a 16 bit ADC for BS2

farsad

dear friends:

do you know if there is any 16 bit analog to digital converter compatible with BS2?

thanks

Mike Green

There are many such devices. One example is www.maxim-ic.com/quick_view2.cfm/qv_pk/3068. Since the Stamps are powered by +5V, you probably will want to use a +5V ADC. Maxim makes a variety of ADCs, some with multiple channels, some with internal reference supplies, some are very fast and some are slower and they have different input ranges. You probably should use a device that uses the SPI or MicroWire serial bus protocol. This requires 3 to 4 I/O pins and uses primarily the SHIFTOUT and SHIFTIN statements to produce a clock pulse and shift in / out the data. Parallax sells several 8 bit analog to digital converter chips which use this protocol and the 16 bit chips work in a similar fashion, just with a shorter data value. Have a look through Maxim's website and datasheets.

farsad

thanks Mike.

if the 16 bit works exactly like the 8 bit, i will have no problem using it.

Tracy Allen

Hi Farsad,

Don't expect it to be quite as easy as the 8 bit converter. You will have a careful reading of the data sheet and application notes if there are any. The digital interface is one thing, as there will usually be several registers to configure and it seems none of these chips are ever "exactly the same" in the interface protocol. And then there is the analog side of things. For example, the Maxim chip that Mike suggested has in input impedance of around 5 kohms, due to the technique that is used to achieve the bipolar input range. The chip has to be driven by an op-amp circuit, unlike your 8 bit converter. This is a relatively fast 16 bit successive approximation converter, single ended (not differential). 16 bits is a lot. Imagine a stack of 131 reams of paper, 500 sheets in each. One bit in that stack is like the thickness of one sheet of paper. The result can be very sensitive to pickup of noise or inaccuracies or thermocouple effects in the input circuitry, and also AC pickup. Many 16 bit ADCs you see are of the Sigma-Delta type instead of successive approximation and are slower because of averaging techniques that reject AC interference. I'm not trying to be discouraging, just pointing out other issues.

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Tracy Allen
www.emesystems.com

Mike Young

Farsad,

Some other tips:· the 16-bitter is likely to have separate analog and digital grounds.· In a high-precision mixed analog/digital system, good engineering practice is to segregate the analog and digital sections of the circuit.· Each should have its own ground plane, and these should come together at only one point.· I usually use a surface mount RF choke to make the link, taking care to ensure it can handle the current on the ground bus.· Extensively decouple the power supply rails on the A/D and op-amp using a mixture of 0.1uF ceramic and 2.2 or 10uF ceramic or tantalum capacitors, keeping the power traces as short and wide as possible.

No all op-amps are created equal, its quite easy to mess up the accuracy of a fantastic A/D through sub-par op-amp architecture.·

I'd suggest you head over to Analog Device's web site and check out the ADISim tools for op-amp evaluation ...

http://designtools.analog.com/dtAPETWeb/dtAPETMain.aspx

Also, all of the major vendors have extensive app-notes on A/D and op-amp usage...

http://www.analog.com/en/DCcList/0,3090,760%255F%255F43,00.html

Good luck!
Mike

farsad

dear friends:
thank you very much for your advise. i decided not to use a 16 bit ADC. because i want to make my job easier but it seems that a 16-bit just mahs every thing more complicated.

thank you again,

Farsad