simple ad converter IC

[stewman]

Hello,

I'm needing to read AD values with a pic, but I need 4 channels and I've only got 3 on the chip (changing the chip isn't an option). I still have 8 io ports available on the pic that I can read if hooked up to a parallel AD converter IC.

Does anyone know of a dirt simple AD converter chip that will continually convert without needing to deal with sending clock signals or reading ready bits or anything like that?

Thanks!
Stewart

[Bigglez]

*** Redacted by poster ***

[Engineer1138]

I think the venerable old 28-pin ADC0809 will run in continuous conversion mode if you hard wire the address and chip select bits. But why oh why would you do that, when there are many modern, easy to use, fast 8 pin ADCs with internal reference that only need clock and data signals?

What's so bad about having to toggle a clock line a few times?

[stewman]

Thanks for the good suggestions, I think the mux switch might be the answer for me.

One issue is for needing a simple solution is that I'm not actually programming the PIC I'm using, i'm communicating with a pre-programmed pic made for input and output operations. it is the Pic IO by jrkerr.com. I communicate with it serially and in one serial poll can have it send back all the pin and AD values (3 AD and 12 IO).

If I need to toggle the ADC pins then I have to send separate serial commands to the chip which takes extra serial port time. If I didn't have to poll it then I could just get all the info in one communication (3 AD values directly and the 8 IO pin values connected to the ADC). I've got a lot of communication going on the serial network so I'm trying to minimized the traffic.

Hope this clears things up. I like the mux idea because if I'm going to complicate things then at least I could multiplex all the ADCs on the chip and save most of my IO pins.

Thanks,
Stewart

[Bigglez]

*** Redacted by poster ***

[MrAl]

Hello Stewart,

If you have two digital PIC outputs available that you are not using
(and you said you did) then you do not need to add any more chips
to your circuit if your accuracy can stand to suffer 1/2 the normal
resolution of the AD converter.

The idea is to form a voltage divider network with four resistors and
connect the center taps to the PIC digital outputs. This forms two
analog inputs either of which can be disabled while the other is being
read by the AD converter.

To read one of the input 'channels' in this way, put the associated
digital output into the high impedance state while also switching the
other digital output to a logical low. The voltage appearing at the
AD input will be approximately 1/2 of the voltage at the actual input
to the network for that channel.
To switch to the other channel, simply put that ones output into the
high impedance state while grounding the other output.
In this way you can read both channels with one AD input.

The drawback is that because the voltage is divided by 1/2 for either
channel you have to make up for that by shifting the AD result left
one time, and that means a loss of resolution of 2 times the normal
resolution. This isnt a problem however for many applications that
dont need super high accuracy. Many times it is also possible to
simply work this factor into the algorithm so no shift is necessary.

Rather than explain the connections i drew up a schematic:

[stewman]

Good and simple solution. I'll give it a try. the inputs are for different joysticks and trim knobs so the resolution isn't too critical.

Thanks again, to everyone's good ideas.

stew