15 Analog Inputs

[manutdfan2004]

Hi,

I posted a while back on connecting a pic to another wirelessly and then onto a PC, the responses were really helpful and I think I know how I am going to go about doing it now.

Problem is I now have realised that I need the first pic to take 15 analog inputs, as it is monitoring the movement of a hand. I know that this is possible by linking the pic's up using i2c as slave devices to a master pic.

But I want to know if anyone has experience of the performance of such a circuit as mine needs to be pretty fast.

Or is there a better way to acheive this, ideally it would be small form factor, which my 3x 28x chips to get the required inputs would not really be.

Cheers
Aaron

 

[boriz]

I just got a couple of these.

•	I2C up to 400kHz 
•	Simple command set 
•	10 channels x 10 bits 
•	Differential input ±1023 
•	2 us Conversion time 
•	5 us Acquisition time 
•	Can continuously monitor all channels 
•	Vref selectable 
•	EEPROM for general use 
•	Sleep mode to save power 
•	Operating voltage 2.0 to 5.5V 
•	Current 1.8mA @ 5V 
•	Sleep current 200uA

Not tried them yet, but it looks pretty fast to me. The items were delivered very quickly.

 

[boriz]

According to the enclosed paper manual, you can read each channel directly by addressing it, or you can set the device to automatically and continuously read all 10 channels and use the Picaxe to interrogate it as frequently/infrequently as you like.

 

[MFB]

I have found that the simplest way to add analog channels to a PICAXE is to use an analog multiplexer. For example, a 16 channel multiplexer could be fed into a single ADC input and its address changed via four digital output lines. You can sample as fast as you can read the ADC and increment the multiplexer channel address. Very little code required and very simple to test.

Something that surprised me recently was the advances made in what seemed a pretty mundane and well-established technology. Analog multiplexers are now available that operate from a single 3 volt supply and have rail-to-rail performance. By placing an amplier between the multiplexer output and PICAXE input, I have found it possible to handle mV level inputs without introducing significant (> 0.1%) errors. Take a look at the Maxim and Analog Devices web sites for examples of the latest multiplexer technology.