ADC frequency limit

Amit

I am starting my first SX project and don't know too much about electronics so please forgive any dumb questions.·

First of all I am planning to read a bunch of devices through an 8 channel 8 bit ADC (the ADC0838 http://www.national.com/ds/AD/ADC0831.pdf·) The spec sheet says that the max clock frequency is 400KHz.· Does this mean that the SX52BD and the chip have to both be clocked at 400KHz or below?· If so, can I use the internal clock set at say 125KHz and use the OSC2 to clock all my devices or would it be better to use an active oscillator to drive the SX and my devices?·

The AD7888 ( http://rocky.digikey.com/WebLib/Analog%20Devices/Web%20Data/AD7888.pdf )·uses the serial clock and as I understand it, that means I can run the SX and the ADC·on two different clocks but I am unclear how this would be connected or how it works.· Can someone please explain this?

I would only need to sample the inputs at about 3Hz so I don't think that I should be limited by my clock frequency.· I would appreciate it if anyone could help clarify this.· Thanks!

-Amit

Bean

Amit,
Normally the SX generates the clock for the ADC from one of it's output pins.
For example if you use the SX/B SHIFTIN command the SX will generate a clock of about 83Khz.
Bean

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·

Paul Baker

They aren't dumb questions, I don't see the 400kHz number you reference, but it doesn't matter. The operating frequency of the ADC does not limit the operating frequency of the SX. Since you are only taking 3 samples per second, the only frequency you need to worry about is the maximum serial clock for your interface with the SX. It is 2 MHz, and if you are using SX/B's builtin SHIFTIN and SHIFTOUT commands, you dont need to worry because they operate at less than 2 MHz, so there should be no issues with you using the AD7888. The only issue you will likely need to deal with is that only surface mount packages are offered for the chip. Sparkfun.com offers breakout boards you can solder the chip to so you can stick it in a breadboard to do your prototyping.

http://www.sparkfun.com/commerce/product_info.php?products_id=495·for the SOIC version of the chip,
http://www.sparkfun.com/commerce/product_info.php?products_id=498·for the TSSOP version

but SOIC is easier to solder·

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·1+1=10

Post Edited (Paul Baker) : 1/24/2006 7:34:28 PM GMT

Electronegativity

Hi Amit.

The clock is generated by raising and lowering the voltage of a pin on the SX that is connected to the clock pin on the device.

If you are transmitting data to a device then the device will read the voltage on its data in pin on either the rising or falling edge of a voltage transition of its clock pin.

If the SX is running at 4 megahertz, and the device has a maximum speed of 400 kilohertz, then you would need keep the voltage on the clock pin high for at least 10 clock cycles before lowering it, and wait at least 10 more clock cycles before raising it again.

I strongly reccomend that you read Guenther Debauch's book.

www.parallax.com/detail.asp?product_id=70002

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I wonder if this wire is hot...

Amit

Thanks for all the help guys! Your info really cleared things up.