How can I improve my line out? - Page 1
Forum section: electronics / projects

author: Starlord
author_rank: Frequent Contributor
timestamp: January 12, 2015, 12:38:54 am
content: .

author: Starlord
author_rank: Frequent Contributor
timestamp: January 12, 2015, 08:10:33 am
content: Well, I've begun running some simulations:http://goo.gl/Dw5r66And it seems that the recommended 100nF cap after the level shifter is unsuitable. The voltage at the output is reduced by half with it in place. I found that reducing it to 10nF makes the problem go away, but I don't know if there's any point to having it in the first place. It seems to act as a low pass filter, which makes sense, since I guess that's what bypass caps are, but would there be any benefit to filtering noise on a line out that's at a frequency beyond what we can hear?I also took another look at that high pass filter on the output, and I plugged in the values 47uF and 10K into the calculator here:http://sim.okawa-denshi.jp/en/CRhikeisan.htmAnd I got 0.3Hz as my corner frequency. That's odd because I thought it was set to 5Hz.Perhaps I assumed the resistance would be 5K because of the 10K resistor to ground on many amplifier's inputs?Hm, no... even then, I still get 0.7Hz for my cutoff frequency.I seem to remember there being a good reason for using the 47uF cap on that output but I designed the circuit two years ago and I can't for the life of me remember what it was.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 13, 2015, 01:16:52 pm
content: Nobody has any insights on how I can improve this?

author: wiss
author_rank: Frequent Contributor
timestamp: January 13, 2015, 02:24:40 pm
content: If the transformer worked you probably have a ground-loop, make only one ground-connection between Amp and DAC.

author: dannyf
author_rank: Super Contributor
timestamp: January 13, 2015, 02:39:46 pm
content: QuoteI think the issue might be with your description of the setup and problems you are encountering.I read no less than 5 times of your original post and I have to say that I still have no clue what you are doing and what you are trying to do.Sounds like you have some voltage through two(?) dacs? And some led drivers and amplifiers got in the way of your doing that?

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 13, 2015, 02:53:19 pm
content: Not nearly enough detail about the whole picture. You concentrate on a small portion of the schematic, but we can't see the whole picture from your limited description.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 13, 2015, 05:37:22 pm
content: Imagine an Arduino with a servo and two TLC5947 LED drivers controlling 48 LEDs, attached to it. The servo and LEDs are powered from the board's 5V rail.Also connected to the Arduino is that DAC there, along with a small 2W amplifier.There is also a line out, to which a 40W amplifier is sometimes connected. Said amplifier is powered by the same 12V battery supplying power to the board.When playing audio through the 2W amplifier, there's some noise from the servo and LEDs, but it's quiet. Well, the LEDs are quiet anyway. The servo static is kinda loud. And if the 40W amplifier is powered with a wall adapter, same deal.But if the 40W amplifier is powered from the same battery as the Arduino, the noise from the LEDs becomes an unbearably loud screeching.Unless a transformer is placed on the line out between Arduino and amplifier. Then all is, relatively, quiet again.Now, I understand that the quiet noise from the LEDs and the louder static from the motor is my bad for not sufficiently decoupling them. And I intend to rectify that. But what I'm curious about is why the noise becomes unbearably loud when the big amplifier is connected to the same battery. It is so loud, am fairly certain that even if I were to stick 100uF and 1000uF capacitors on both LED drivers and next to the motor, that the noise would not subside.I suspected a ground loop was at issue, but people have amplifiers in their cars running off the noisy supply there and the amplifier doesn't go crazy. And the first amplifier this happened on was one designed for a motorcycle. Also, I cannot obviously remove the ground connection between amp and battery, and I am pretty sure one of the first things I tried was simply disconnecting the ground side of the line out cable and then I got nothing.And if I have a ground loop here, what causes that exactly? How could the microcontroller's ground end up at a different potential than the amp's when the ground is connected in two places and they're a few feet away from each other? And why would that cause this horrible screeching? Is it like, the ground is noisy, and the line in is noisy, and the noise from both are adding up or something?

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 13, 2015, 05:49:00 pm
content: Imagination can take us only so far. Things like HOW you are connecting everything together become critical in systems like you are describing. For example, using a strict "Star" grounding scheme can prevent switching noise returning through common ground wires from affecting the audio, etc. etc. etc. You may be taking the overall system wiring scheme too casually.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 13, 2015, 07:02:58 pm
content: Quote from: Richard Crowley on January 13, 2015, 05:49:00 pmI can't use a strict star grounding scheme. For one, the fact that the line out has a ground at all violates that. For another, the LED drivers are powered from the board's 5V supply but must be located some way away from it. I also have little control over how my end users wire up the battery and amp, so I have to work under the assumption that the wiring won't be great.It also wouldn't do much good to show you the board's old layout, because the new layout is completely different. I need general advice on how to make the line out and my audio section more immune to noise.Take this board for example:https://learn.adafruit.com/system/assets/assets/000/016/642/original/adafruit_products_schem.pngWhy do they have ferrite beads joining avcc, vcc, agnd, and gnd, if ferrite beads filter noise in the mhz range, and that is well outside the frequency humans can hear? Do they somehow filter out spikes that could cause static? On my old board, I fenced off the analog portion of my ground plane connecting it at only one point, over which I routed my DAC's data lines, but I did not have a bead or anything at the point where they joined. I'd rather not put one bead on the bottom of the board and pay for two sided assembly for a single component though. I guess I'd have to use some vias or something there but then the data lines would not pass over the place where the planes join which is bad...And on the line out, what function do R6 and R7 perform, and what are the two resistors and capacitors going to agnd doing?

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 13, 2015, 07:15:20 pm
content: Using a PC board, and even using common ground for the audio portion and the noisy digital portion do not preclude using proper star grounding methods.Ferrite beads/chokes are used where additional filtering (especially into RF regions as you say) are required. They seem unlikely to be very helpful with the problem as described here.I am unable to comment further on this question because so much pertinent detail has not been disclosed.I think a much more holistic approach is required here. Good luck.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 13, 2015, 08:51:06 pm
content: Quote from: 3roomlab on January 13, 2015, 06:42:36 pmAdding more capacitors is one of the first things I tried.Adding a 1000uF to the main power connector on the board didn't do a whole lot. Placing 100uF capacitors next to the LED drivers did help, but I did that test while using the on board amplifier, so while the noise was cut significantly, it's so much louder with an external amp that I'm sure the problem would not have been abated significantly.That jumper I added to separate the ground for the voltage divider I have a good feeling about. But I won't know if it works till I have a new PCB made, it's impossible to modify the old PCBs to test it. It won't clean up the board's power supply, but the theory is that referencing the divider to the amplifier's ground will cancel out the noise, somehow. Anyway the guy that suggested it seemed pretty confident. And if it works I can live with that, but if there's anything else I could do to improve my chances of making this thing work right on the first try, I'd love to know about it. I really can't afford to do a bunch of PCB revisions to get it right.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 13, 2015, 09:32:08 pm
content: As others have pointed out, you are only showing us half the picture. However, reading between the lines and making a few guesses:Quote from: Starlord on January 13, 2015, 05:37:22 pmsuggests that you have tied the ground to the amplifier to both the battery and the line-out point - i.e. you've created a loop. Ground loops are always bad news when dealing with low-level signals - like audio. I am guessing that your problems are being caused by the current from the LEDs and motor returning via the amplifier's ground. If you carefully examine the grounds of the car and motorbike systems I think you will find that the audio signal path is not shared with any other system, although they may be connected together at a point - i.e no loops.As you have discovered you need to break the ground loop. The easiest method is using a transformer. Alternatively, making the amplifier input differential should also work - provided it's implemented correctly.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 14, 2015, 07:01:57 am
content: Quote from: Andy Watson on January 13, 2015, 09:32:08 pmYou guessed correctly. The amplifier is connected to the same battery, so it shares a ground with the board, and there is a line out on the board, which also has a ground.As for your suggestions...The whole point of this exercise is to get rid of the transformer. My board needs to be small so I can't include one on my board, they're expensive, and I have no idea how to spec one for audio. But up till now, that's the solution I've been stuck with.And the amplifier... well, I can't modify it, so I'm not sure how I could make the input differential. And I really just want to have a standard line-out on my board.Is there no other way to get this current to flow through normal channels? One fellow suggested that when I add decoupling capacitors to my LED drivers that I should add a resistor on the power line if I could accept the accompanying voltage drop. The output of my DAC is 5V to begin with and I have to drop it with a voltage divider, so could I potentially add a resistor between the line out port ground pin and my board's ground and still have this work but discourage the current for the rest of the circuit from flowing through there?I don't have a working board at the moment to test this with, or I'd try it, but if someone thinks it would work I could borrow one from a friend and test it out, since I should just be able to modify a 3.5mm cable to do it.

author: wiss
author_rank: Frequent Contributor
timestamp: January 14, 2015, 08:48:05 am
content: Cut the audio screen close to the amp.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 14, 2015, 09:38:07 am
content: Quote from: wiss on January 14, 2015, 08:48:05 amDo you mean the cable shielding? I assume your intent is to disconnect the ground from the line out completely. But I seem to recall that didn't work, though I can't remember if the audio stopped working completely or the noise failed to go away.Anyway, I don't want to require the user to use a modified 3.5mm cable to make this work.

author: fcb
author_rank: Super Contributor
timestamp: January 14, 2015, 09:38:39 am
content: 1. Layout - grounding paths are critical.2. Filtering in both the audio (stopping PSU noise from entering the audio section) and motor/LED drive (stopping noise from getting back upto the PSU).3. Coupled noise - have you got noisy tracks running parallel to audio lines?Publish your layout.

author: wiss
author_rank: Frequent Contributor
timestamp: January 14, 2015, 11:28:18 am
content: Quote from: Starlord on January 14, 2015, 09:38:07 amWith the battery you will still have ground through the power-supply.QuoteThen you have to go balanced (a trafo will do), or remove what ever it is that causes large currents through the ground-loop.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 14, 2015, 12:32:13 pm
content: .

author: wiss
author_rank: Frequent Contributor
timestamp: January 14, 2015, 01:00:22 pm
content: On the board, is "LINE OUT -" connected to anything more than the resistor?

author: Starlord
author_rank: Frequent Contributor
timestamp: January 14, 2015, 01:13:53 pm
content: Quote from: wiss on January 14, 2015, 01:00:22 pmI don't understand the question. There's a bunch of stuff connected to it, which can be seen in the schematic.As for what would be connected to the pins themselves, with this version of the board I didn't have a 3.5mm jack, so I had to make custom cables with a 3.5mm jack at one end and a standard female connector for a pin header on the other.

author: wiss
author_rank: Frequent Contributor
timestamp: January 14, 2015, 02:24:56 pm
content: The blue stuff is the opposite side and also ground for that pin?

author: Starlord
author_rank: Frequent Contributor
timestamp: January 14, 2015, 03:02:49 pm
content: Yes. It's a two layer board.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 05:31:32 am
content: I don't own a scope. Can't afford one yet. I'm flying blind.I hope to get a 1054Z soon though. So glad I didn't spend my money on the older model last year.Anyway, it's not a broken chip, I've built a lot of these, they all had the same issue. Even with different batches of chips.Also that PCM programmer dude was talking nonsense. You just need to pull the LDAC pin low and send bytes to the chip and they're latched in when all 8 bits are received. This is how my boards work, and how Adafruit's Wave Sheild works. And I don't actually know what the BUF bit does, but there is no way to read it from the chip to make sure a byte has been transmitted as the chip has no MISO line. The datasheet seems to indicate it has some effect on the Vref pin, but I don't know what. I've tried setting it and hear no difference in the output.

author: fcb
author_rank: Super Contributor
timestamp: January 15, 2015, 01:59:59 pm
content: So your top layer is +V and your bottom layer is GND/0V?Things to try:1. Break the top layer at the bridge area between the analog and digital sections? Try an inductor or resistor, does the LINEOUT noise reduce? (you won't be able to use the PA as the drop across a series resistor will Fxxx things up).2. Have you tried removing the PA IC?3. Bulk capacitance - try adding a 1000uF low esr capacitor on the analog section rail.4. There's no LPF on the output of the DAC - what sample rate are you running at - could this be upsetting the PA?5. What is the jitter on your SPI bus? Are you certain your artifacts aren't embedded elsewhere?

author: LukeW
author_rank: Frequent Contributor
timestamp: January 15, 2015, 04:13:59 pm
content: The top-layer Vcc pour plane seems a little unusual, but other than that I would really say your layout is not bad.And it looks like your schematic is really just a close derivative of the Adafruit one, so the performance should be similar.Try laying out the Vcc nets in the usual way, putting a groundplane on both layers and using good stitching between the planes with vias.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 04:26:47 pm
content: Quote from: fcb on January 15, 2015, 01:59:59 pmYes.QuoteWill an inductor mess things up as well? I'm pretty sure it won't, but...How small of an inductor can I use? They're not the cheapest component and I don't have a ton of space on the board.How big of a resistor are we talking? By my calculations if the amp is drawing 500mA, then a 1 ohm resistor would result in a 0.5V drop. Not exactly ideal but the amp should work down to 2.5V and I don't have any 5V signals connected to it...Hm, but I do have 5V signals going to the DAC... And that liek the amp has inputs rated only up to Vdd+0.3V. And that's an absolute max rating, so really going above Vdd at all would be bad.If I don't try to use the amp, it's quieseent current is still up to 5mA, and if I assume the DAC and whatever pulldown resistors I have aren't drawing more than 25mA when active, then I might be safe at up to 10 ohms. But jeez... Seems easy to destroy the chip accidentally that way.Unless... I put the resistor on the ground? My intuition says that would reduce the signals and Vcc equally. I'm not sure though of the repercussions of doing that though. Would it cause the ground in that section to float above 0V instead of pulling Vcc and the signals down? Would it break something when an amp is connected, with it's ground? Or would it work just fine, floating a bit when no amp is connected, and being referenced to the amp's ground when an amp is connected?QuoteNo, but I'm fairly certain it's not that. The small amp isn't used at the same time as a big amp is attached. And even if it could have some effect just by being in the circuit, I can tell by the sound that's made what's making it. For example, if I have a bargraph, as it fills, the screeching noise gets louder. Also, the screeching noise goes away if the LEDs are at 100% brightness, indicating it is the PWM of the LED driver causing it.QuoteI don't think I can fit such a large capacitor, but I could add a footprint for a 100-220uF there.Quote22,050Hz. Why would this affect the PA?QuoteI don't know what the jitter is. I don't have a scope to measure it. I don't think that's the issue though. Like I mentioned before, the noise is very clearly tied to the PWM of the LEDs and static occurs when a servo is moving. If there is noise from the SPI bus, I can't hear it over those. Also the noise issue is separate from whatever the issue is with the line out that causes the noise to be massively amplified over the signal. If I made the noise 10x quieter, I'm sure I would still hear it over the music playing.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 04:45:02 pm
content: Quote from: LukeW on January 15, 2015, 04:13:59 pmDo you mean the shape of it or the fact that I have one?QuoteThe design was derived from their WaveShield, but I use a different level shifter for the SD card, and an audio amplifier instead of an op amp. Because I don't have the op amp, the line out portion is different. I also used a different LPF on the Dac's VRef if I recall correctly.Unfortunately what I didn't do was stick the SD card on the SPI bus, instead opting to put the DAC there, which screwed everything up. I had to modify the libraries and it was a huge pain in the ass getting the USART to work in SPI mode. And then I found I couldn't send data to my LED drivers fast enough with highly optimized SPI code, to send them data between samples for the DAC, and I could not send partial data to them and then feed the DAC a sample because they had no chip select. So I ended up having to cut the traces to the LED drivers and wire them to that spare USART port I put there for debugging. Was a mess. I've fixed all this in the new design though. SD card on SPI bus, DAC on it's own bus, and LED drivers on their own bus. I can bit bang the latter two if I have to. In fact the WaveHC lib bit bangs the dac by default.QuoteWhat do you mean by the usual way? What do you mean by put a ground plane on both layers? Are you saying I should have ground planes on both the top and bottom of the board, and then route individual traces everywhere for Vcc? I've never seen boards done that way. Adafruit doesn't do it that way. I don't think Sparkfun does either.I did plan though to split the power plane on the left side, and I've moved the LED connector over there, so that the current for the most power hungry devices would be forced to stay in their section well away from the audio stuff that will be over on the right side of the board.Of course, I don't know if that will help at all. There still needs to be a connection at the regulator. But if I put a capacitor in that audio portion, and by that power hungry 5V section, and at the 5-16V input to the board... I'm hoping that will be sufficient.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 15, 2015, 05:03:40 pm
content: Quote from: Starlord on January 15, 2015, 04:45:02 pmUnless you break the ground loop you will be wasting your time. With the loop in place your amplifier will be amplifying the noise generated by the LEDs, motor and logic, plus any other EM noise that cuts the loop.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 05:54:53 pm
content: Quote from: Andy Watson on January 15, 2015, 05:03:40 pmSo, how do I accomplish that?I'm pretty sure simply disconnecting the ground entirely from the line out pins doesn't work. I think I've tried that. I don't remember if I get no sound at all, or if the problem simply doesn't go away, or if it got worse though.I had one fellow suggest I reference the ground for the voltage divider to the amps ground by connecting it to the line out, but breaking the connection to the board's ground. I haven't been able to try that yet, but he said it should cancel out the noise. I'll probably include a jumper pad in my next design for that purpose.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 15, 2015, 06:30:37 pm
content: Quote from: Starlord on January 15, 2015, 05:54:53 pmYou've already rejected the best option, i.e. a transformer.You could float the audio section - i.e. opto-isolate the input to the DAC and provide it with its own power supply - makes the transformer option look more attractive.I understand why you don't want to make the input to the external amplifier differential. You could make the line-out into a pseudo-balanced output. This technique is often used in mixing desk outputs where it is impossible to know (in advance) what the desk will be feeding and whether it will be grounded. Basically, you take the DAC signal and convert it to a balanced signal, then reference it to the ground of the line-out jack. However, it is a compromise may degrade signal quality when the external amplifier isnotgrounded. Also, it might not be as effective in rejecting the high-frequencies generated by the LED and motor switching.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 07:27:26 pm
content: Quote from: 3roomlab on January 15, 2015, 07:14:28 pmI don't have any way to test that at this time.But are you interested in the interference on the on board amp's output, or the interference heard on the line out?

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 08:00:27 pm
content: .

author: Dongulus
author_rank: Regular Contributor
timestamp: January 15, 2015, 08:13:52 pm
content: I'm not really an expert at layout, so take this for what it's worth.Something that concerned me when I looked at your layout is that the return path for the electronics in your audio island look quite constricted. The grounds reach the ground plane on the bottom side through vias, but the path back to the supply ground is blocked by many signal traces on the bottom side. If you follow how the current has to travel back to the supply ground, there are only two paths, each with bottlenecks: 1.) Right between the LED header and the board edge on the right 2.) The bottom edge by the cutout. The path that passes by the LED header also coincides with the return path for the LED driver. I think it's possible that the LED driver return current could influence the ground level in the audio circuitry because of this.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 15, 2015, 08:24:38 pm
content: Quote from: Starlord on January 15, 2015, 08:00:27 pmNo, not really. I think I see where you're going with that but you need to be a bit more "active" in referencing the DAC output to the ground of the line jack.QuoteYes, that is what I meant.QuoteGood decoupling will help but it won't entirely solve the problem.When placing the decoupling consider the path(s) that the current(s) will take - through the LED (or whatever) and through its associated decoupling - it will form a loop. Try to minimise the area of the loop and try to avoid having overlapping loops.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 08:32:58 pm
content: .

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 08:41:13 pm
content: Quote from: Andy Watson on January 15, 2015, 08:24:38 pmI don't know how I can be more "active" in referencing the output of the DAC to the ground of the line jack than connecting it directly to it and breaking the connection to my board's ground.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 08:59:51 pm
content: .

author: Starlord
author_rank: Frequent Contributor
timestamp: January 15, 2015, 11:48:10 pm
content: I believe when Mr. Watson cryptically referred above to a more "active" ground reference on my line out, he was referring to an active high pass filter like the one seen here:http://en.wikipedia.org/wiki/High-pass_filterOnly problem is I don't know much of anything about op amps, nor do I know how using this would help. I mean it might be a more effective high pass filter, but so would adding another resistor capacitor pair to make a second order filter. I haven't researched exactly how much more effective an active high pass filter can be.Still, I'm not sure if this advice is intended to correct the ground loop, or just filter out a lot more noise. Or if it even is what he was hinting at.

author: fcb
author_rank: Super Contributor
timestamp: January 16, 2015, 11:16:54 am
content: Don't bother with a new layout until you have fixed your existing problem.How about working out if the noise is caused by LED's directly (remove the drive to the LED's)? Does the noise go away?What if you feed the LED's from a totally separate supply. Does the noise change?Are you sure that the noise isn't embedded in the DAC output, perhaps by an artifact caused by the code (maybe the display driver does something weird as it changes - check your ISR's are working properly).?

author: Starlord
author_rank: Frequent Contributor
timestamp: January 16, 2015, 12:03:48 pm
content: Quote from: fcb on January 16, 2015, 11:16:54 amThere are two issues here. Power supply noise and the ground loop / noise amplification.When the LEDs aren't running there's no power supply noise. Unless I'm running a servo. Then there's power supply noise from that.QuoteI'm sure the power supply noise would go away. But the ground loop issue would not.QuoteYes, I am certain of that.All I want to know in regards to the LEDs is what measures I could take to reduce the noise from them. Forget that the DAC even exists. I know for example that putting some large capacitors on the LED supply helps. I asked about using inductors but people steered me away from that because they could oscillate and make the problem worse. Someone suggested I could put a resistor in series on the power rail to the LEDs, but I don't know if that will do anything, whether it should be on VCC or GND, or how large a value I should use.I did some calculations and I think that if I put it on the Vcc line then the voltage drop could cause the data lines to exceed the ratings for the input pins. I think such a resistor would have to be less than 0.6 ohms to avoid a voltage drop of more than 0.3V with a 500mA draw. I don't know if a resistor that small would help at all.On the other hand, if I put said resistor on the ground side, then while that might lift the ground, it would affect the power and data lines equally, so I could use a larger resistor, so long as I don't lift the ground so much that I don't have too little voltage to drive the IC and LEDs.Then again, it occurs to me that there's probably a minimum below which I cannot go on the data pins as well, so if they were set low, then their ground might be below the ground of the IC, and that could destroy it too.So it would seem the only safe thing to do would be to put resistors on Vcc AND the data lines. Except... the voltage drop would be greater on Vcc than on the data lines because the current draw would be higher, wouldn't it?Ugh. Perhaps an inductor is the only answer, if the capacitors alone are insufficient.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 17, 2015, 10:14:44 pm
content: Aha... I've found it!http://www.ti.com/lit/an/sloa143/sloa143.pdfNow I just need to understand it.

author: wiss
author_rank: Frequent Contributor
timestamp: January 17, 2015, 11:23:58 pm
content: Usually, these things are not difficult, only a bit complicated...

author: Starlord
author_rank: Frequent Contributor
timestamp: January 17, 2015, 11:45:16 pm
content: Well I read through the datasheet and I kinda understand it, but I'm not sure if I'm interpreting these weird schematics right, and I'm not sure how to implement this.It seems like I'd either need a different DAC (I think I've seen differential dacs in my searches, though I don't know if they'd be useful in this circuit), or another chip. An op amp perhaps? I have no experience with them and have no idea how to select a suitable one.On page 4 of the data sheet, is that circle on the left where I'd connect the output from my DAC? It doesn't make sense the way it's wired.I found this drawing of a differential amp:http://en.wikipedia.org/wiki/Differential_amplifierAnd it looks like the DAC would connect to both + and - which is weird. But it also looks like it would be connected to the shared ground between my board and the amp. But I can't short the output of the DAC to ground that would destroy it.I think the resistor RGBK, SGND and the DAC output are all connected to Vs- as well?Why doesn't this schematic make any sense?

author: Starlord
author_rank: Frequent Contributor
timestamp: January 19, 2015, 01:26:18 pm
content: So does anyone have any suggestions how I can move forward with that TI design? At this point this is the only thing holding me back. I don't know how to select an appropriate chip (well, TI suggests a few but they're fairly expensive and more than I think I need), I don't fully understand their schematic. :/

author: Starlord
author_rank: Frequent Contributor
timestamp: January 19, 2015, 02:08:22 pm
content: Well I think the reason that schematic didn't make any sense is because it's not a schematic, it's a block diagram, something I'm not terribly familiar with.The circle I guess is supposed to be my microcontroller. And I think the diagram is telling me the - side of the differential amp is supposed to be connected to the microcontroller's ground and to the line out ground through RGBK.As for the line coming out of the tip of the differential amp, is that supposed to be the + side? In the other drawings of differential amps I've seen there's nothing connected to the tip, there's a + and - output on top and bottom edges of the triangle.