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Old 18th February 2016, 11:30 PM   #1
donovas is offline donovas  South Korea
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Default help me with my dac design?

i'm trying to create an ad844 dac based on Using the AD844 as an I/V

rough block diagram as seen on the pic. currently only tried it on a breadboard but every iteration i've made so far has a lot of noise.

there's a high pitched hum when i2s is fed to the dac chips- otherwise the whole thing is silent when cs8412 is taken out.

then there's high pitched noise that modulates with high frequency signal- when 12khz, 14khz (and above) tones are played, 8khz (ish, i'm not so sure) noise is overlaid with the signal. this is also heard in the music, obviously.

then there's static noise with the right channel signal of the dac. this is a radio static-like noise that gets excited with the music. and no matter what i swap it is always with the right signal so the noise is in the signal domain- a design error, not a construction error.

as you can see in the diagram, i've tried to bias the - input of ad844 with the negative polarity of tda1387 in a differential mode, instead of zero dc offset then input to the ground as it is done in the original ad844 thread. i only get about 10mv offset at the pin 5 output of ad844. but with horrible sound as described above....

what have i done wrong? i've breadboarded george's ad844 implementations before and they all came out wonderful.

everything is properly decoupled on the board and the ad844 i/v resistors are paralleled with 1nf caps.

Click the image to open in full size.
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Last edited by donovas; 18th February 2016 at 11:34 PM.
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Old 19th February 2016, 12:41 AM   #2
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I might be able to offer some help but your schematic is too small to read.
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Old 19th February 2016, 09:16 PM   #3
donovas is offline donovas  South Korea
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Click the image to open in full size.
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Old 19th February 2016, 09:19 PM   #4
donovas is offline donovas  South Korea
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hopefully this works.

cs8412 i2s>hex inverter data flipflop> 2 tda1387 differential mode> signal r+/r- pin 2/3 on ad844

thank you in advance.
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Old 19th February 2016, 11:21 PM   #5
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There are some important details apparently missing there. Where are the resistors between the DACs and the AD844s? The DAC's current out so there's a need for a resistor to do I/V before its output can be fed into the +ve pin of the AD844.
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Old 20th February 2016, 04:13 AM   #6
donovas is offline donovas  South Korea
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Im using ad844 as transimpedance amp, the same way its used in ad844 i/v thread- except that the - pin is biased by the out of phase 1387, instead of going to ground.
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Old 20th February 2016, 06:04 AM   #7
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If you'd like help with your design you'll have to show all the details in your schematic.
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Old 20th February 2016, 06:17 PM   #8
donovas is offline donovas  South Korea
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sorry, abraxaito, i don't know how to draw schematics, and the implementation is crude as it is. it's basically differential tda1387 with ad844 i/v stage as described above.

Click the image to open in full size.

no different from this except pin 3 gets fed negative phase signal instead of going to ground, and no dc offset circuit.
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Old 21st February 2016, 12:00 AM   #9
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If you connect pin3 of the AD844 to one of the outputs of the TDA1387, you'll have a problem if there's no resistor to convert the I to V. Also probably the JFET current source is providing too much current - just a guess. A circuit for the TDA1541A isn't going to be directly compatible with TDA1387.
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Old 21st February 2016, 10:19 AM   #10
donovas is offline donovas  South Korea
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as you know, 1.5k ohm in that circuit above is the i/v resistor, the rest of the opamp being a current mirror(buffer?). but you're saying i should feed voltage into pin 3?

and im not using a jfet current source.
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