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Grossly parallel multibit DAC adventures
Grossly parallel multibit DAC adventures
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Old 15th October 2019, 08:54 PM   #11
nautibuoy is offline nautibuoy  United Kingdom
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Originally Posted by abraxalito View Post
As I set out on the lingDAC thread, I'm not sure that a reflow approach would in fact be more efficient in time. Partly because I prefer to test the DACs first, with only power applied and partly because these chips are already desoldered and they're not mechanically consistent from part to part as fresh parts off the reel would be. They have excess solder on many pins and on the sides of the package which sometimes leads to shorts between pins. The pins are also bent out of shape in many cases so they're not all in contact with the PCB which makes me wonder if they'll be amenable to reflow.
Ah, OK. Fair enough. I didn't clock that properly before. I guess it would also ask questions about the resilience of the chips to repeated heating.

I had a look at the gerbers in a viewer - the PCBs are smaller than I imagined!
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Old 16th October 2019, 02:11 PM   #12
sgrossklass is offline sgrossklass  Germany
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AoE 3rd mentions a current input topology named a "regulated cascode", which finds use in high-bandwidth photodiode transimpedance amplifiers (up to dozens or hundreds of MHz). In this one, input voltage noise is determined by one transistor while current noise is determined by the other. Seeing your thread, maybe this would be something of use for an I/V? Transformers, while undeniably useful, are notoriously messy.
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Old 17th October 2019, 01:10 AM   #13
abraxalito is offline abraxalito  United Kingdom
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Thanks for the suggestion - in the case of active I/V I nowadays always like to have a filter between it and the DAC's output. This rather constrains I/V circuits in that they need to have a well-defined, constant input impedance (vs frequency and vs amplitude) and the lower, the better. Putting a trafo into the circuit was my solution to the conundrum of the DAC wanting a low load impedance and the I/V circuit wanting to be relatively higher input impedance.

I set out some background thinking here : How a transformer helps... | Details | Hackaday.io
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Old 18th October 2019, 02:54 AM   #14
abraxalito is offline abraxalito  United Kingdom
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In theory its possible to accommodate even more than 144 TDA1387s although I've yet to figure out a way of fitting the thicker wire on the transformer's bobbin. There has to be some way though.....
I came up with a way to get fewer turns on the transformer - add the CT later with thinner wire. Here I have 6+6 T for the primary (0.62mm) and there's enough room left for 4000T of wire for the secondary (0.05mm). This gives an impedance ratio of ~111k meaning an 11k I/V resistor on the secondary represents 0.1ohm to the DAC array. Initial listening indicates a slight improvement in LF - the price is the need for a lot more DAC chips - at least 128.
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