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| decent but affordable 16 bit DAC |
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| David Hess:
--- Quote from: OM222O on June 10, 2019, 08:46:57 pm --- --- Quote from: David Hess on June 10, 2019, 05:20:55 am ---The tricky implementations of gain switching compensate for or are invariant of the series resistance of the switch. https://www.analog.com/media/en/training-seminars/tutorials/MT-072.pdf --- End quote --- That is why I recommended different fets. Higher Rdson means lower capacitance and better transients, etc. but can introduce some errors. Higher capacitance has the opposite effect, therefore it's a tradeoff. Anyhow if the resistors are chosen to be large enough, often the Rds is orders of magnitude better than the resistor tolerance itself, making it a non issue ;D --- End quote --- I am just point out that with a slightly different circuit configuration, FET resistance becomes irrelevant because only the operational amplfier's input bias current passes through it. Now JFETs or integrated CMOS multiplexers can be used without consideration of series resistance. |
| OM222O:
Interesting ... Can you please provide a schematic for that configuration? So far I have only came across the example which I provided ??? |
| David Hess:
--- Quote from: OM222O on June 11, 2019, 09:33:59 pm ---Interesting ... Can you please provide a schematic for that configuration? So far I have only came across the example which I provided ??? --- End quote --- Figure 7 on PDF page 5 of the document I linked as shown below shows the basic idea: https://www.analog.com/media/en/training-seminars/tutorials/MT-072.pdf Note that separate dividers in parallel with the output could also be used instead of one stacked divider. Input resistance matching to remove the contribution of input bias current to input offset voltage can be done by adding a compensating resistance in series with each switch if necessary; the AD797 shown uses input bias current cancellation so this was not shown here but it might be done with a 741 or 324. A low input bias current part would not require this either. |
| OM222O:
Sorry but I can't see how this is different from the schematic that I provided. All the drains are commoned and will go to the inverting input (R+ net), the gates are the "TTL gain control" and each source goes to one resistor in the chain. My idea used a fixed resistor in the feedback path as well, but now that I've seen that diagram, I realized it was just an extra part and you could remove it alltogether :-+ please correct me if I'm wrong and the diagram in AD app notes is different from my schematic. Edit: in hindsight I only provided the schematic for the range switching and not the entire circuit as I didn't have time and that schematic was already abailable to me quickly. Maybe that's where the confusion comes from. If so, my bad and I will create full schematics for the specific implementation rather than using words to describe it from now on ;D |
| nemail2:
Hi sorry I'm on vacation currently with (gladly :-DD) limited access to a PC and writing stuff on the phone is just... meeeeeeeeeeeeeh :) --- Quote from: OM222O on June 11, 2019, 04:53:03 am ---The parts choices seem reasonable :-+ But that layout though :-DD no offense but it seems very willy nilly without any rhyme or reason. Usually the layout follows the schematic, i.e. you have sections that do specific things, rather than just running traces all over the shop. What software are you using for the pcb design? If it's easy eda I can help you clean it up. Otherwise I can upload some pictures as hints. It would be beet to seperate your analog and digital supplies btw. LDOs are so cheap these days that there isn't an excuse for not doing so. You also won't need the ferrite beeds if you have a clean supply to start with :-/O --- End quote --- I appreciate your feedback and I knew that the layout is not ideal. I wasn't aware that it is THAT horrible, though :) The lower part is digital stuff, display, MCU power supply, relais switching, pwm fan control, etc. Then there are big fat power traces for the series pass elements and to the output connectors and in between from left to right) there is the current measurement circuitry, the CC/CV opamp control loop and the onboard voltage measurement as well as the external voltage sense resistors. I do have quite many LDOs and Vregs on the board, especially in the analog part I didn't go tight and did rather put in another LDO than going all over the board with a trace. when crossing traces, I tried to only do it in 90 degrees and I tried to separate digital and analog stuff as much as possible and I tried to keep traces as short as possible, especially for the critical (analog) parts. So.. I thought it was halfway decent but appearently not :-DD so I'll gladly take any advice on how to optimize the layout. I don't really know what to optimize there, apart from relayouting everything. However if I'd do that, it might end up worse than it is now. Or quite the same :-DD |
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