| Electronics > Projects, Designs, and Technical Stuff |
| FAST A>D conversion |
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| NorthGuy:
--- Quote from: stj on April 06, 2019, 12:07:20 am ---this may be a good time to add that i'm trying to keep it small. i was thinking of just streaming the adc to a latching buffer, then a 5bit binary>decimal convertor and finally an led bargraph, using the 6th bit to change the led colour to indicate 0-2.5 or 2.5-5v --- End quote --- Are you going to update the bargraph at 20 MHz? |
| Someone:
--- Quote from: NorthGuy on April 06, 2019, 01:18:19 am --- --- Quote from: stj on April 06, 2019, 12:07:20 am ---this may be a good time to add that i'm trying to keep it small. i was thinking of just streaming the adc to a latching buffer, then a 5bit binary>decimal convertor and finally an led bargraph, using the 6th bit to change the led colour to indicate 0-2.5 or 2.5-5v --- End quote --- Are you going to update the bargraph at 20 MHz? --- End quote --- Charlieplexed to keep the pincount low.... |
| splin:
--- Quote from: stj on April 06, 2019, 12:07:20 am ---this may be a good time to add that i'm trying to keep it small. --- End quote --- Most of the ADCs seem to be at least 24 pin TSSOPs; the 42MSPS 8 bit ADC08351 is cheap ($1.45 @ 1K) and is available in WQFN-24 but is a 3V part. --- Quote ---i was thinking of just streaming the adc to a latching buffer, then a 5bit binary>decimal convertor and finally an led bargraph, using the 6th bit to change the led colour to indicate 0-2.5 or 2.5-5v --- End quote --- You also need an oscillator and possibly an i/p buffer depending on the source impedance. How about the STM32L412K8? It has 2 ADCs which can run up to 8.88MSPS in 6 bit mode. They can be interleaved to give you 17.76MSPS. It is < $2 which is probably cheaper than the ADC solution as you wouldn't need an oscillator or binary to dec convertor. You would likely need a buffer as the maximum source resistance is 220 ohms at this speed. It is available in a 5mm x 5mm UFQFPN - 32. It is a 3.3V part only but it would give you a lot of flexibility for handling glitch detection - the ADCs have 3 'analog watchdog' comparators which can generate interrupts when user specified thresholds are crossed. You could for example extend the LED on time when a glitch is detected to improve its visibility (a 50ns LED pulse would be very hard to see without overdriving the LED - but you could use a capacitor to do that easily enough I guess).. You might also be able to detect oscillation and indicate its frequency on another bargraph. You'd not have much processing time for each sample as you'd only get 4.5 clocks per sample @ 80MHz clock but you could use the DSP instructions to process 4 x 8 bit samples per instruction. I'm not sure if it can be configured to DMA the ADC data to a GPIO port directly to drive the LEDs - that would remove a lot of software overhead. If not you could use a second DMA to transfer the samples from memory to a GPIO. |
| OwO:
--- Quote from: blueskull on April 06, 2019, 02:49:27 am --- --- Quote from: OwO on April 04, 2019, 03:14:31 pm ---AD9200 is only $1 and gets you 10 bits @ 20MSPS. --- End quote --- Where to buy? I usually ignore Taobao and other Chinese non-authorized sellers. Digikey has it listed at $8, LCSC at $9, and even ADI official store at $3. The cheapest authorized reseller is Quest, at $2.7@1k pcs, or Chip One Stop Japan (an Arrow company), at $2.8@1 pcs. --- End quote --- It's around $1 on aliexpress and $0.5 on taobao. It probably is a Chinese replacement but I have used over 300pcs from taobao from 3 different sellers and every single one performed to spec over the specified environment and I haven't had any fail after year. I don't care about getting genuine ADI parts, only parts that perform to spec and don't fail. Anyone should be allowed to make a AD9200 and sell it as AD9200 if it adheres to the datasheet specs. |
| stj:
to answer some questions, no mutiplexing, just 30 leds wired direct to the binary>decimal convertor. update rate of the leds would be the rate the data under the probe changes state - that's why the latch is between the dac and binary>decimal decoder. now 20MHz is kind of worse case - most signals would be 4MHz or less - i picked 20 because it's kind of the max you would see a 74ALS series being run. i know they can go higher but it's very rare. |
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