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Analog Multiplexer


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#1 ionic-fire

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Posted 29 September 2010 - 01:35 AM

I have the U6 with the CB37, and am almost out of analog inputs. I understand from reading the manual that the DG408 is a recommended multiplexer. My question is how to connect the DG408 to the CB37: the DG408 is a chip, so I assume I would need to set up a breadboard-type setup or through-hole circuit board with screw terminals for in and for out? And a socket for the chip too? Where do I connect the DG408 into the CB37 (aka what pins do I connect to)? And how is it possible to get 112 total AIN's by using multiplexers? I have pasted the pin description below for the 408, and I understand the S1-S8 pins--that is where I connect the input signal. But then it talks about a logic decode input (pins 1,15,16), enable input (pin2), V+ and V- (pins 3 and 13). The documentation says it needs a nominal input voltage of +5V so this would be the V+, but then what is used for V-? Where do I get this from? It also lists a test schematic whereby A0,A1,and A2 are connected in parallel to a logic input--is this where the truth table comes into play? It also has a +2.4V for the ENABLE input on pin 2. Plus, how do I know which input is which inside the DF program? In other words if I add two 408's, how do I know which AIN corresponds to which chip? I am also using DAQFactory, and the U6 driver should natively support all these extended AIN channels, I seem to remember reading in the manual. Any insight would be fantastic! Thanks much! DG408 PIN SYMBOL DESCRIPTION 1 A0 Logic Decode Input (Bit 0, LSB) 2 EN Enable Input 3 V- Negative Power Supply Terminal 4 S1 Source (Input) for Channel 1 5 S2 Source (Input) for Channel 2 6 S3 Source (Input) for Channel 3 7 S4 Source (Input) for Channel 4 8 D Drain (Output) 9 S8 Source (Input) for Channel 8 10 S7 Source (Input) for Channel 7 11 S6 Source (Input) for Channel 6 12 S5 Source (Input) for Channel 5 13 V+ Positive Power Supply Terminal (Substrate) 14 GND Ground Terminal (Logic Common) 15 A2 Logic Decode Input (Bit 2, MSB) 16 A1 Logic Decode Input (Bit 1)

#2 LabJack Support

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Posted 29 September 2010 - 12:54 PM

If you have a breadboard, or use the EB37 we sell, you can plug the chips into that and then use jumper wires for all the connections:

http://labjack.com/c...xperiment-board


And how is it possible to get 112 total AIN's by using multiplexers?

Each mux connects 8 lines to a single analog input. 8 times the 14 analog inputs on the U6 gives you 112 possible lines.


But then it talks about a logic decode input (pins 1,15,16), enable input (pin2), V+ and V- (pins 3 and 13). The documentation says it needs a nominal input voltage of +5V so this would be the V+, but then what is used for V-? Where do I get this from? It also lists a test schematic whereby A0,A1,and A2 are connected in parallel to a logic input--is this where the truth table comes into play? It also has a +2.4V for the ENABLE input on pin 2.


The diagram in Section 2.6.1 shows most of this:

http://labjack.com/s...ers-guide/2.6.1

This diagram is multiplexing AIN12 and AIN13, so you would use channel numbers 112-127.

You connect the 3 MIO lines from the U6 to A0/A1/A2. This tells the mux which channel to choose.

V+ and V- need to be at least the range of your signals. If all your signals are within 0-5 volts, you can use VS and GND. If your signals are +/-10 volts, however, you can use Vm+/Vm-, in which case you have to consider if you can get enough current from Vm+/Vm- to power the number of muxes you want to use.

You don't need to disable the muxes, so just tie the EN pin to VS.


Plus, how do I know which input is which inside the DF program? In other words if I add two 408's, how do I know which AIN corresponds to which chip? I am also using DAQFactory, and the U6 driver should natively support all these extended AIN channels, I seem to remember reading in the manual.

When you use a channel number from 16-127 the U6 automatically controls the MIO lines. For example, if you ask for channel 112 or 120, the MIO lines are set to 000 selecting the first input on both muxes. If you ask for channel 112 the U6 measures AIN12, while if you ask for 120 the U6 measures AIN13.

#3 LabJack Support

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Posted 29 September 2010 - 12:54 PM

Also, we are working on a mux board to sell. Contact us at [email protected] for more info.

#4 Sahm

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Posted 28 October 2014 - 03:23 PM

Hi

 

I have a question about the multiplexer board that you described here.

I have made a board with Intersil DG408 as you described, but when I read the thermocouples on a single IC ( 8 channels attached to that multiplexer) their values are different although they are in the same temperature.

 

Could you tell me what could be the problem? or what I should check in my board.

 

I have tested the ICs and they are ok (changed them with each other, and their response is the same), I have tested the thermocouples ( changed them with each other), but still the channels 16-19 are normal, 20-23 are 10 deg higher. For other channels their values are even higher.

 

I can send you pictures of the board I have made ( it's a mess).

 

Thanks,

SAHM.



#5 LabJack Support

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Posted 29 October 2014 - 01:24 PM

See the complications and troubleshooting tips on the Thermocouples App Note:

 

http://labjack.com/s...s/thermocouples

 

 

If you just have 23 connected and remove everything else, does it still read 10 degrees high?

 

 

10 degrees C high is a voltage error of roughly +400uV.  Do you have a DMM that can measure to that level?  If so, poke around and see if you do see that 23 reads 400uV higher than 16.




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