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buffering and multiplexing 8-ch 16-bit ADC

G

Glenn

Jan 1, 1970
0
I am currently using the ADS8344 8-ch 16-bit ADC connected
to an Atmel AVR microcontroller to interface various types
of pots, bend sensors, touch sensors, etc. and using 0-5V.

Would like to know what the best way is to increase this to
16 channels? What multiplexer chips would you recommend to
switch two sets of 8 channels?

Where would a unity gain buffer and low-pass RC filter be
placed - in between the MUX and the ADC, or on each input
channel of the MUX? Any recommended buffer chips?


And just wondering -- does there not exist MUX chips with
on-board unity gain buffers?

Any suggestions would be much appreciated -- alternate
approaches welcome.


Thanks.
 
T

The Real Andy

Jan 1, 1970
0
I am currently using the ADS8344 8-ch 16-bit ADC connected
to an Atmel AVR microcontroller to interface various types
of pots, bend sensors, touch sensors, etc. and using 0-5V.

Would like to know what the best way is to increase this to
16 channels? What multiplexer chips would you recommend to
switch two sets of 8 channels?

Where would a unity gain buffer and low-pass RC filter be
placed - in between the MUX and the ADC, or on each input
channel of the MUX? Any recommended buffer chips?


And just wondering -- does there not exist MUX chips with
on-board unity gain buffers?

Any suggestions would be much appreciated -- alternate
approaches welcome.

Its probably going to be easier to add another ADC.
 
D

David L. Jones

Jan 1, 1970
0
Glenn said:
I am currently using the ADS8344 8-ch 16-bit ADC connected
to an Atmel AVR microcontroller to interface various types
of pots, bend sensors, touch sensors, etc. and using 0-5V.

Would like to know what the best way is to increase this to
16 channels? What multiplexer chips would you recommend to
switch two sets of 8 channels?

Where would a unity gain buffer and low-pass RC filter be
placed - in between the MUX and the ADC, or on each input
channel of the MUX? Any recommended buffer chips?


And just wondering -- does there not exist MUX chips with
on-board unity gain buffers?

Any suggestions would be much appreciated -- alternate
approaches welcome.


Thanks.

Do you need 16bit resolution for all your inputs?, for example the
pot.
If not then how about using an AVR with an 8 channel 10bit ADC, even
the ATtiny26 has that.

The problem with a single RC filter after the mux is that the ADC
input needs more time to stabalise after switching. But if you don't
have to scan too fast then that saves you 15 RC filters. There are
pros and cons to both approaches.

Regards
Dave :)
 
G

Glenn

Jan 1, 1970
0
Do you need 16bit resolution for all your inputs?, for example the
pot.
If not then how about using an AVR with an 8 channel 10bit ADC, even
the ATtiny26 has that.

The problem with a single RC filter after the mux is that the ADC
input needs more time to stabalise after switching. But if you don't
have to scan too fast then that saves you 15 RC filters. There are
pros and cons to both approaches.

Regards
Dave :)

I do need the 16-bit resolution for the 16 external
channels, and will be using the on-board 10-bit ADCs already
for less critical stuff. Even for the pots, we need a good
resolution as often the pots are physically using only a
smaller portion of their full range.

The sampling needs to be kept as quick as possible - I will
be polling the uC at around 60hz and need to return all the
sampled AD channels with a minimum of latency. Also, I want
to be able to oversample certain channels if necessary,
without blowing the 60hz sampling period.

I see what you mean about the RC filter after the MUX - I
wouldn't be able to filter as much compared to having 16
seperate RC filters.

What about the opamp buffer? Would this go between the MUX
and the ADC, and how would that affect the scanning /
crosstalk, as opposed to having 16 seperate buffers?


A second ADC could work but would just cost a bit more. For
some applications it would be fine, for others, it might be
better to add lots of multiplexers, and in the future, I may
need even more channels. I will probably use both
approaches at different times - any idea what multiplexer
might be suitable?


Thanks for all your help,

Glenn
 
R

Rich Grise

Jan 1, 1970
0
I do need the 16-bit resolution for the 16 external
channels, and will be using the on-board 10-bit ADCs already
for less critical stuff. Even for the pots, we need a good
resolution as often the pots are physically using only a
smaller portion of their full range. ....
What about the opamp buffer? Would this go between the MUX
and the ADC, and how would that affect the scanning /
crosstalk, as opposed to having 16 seperate buffers?

Don't put anything between the mux and the ADC. Buffer each
signal individually. To 'splain, just think if two adjacent
signals were, say, 4.5v apart? Your buffer would have to
slew that, and then pick up on the signal itself. Buffer
before you select.

But truly, it sounds like a second DAC is the way to go,
especially given your sampling requirements.
 
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