Analog to digital conversion with multiple channels

[bubulindo]

Hello,

Upfront i would like to tell that the Subject hasn't got much to do with my doubt.

I'm using the SAB80C515 and i would like to know if it is possible to connect all the analog inputs of the A/D to the same point?

Will it affect the accuracy of the readings??



Best regards, Carlos Cardoso

 

[nbucska]

Hi Carlos:
Without knowing the MFR and exact type# : one analog input
pin can be connected to one analog signal only but you may
add an analog ( 1 of 4, 1 of 8 etc.)usually MOSFET selector
before a pin to increase the number of signals so you can
connect any of the 4,8,etc. signals to an input pin -- one
at a time.

The SAB80C515 I looked up has a built-in selector so it has
8 analog input pins. It is usual for uC-s to use a pin selectably either as analog OR digital I/O. If you need
more, tell me the make and exact type#.






<nbucska@pcperipherals.com>

 

[buzzp]

If the chip doesn't have multiple input channels, use a multiplexer to select which channel you want to read from. This is easily accomplished using microcontrollers and a mux. Most micros I have used with A/D do not have the ability to resolve two channels at the same time since only one bit is set when an A/D is complete or only one interrupt is used for multiple A/D channels.
A mux can be had to have many I/O's connected to it. You select the channel using as little as 2 bits (depending on how many input channels you have into the mux- 2 bits would be for 3 inputs, 3 bits for 7, etc)

 

[nbucska]

2 bits =4 channels
3 bits =8
n = 2 to the n


<nbucska@pcperipherals.com>

 

[buzzp]

Yes of course. Sorry for that, hehe. You would think that would be drilled in my little pee brain.

 

[cbarn24050]

Hi, that would be rather pointless. If you just want 1 channel just use 1 channel.

 

[buzzp]

I believe the poster has multiple signals he wants to convert to digital. After reading the post several more times, it is not clear if this is the case. I would assume so or why ask those questions unless he might be referring to the reference voltage for the A/D measurements. I guess we will see when he/she replies.

 

[bubulindo]

I want to connect these all channels to the same point in order to get higher accuraccy with less effort. If you saw in 80c515 i can select the reference tensions for the A/D maximizing accuracy.



Best regards, Carlos Cardoso

 

[cbarn24050]

Hi, there is only 1 ad converter so you can only have 1 set of ref voltages active at any time, in other words you can't have different channels using different ref voltages at the same time. To get an accurate result you have to convert with 5-0v then convert again with a different ref based on the result of the first conversion.

 

[buzzp]

Why not mux them then change the reference voltage based on the channel your converting? I do not see how tying all the signals to the one A/D channel is going to improve accuracy.
I have not worked with microprocessors but with microcontrollers that I use the reference voltage can be changed easily in software to be internal or external
(assuming you have an external reference voltage source).

 

[IRstuff]

Why do you think that you'll get better accuracy?

Oversampling with post filtering could reduce the noise; is that what you mean?

But the fundamental accuracy is limited by the reference signal and the A/D circuitry itself, regardless of the number of inputs used.

TTFN

 

[Heydave]

In general terms, the accuracy resulting from connecting all the inputs to one source should not be effected. The only issues might be impedance loading on the source or increased noise injection but I doubt either would be a problem.