Hi all, I'm making this current output in fixed ratio's... 1,2,4... etc. I'm using 0.1% RG series from susumu. There are lots of 1:2 ratio's, But the only 1:2:4 that I find (in stock at DK) is 75/150/300. are there any others? (There is 140/280/560, but 280 is a non-stock item at DK.)
75/150/300 also has 1.2K so a 1:2:4..16 ratio, which look nice.
255/510/1020 is nominally exact. (Susumu sells the E24 values in their precision series as well, iirc.)
Cheers
Phil Hobbs
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Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
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hobbs at electrooptical dot net
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Tim, Phil, thanks. I had a total brain fart today. It's an opamp -> fet -> load/resistor_feedback current source. I wanted to switch R (with rotary switch) and I thought I could do it in shunt/ parallel, until I tried to lay it out.. oops. (I hope the odor did not escape my room.)
Max current is in the 200mA range. (limited by switch...) Now I'm thinking about ditching the rotary switch and using a latching relays.. ~$2-3 each (in 100) Seems like I could do a lot with 3-4. I'd still have to have inputs.. push buttons I guess.. and led indicators. A front panel pcb... (rotary switches are on the shelf.)
Sorry for thinking out loud.
George H .
PS, Phil, Re: 249,499, 1k, 2k, 4.02k I wish we could "bend the log a bit" and have 250,500,1k,2k,4k..
Yeah it would be much better to do most of the switching at the front end.... I need less than 2 decades. (And I'm feeling like someone will want more current.)
My recollection is that R-2R ladder DACs are far less sensitive to resistor tolerance than 1-2-4-8R-type DACs. Back in the Olden Days of DOS computer audio, an 8-bit R-2R ladder made from 10K-20K 5% resistors (driven by a printer port... remember those?) gave pretty decent sound.
You could even make an 8-bit ADC by wiring the ladder output to a status input pin on the port. Wire the input signal through a summing resistor to the same pin, and use SAR logic to balance the ladder output against the signal at the gate threshold voltage. The whole thing could be wired into the parallel port connector hood to get a "passive" ADC (since the active part was inside the computer).
Ahh, DOS were da days!
Best regards,
Bob Masta DAQARTA v9.00 Data AcQuisition And Real-Time Analysis
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I was playing with this idea in bed last night... The onerous part (for me) would be the user interface. Push buttons and led indicators.. It's not something I've done, so I'd have to research, copy and adapt, screw-up, etc...
The switch is too easy... I do still have a bug up my butt with grayhill switches.. so doing something else is appealing.
Say I was looking through AoE3 for a precision opamp with a bit of speed, and I cam across the opa192.
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Which has got some sweet specs! If someone mentioned this opamp before then I missed it. Except for this weird extra noise when the input is near the positive rail, it looks almost "perfect".
Yikes, that is pricey. Serial DACs are cheap, but I guess you want parallel. We're paying $4.40 for AD5440, a dual 10-bit parallel DAC. There are probably cheaper singles, maybe 8 bits. That DAC0808 is 69 cents in quantity and might be accurate enough.
Chopamps: ADA4638 handles +-15 supplies and costs about $2. Lower voltage chopamps like AD8628 are cheaper.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
We've found that designing our own box, and having a shop punch/bend/anodize the sheet metal, is cheaper than buying a commercial box, much less machining it.
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John Larkin Highland Technology, Inc
picosecond timing precision measurement
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