Designing a FPGA Board, and some questions. - Page 1
Forum section: electronics / beginners

author: antercreeper
author_rank: Contributor
timestamp: April 05, 2024, 07:29:43 am
content: where to place esd device(tvs, mov, etc...)I have seen both. Which is correct?

author: selcuk
author_rank: Frequent Contributor
timestamp: April 05, 2024, 11:27:37 am
content: You normally place TVS or varistors right after the connector. You may want to limit inrush current with a resistor to decrease the stress on the TVS but this normally leads to burnt resistors after transient events. I recommend you to place connector + TVS + resistors, so that resistors can limit the current into i.e. IC but don't interfere with the inrush TVS current.There are "Pulse Withstanding" resistors on the market. If you build a surge protection (not ESD) circuit with both a MOV and TVS, you can place connector + MOV + resistor + TVS. This time it limits the TVS inrush current but still the first component after the connector is a protection element not the resistor. There are ferite beads for this purpose as well.

author: xvr
author_rank: Frequent Contributor
timestamp: April 05, 2024, 11:39:32 am
content: Generally TVS placed directly after socket, because it is energy absorption device. But not be misleading by TVS functionality. If you see '5V' as TVS rated voltage it doesn't mean, that no more than 5V will be AFTER TVS. It is only mean that TVS will be closed (or almost closed) up to 5V on input. When you got ESD spike TVS will absorb it, but voltage after TVS will be much more, than its rated voltage (may be 20V or so).So highly recommend to add second level of protection after TVS - just current limiting resistor (together with builtin protection diodes on IC input) could be enough.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 05, 2024, 12:28:54 pm
content: Is that a phono jack? Hardly matters as it's highly unlikely the contacts get hit without the outer collar (ground) getting hit first -- connectors are a valid and effective way to mitigate ESD at the source itself. See also USB, HDMI, etc.: these operate at speeds where the capacitance of a diode that can survive ESD at all becomes questionable. Maybe this depends on the type of connector; metallic housing types are preferable for obvious(?) reasons.Now, if the connector is floating (isolated from chassis, or chassis is nonmetallic), the return path for an ESD strike will couple through the connecting wires, into the signals -- shielding is required all around for the above to be true.This includes the use of ground plane at the PCB level, which acts as a partial shield itself. No ground plane, means basically your whole board, everything on it, is subject to ESD.I'm guessing it's not chassis mounted, or not on leads, given the on-board symbol. For board-mounted connectors, preferably place near the edge of the board (ESD currents prefer to flow around the edge of the ground plane, it's a skin effect and EM fields thing), and place TVS components nearby; preferably avoid placing components or routing near the board edge, but have signals come inward towards the center, in which case you would have TVS midway along the route between connector and core circuitry.If using a 2-layer board, pour ground both sides, add stitching vias, and ensure contiguous ground wrapping around every trace, prioritize stitching around peninsulas, trace crossings, etc. Prohibit overlapping traces (any trace crossing/overlap creates negative space through top and bottom, where no ground can pour around either trace), and make trace crossings perpendicular where possible; group traces into buses to minimize crossing area.If 4+ layer, inner GND/VCC plane is most commonly chosen, and makes an effective and simple design; pouring signal layers with GND can be done but the improvement is marginal while the complexity of routing and stitching it goes up significantly.Tim

author: antercreeper
author_rank: Contributor
timestamp: April 06, 2024, 02:06:06 am
content: You are right. No shield :'(6 layers board.Wifi module is just a model for estimating the area

author: antercreeper
author_rank: Contributor
timestamp: April 06, 2024, 03:02:38 am
content: after modified

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 06, 2024, 01:52:04 pm
content: Yeah, I like that. Both TVS and connector going right into plane (and having lots of layers including at least a stitched top, at that) gives the best chances of success.An RC (or more) filter, going back to the chip, might be desirable for various reasons, but I don't know what the chip is, or if that would matter. For example, a mic or line input almost certainly wants filtering to avoid RFI turning into rectified offsets due to input clamp diodes or internal effects, and a sampling filter on this apparently digital board (--> is it an ADC?) would have obvious value. If it's an output (headphone driver?), a linear amp might still be overwhelmed by RF on the output, though less sensitive than to an input; if class D amp, EMI filtering might be desirable to reduce emissions on the cable -- the resistors are a start, but only taking the edge off, mitigating ringing so you won't have excess peaks, but the bulk of the step remains.Or maybe it's RS-232 on the uncommon/rare/old phone jack, in which case a little bit of filtering might still be desirable but likely isn't required as the transmitter slew rate handles that pretty well for a start. You wouldn't want to put raw logic levels into there, and RS-422/485 I wouldn't trust on typical phono cables (the kind of shielding they have) but maybe you'd get away with it, and slew-rate limited transmitters are available which would help.And... I don't know what else, that most likely covers, in some way or another, anything you'd want to stuff into a phono jack, heh.Tim

author: antercreeper
author_rank: Contributor
timestamp: April 06, 2024, 02:58:31 pm
content: DAC, ADI MAX9850. can work as low as 1.8V, one I2S input and one analog input mixed, and one analog & one headphone(has amp with no capacitor pop) output.It provide all what u need.I like this chip, which has a integrated amp and has very good sound. (amp can seperate the influence from the headphone, instead of direct connect to DAC. I add 200 ohms resistors because It's too loud. Maybe very suitable for those headphone with low sensitive and high impedance, or some very long cables)The I2C register is very simple.Emm. Maybe I will open source this FPGA board after finishing the test.(however bad quality)

author: antercreeper
author_rank: Contributor
timestamp: April 06, 2024, 03:23:20 pm
content: By now, a big problem is how to deal two USB input which comes from different adapter(I am about to make a Or-ing with mosfets so that you can use a larger 9V or 15V PD from one port if you need more currents, or use 5V 3A fallback from the other port)But there are about 60 volts between those two grounds......for example one is connected to computer and perfectly grounded, and the other is 2 pin cell phone adapter.And during hotplug, it indeed cause mistriggered on FPGA input.But isolate the ground will affect the return path of USB2.0 signal. not fully differential ):The D3 is indeed a TVS diode to blow the fuse if overvoltage. I am sure the current design here is bad and I am confused.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 06, 2024, 03:53:02 pm
content: 60V where? DMM on AC? 120V mains? That's through the 'Y' / EMI capacitor, it's high impedance and safe to ground.USB without shield tied to ground, means inviting all the trash on the shield, into the signal/pairs. So, immunity drops to a volt or two, maybe.Putting CMC on the power line and lifting shield, removes any ground reference for the pair, and immunity drops under a volt probably.I haven't seen a situation where it isn't acceptable to hard ground shield as well as power pins (i.e. 5V is also a ground for AC / EMI purposes), but in the event you need to float it, it can still be AC-grounded by placing many bypass caps around the ground pins. Multiple are required to get the ESL low enough to maintain shield effectiveness.Also just straight up isolation is a possibility (USB data isolator), obviously you don't get power that way and would need a DC-DC as well.Tim

author: antercreeper
author_rank: Contributor
timestamp: April 07, 2024, 02:22:27 am
content: Thanks. You clear my doubts. at the very start I am worried about the USB data lines.I have made this conclusion.I will make a Power only PD 9V ~ 20V input with CMC(just Barrel Connector Replacement), with a USB to UART. I cannot isolate USB but I can isolate the UART with some devices.And the other is USB2.0 data + 5V 3A, and with all things grounded.

author: BrianHG
author_rank: Super Contributor
timestamp: April 07, 2024, 03:51:45 am
content: Quote from: antercreeper on April 06, 2024, 02:58:31 pm200 Ohm resistor, why? The DAC you chose has a built in separate analog volume control for the headphone output in the I2C registers. Is it possible you set the headphone output volume gain to the wrong setting? It was designed to be able to boost the sound by +6db. Also, with a 200ohm series resistor, what happens to your output's 'audio damping factor' when driving the headphones?

author: antercreeper
author_rank: Contributor
timestamp: April 08, 2024, 02:13:06 am
content: I know that, I have set the internal gain register. (In fact it is a historical problem...... at the beginning my i2c controller is utter grabage and ignore the first command which is to set the volume.)In fact you can change it to a lower or 0 ohms.It is not very important.I can't tell if there will cause huge impact, but adding resistors is not bad.I add it from several reasons.The first is the ESD surge, and the second is shortage(although the DAC can handle it).The third is from some mistierious guidance and Hifi forum's suggestions.My understanding is, what you need is a current amp(The headphone is drived by currents!)however the op amp is based on voltage, input output closed loop. In fact I cannot tell if MAX9850 behaves as voltages or currents source.So adding resistors will reduce the impact of headphone's inductive impedance, make the whole load looks like a resistors, and from the voltage output you will get your current. Or you can calculate or draw bode plots of the total closed loop.