Multichannel ADC chip - Page 1
Forum section: electronics / projects

author: SimonST
author_rank: Newbie
timestamp: January 21, 2019, 10:34:27 pm
content: Hi!I started a project to get better at non-RF electronics and I am looking for an IC or design trick.I wanted to try to build pyrotechnic firing system for fun. It is basically a box which provides power to different outputs depending on a control signal, to the outputs you connect an e-match which when about 500mA-1000mA is provided for a short while it burns. One nice feature to include is the ability to measure the resistance of an e-match. With that information you can deduce if you most likely have connected the correct pyro to the correct port or not and the health of the e-match. The level of accuracy needed is quite low. everything below .1ohms is considered a short, then measure which range the circuit is in 1 to 5 ohm, 5 to 10 ohm, 30-50 Ohms or >50Ohms. The measuring current has to be below 10mA.Does anyone know of a cheap multichannel (8-16 channels) ADC that could do something like this? Or some other way to do it. I imagined it would be easy to find a chip used for multi select buttons but have not been able to find one. Any tips?

author: ataradov
author_rank: Super Contributor
timestamp: January 21, 2019, 10:53:37 pm
content: A lot of modern MCUs will have at least 8 channels. Put a couple of them to get 16.SAM D09 in QFN-24 package is $1 in single quantity and has 10 ADC channels, for example.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 22, 2019, 12:18:39 am
content: ADC has to be interfaced with a uCPU, and based on the question I think OP is not ready yet to design PCB for QFN. Better to get arduino MEGA2560, there a lot of examples and finished projects like that.

author: Bassman59
author_rank: Super Contributor
timestamp: January 22, 2019, 02:32:31 am
content: Do you need simultaneous sampling or can you deal with sequentially sampling each channel?Do you need to read data from all channels simultaneously or can you read each channel’s result sequentially?

author: jaycee
author_rank: Regular Contributor
timestamp: January 22, 2019, 03:30:47 am
content: Microchip MCP3008 has 8 channels. Cascade 2 of them to get 16 channels. They are SPI devices so you can control them with an Arduino easily.

author: ogden
author_rank: Super Contributor
timestamp: January 22, 2019, 04:19:20 am
content: If MCU of your choice does not have enough ADC inputs - use analog MUX'es. One to mention: CD74HC4051.

author: SimonST
author_rank: Newbie
timestamp: January 22, 2019, 10:56:59 am
content: Thank you for all the feedback!More info:I am working with STM32F042 at the moment. Might move up or down if need more/less.I would like to avoid QFN due to problems with solder joint cracking with vibrations (part of the fun for me with this project is to design it similar to a product (100-1000 pcs at least) and not a one-off.).Sequential sampling is OK. I do not know why I did not think about using analog MUXes. Guess that would be the most cost effective even CD74HC4051 costs about .5 USD.The "problem" with using a cheap MCU is that I might have to deal with several boot loaders and a more advanced software update scheme. I am not sure I am ready for that yet. But might be a nice challenge.

author: aheid
author_rank: Regular Contributor
timestamp: January 22, 2019, 02:50:29 pm
content: The combination of low resistances and low max current makes it difficult for using a 12bit ADC straight up, like many built-in ADCs.One option would be to use an uC with 12bit built-in ADC and external V_REF, like one of the STM32s in the right package, in combination with a 1.2V voltage regulator. That would give you an LSB of about 0.3mV. The regulator could be made using a voltage reference, opamp and a pass transistor.Then you could put the e-fuse in a simple resistor divider circuit with say a series resistance of 150 Ohm, a 150 Ohm resistor in parallel with the e-fuse. That would give you relatively easy <1 Ohm discrimination, yet still easily discernible readings in the 1-100+ Ohm range. Max current through e-fuse would be ~8mA.One advantage of this setup as I see it is that if you have a beefy enough regulator (10mA per channel) you don't need to mess with analog multiplexers by just powering all the e-fuses at the same time, using the plentiful analog channels on the uC for measuring. It should also be relatively easy to use in conjunction with the high-current "fun time" driver circuit, using back to back mosfets to "isolate" the measuring circuit from the e-fuse.Then again I'm no expert so

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 22, 2019, 03:49:39 pm
content: AD7490:https://www.analog.com/en/products/ad7490.html16 input channels, 12-bit, TSSOP package, SPI.Don't forget to add proper protection to the inputs.

author: Yansi
author_rank: Super Contributor
timestamp: January 22, 2019, 03:57:00 pm
content: AD7490... jeez. That cost an arm and leg for what is here really needed here.Just slap there a few MUXes and use the internal ADC in your MCU.

author: ogden
author_rank: Super Contributor
timestamp: January 22, 2019, 05:02:38 pm
content: Quote from: Yansi on January 22, 2019, 03:57:00 pmFYI many 64-pin stm32's have 16 external ADC inputs. For example STM32F030R8T6 qty1 price: 2.5$ or lower, 16 channel AD7490 qty1 ... around 12$