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More analog inputs for a very analog world :)


#1

Hello

For my project I will use 8 accelerometer(50G, 100G, 250G) (5-7V) and 1 pressure sensor (70PSI) (10-15V). Unfortunately the USBizi processor provides only 8 analog inputs.

I’m not very good with electronics, but I saw in this forum, that there is the possibility to use an ADC on the SPI to create more analog inputs. I would like to have a very high sampling rate on the inputs. Unforuntately I can not find the ADC :frowning:

Can anyone help me or recommend me a suitable part?

Regards,

Lionel


#2

MCP3208. http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010534. I just searched on “SPI ADC” and found this link http://extremeelectronics.co.in/avr-tutorials/interfacing-12-bit-spi-adc-mcp3204-with-avr-micro/ in the first page of results.


#3

Thank you very much, this is just the kind of input I needet. Also with good explanation!

I will do some research in the tutorials there. Are there any restrictions from FEZ Panda, which I have to take into consideration?

Thanks again for the quick answer :slight_smile:


#4

nope. I’d just use one of these connected to a Panda on SPI, handling your 8 accelerometers, and one I’d use the Panda ADC for.

SPI is pretty easy to get working in Fez - some would say Freakin’ easy ;D :D!


#5

USBizi 100 documentations says that the SPI is 8/16bit. You sent me a link to a 12bit ADC. I found a 8 bit chip on the website of a local supplyer.

single-ended
Serial, SPI
2.7 mA Power Supply (min 2.35V max 5.5V)

is this compatible? I could not find any information about the SPI power Supply of USBizi 100


#6

the 12bit refers to how many bits the ADC reads at. 8 bit will only get you 256 variations of value, the FEZ has 10 bit, or 1024 possibly readouts, 12 bit gives you 4,096 variations of value.

Think of it as a scale… with a 2 bit ADC you have possible values of:
1, 2, 3, 4.
Not very useful. If you had a voltage range of 0-4.0v, you’d only be able to read 1, 2, 3 or 4v… nothing in between.

With a 4 bit ADC you could read 16 values… so if reading 1-4v, you’d get upto 1/4 volt accuracy.

Now go to 8 bit… and you have 256 values, from 0-3.3v, you get ~12mv accuracy. Not so accurate.

10 bit over 0-3.3v gives you ~3mv accuracy.

12 bit from 0-3.3v gives you ~0.8mv accuracy - now we’re talking!

Probably not very well explained, rather tired and have a headache. Check out wikipedia: http://en.wikipedia.org/wiki/Analog-to-digital_converter

You’ll supply the ADC with power from the 3.3v supply pin on the FEZ. The power supply is for the IC and is independent of SPI completely.


#7

As Mark said, the bit-rating on ADCs show how small an increment the measurement can take. The larger the number, the closer the gap between each successive measurement is. The question for you is - how accurate do you need it to be???

SPI is “serial Peripheral Interface”. Serial means single bit at a time.

If I were you I’d do two things. Ditch the idea that you want so many devices at once, and connect a pressure sensor and an accelerometer directly to your Fez and make that work. In the meantime order the chip I mentioned (or any other one you decide). Then when it arrives you’ll have figured out measurement with the ADC directly on Fez, and you will be better able handle the SPI connection :slight_smile:


#8

Why not just use digital accelerometers?


#9

Guys, thank you so much for the good advice and good links. I’m going to spend now sometime in the internet and find out what I really need :slight_smile:

@ Gus: The digital accelerometer go for some reason not high enaugh. Max I saw was like 18G, but we would like to be accurate at around 40G and again at around 200G. I found a digital pressure sensor, but its just twice as expensive as the analog sensor :frowning: (and we are probably going to destroy some sensors during development…)


#10

I am not an expert at accelerometers :frowning: I only wanted to point out that you may not necessary need analog inputs if you use I2C or SPI accelerometer.


#11

What about this board ?