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Using analog signal MUX circuits for A/D board

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tjalps

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Hello!

I'm making one A/D moard with mc and I need more pins. Sounds familiar? I'm in dilema if I should use MUX CD4067, but there is quite big resistance Ron. Does anyone have any experience with use of this MUX circuits in real aplications. Any better solution?

Regards!
 

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signal mux

Place a high input impedance buffer/antialiasing filter between the MUX and the ADC (U have to do this anyway in order to provide antialiasing filter at half the maximuum sampling frequency of the ADC if not an SD type is used), or use ready data acquisition IC.
 

tjalps

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4067 mux

Hello!

For that you have two options:

1 Double reflow with selectiv soldering

You can first apply solder paste on let say top side of PCB, then you place SMT components and reflow that. You shouldn't put any heavy components on this side( 0805, 1206, so8 .. are OK ). After you do same on the other side. That SMT components are not displaced on PCB you can use two different solder pastes with diferent melting points. For that you also have to adjust temperature profile on reflow owen. After you can solder selective THT components.

For more info
Google: double reflow

2 Reflow on Top + Wave on bottom

As in previous post mentioned you apply paste on top of the board and then also place SMT components. After reflow you turn PCB around and then aplly glue points on the PCB at the places, where components will be placed. After that you should place all components on PCB and reflow that. At this step there is still no solder joint on the bottom. After you can place also all THT components and then drive this board thrue vawe solderin machine. You must be careful that on bottom side you don't have any high components, fine pitch.. After that you only should check the board.

Because I'm not so good in english, I hope that you will understand this. In case of any aditional questions please, do not hesitate.

Best regards!
 

tjalps

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4051 mux

Sorry for mix.

Hello!

In this case I'd like to sample DC. So you think that this MUX 4051 0r 4067 is quite usable? On what should I be careful?
I now this rule fsamp=2 * fact. But is it realy necessary to put anti aliasing filter on front of ADC if you now that all frequencies will not go over that values. Do you have any good proposal for IC?

Regards!
 

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analog mux buffer as well

Well let us be realistic. The inportant things of the MUX selection is the ratio of the ON/OFF impedances, the aperture time and the charge injection.

For slow process sampling (like the DC in your case) you may sample reasonably slow, then the aperture time and the charge injection aren't an issue, because you may wait long enough after the channel selection for the system to settle before initiating an ADC conversion. Hence in this case the most important criteria is the ON/OFF impedance ratio. Then how could some put a number on a OPTIMAL one (the most cheap that still do the job) ? Well all depend on the inter-channel xtalk you are allowed to have, and/or of corse on the resolution of your ADC (taking in account eventual oversampling).

Also when measuring DC, you will need good long-therm stability. Since every MUX impedance is thermaly dependent, U should think of implementing differential MUX (preferably on the same die istead of two items), and make sure the system is well thermally insulated from the air convection.

At last (but not least) the proper grounding could be an issue. Try to allways keep the analogue part on a big fat ground plane, separated from the digital ground. Those two usually are tied together @ the ADC.
 

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