12Bit 8 Channel ADC

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#1
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Hi All,

I have a project where i need to build a data logger which will log temperature from 8 sources and send it via uart to the PC. The temperature sensor will be a thermocouple.

But all AVRs are 10bit. So I cannot use the internal ADC of the AVR chips.

I am looking for at least 12 Bits and 8 Channels preferably on an I2C bus. Can someone suggest me a part number??

Thanks

_____________________ Love and Peace keeps PrOgraMmerS happy :)

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Is a one chip solution OK?

The XmegaE5 series have a 12-Bit, 300 KSamples/Sec, ADC with either 8 differential or 16 single ended channels.

Perhaps worth a test drive?

JC

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Hi DocJC,

One chip would be the best solution, but I have never worked with XMEGAS.
maybe for future designs i will consider the XMEGA series.

_____________________ Love and Peace keeps PrOgraMmerS happy :)

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The MAX31855s read thermocouples with fraction of a degree resolution. SPI interface, 3.3V

Imagecraft compiler user

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hi bob,
The MAX31855s are costly and is single channel.

I was looking at AD7998 from Analog Devices

_____________________ Love and Peace keeps PrOgraMmerS happy :)

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have you seen the LTC2309 ? 6€ per chip in farnell.

Here is a link to a comparator of Linear Technology ADC:

http://www.linear.com/parametric...(adc)

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The ADS7828 from TI?

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The ADS7828 looks good to me. i will finalize on this.
Thanks sbennett

_____________________ Love and Peace keeps PrOgraMmerS happy :)

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Looks like a MAX31855 is $4.50, so 8 would cost $36, and you get 14 bits on the temperature side. I see the ADS 7828 is about $8.50, but will you need 8 thermocouple preamp chips also? I don't see any gain control in the ADS7828 data sheet. Need to get down into the microvolts on thermocouples.

Imagecraft compiler user

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Don't forget that you need cold junction compensation to get accuracy better than 20C. Of course, if you are measuring 300C, it might not be a big deal. But at temperatures most of us deal with, it will be a big deal.

At the very least, I would add a thermistor to read the temperature of the "cold junction", then use that to modify the temperature indicated by the thermocouples.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
Don't forget that you need cold junction compensation

Yes i remember that. I planned to use the internal ADC of the AVR with LM35.

_____________________ Love and Peace keeps PrOgraMmerS happy :)

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Why not use 2 4-channel ADC's like MCP3428? The sampling speed is much lower than the data transfer speed so why bother?

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MCP3428 is also a good choice. and it has a PGA too

_____________________ Love and Peace keeps PrOgraMmerS happy :)

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Don't you need to do a 3rd order polynomial with ln of the volts cubed like the Steinhart equation for thermistors when you linearize the tc volts to temperature? Unless your programmer works for free its going to cost more to do the tc equation than the chips cost. Unless you're building 1000+ of them, then pushing the hw stuff into sw pays off.

Imagecraft compiler user

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It is pretty fast and easy to linearize TCs with a simple LUT

Tom Pappano
Tulsa, Oklahoma

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I've used the MCP3208 and it works well in my application (reading temperature sensors).