supply, bypassing the op-amps and replacing some regulators.

Regards,
N.S. Ng
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Old 21st October 2001, 06:01 AM   #9
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Where can i buy these? do you have any links?
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Old 22nd October 2001, 02:23 AM   #10
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Also, has anyone made this DAC (http://www.geocities.com/d_broadhurst/pcbs.html) or heard it. it has the chipset i was already looking at all in one package... looks pretty nice with upsampling and 24/96 DACs. I also really like the jitter reduction scheme in Crystal Semi-conductor's CS8420.
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