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CD Datasheet, CD Single 8-channel Multiplexer/Demultiplexer Datasheet, buy CD Data sheet acquired from Harris Semiconductor .. CD MHz. VOS at Any Channel. MHz. Electrical Specifications Common Conditions Here: If. CDBMS, CDBMS and CDBMS analog multi- plexers/ demultiplexers are digitally controlled analog switches having low ON impedance and very.

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By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. I am interested in developing a generic 8-channel 4-wire PT interface for RaspberryPi. Needed accuracy must be better than 1degC and range from degC. Additionally i need lead wire compensation. Here is datasheet plan: I am guessing this should work: Then B measure voltage drop across PT Now my problems are: So the current may not be that constant.

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I will hand pick the resistors but what about temperature compensation? In general how much current variation can i expect?

As i have read in distributed posts here as well as in some papers eg. But i could never find how they practically implemented this or if its performance suffered and so on. This is so because 1mA is passing through the 1st MUX. Kindly let me know if there are any mistakes or improvements in the above scheme. I am a novice mechanical eng. I am only now interested in 3-wire PT sensors as these are what available locally at affordable prices. All the rest of the requirements remain the same.

So the problem is clearer now: In this figure, the first wire of pt is supplied by 1mA current through a HC see footnote. Since i need to make a differential measurement between the first two leads of the pt while dc4051 rds on dafasheet HC, i attached A0 of the ADC directly to the first wire through a schottky diode.

The diode prevents the activation of other PT channels not activated by HC Am i wrong or OK in this regard? Is leakage current going to be a problem? I have also included a resistance 2. This i presume will help make a better differential measurement at the ADC C4051 and A3 while avoiding any current passage through HC Does this make sense? I will take Sephro’s suggestion of HC The Texas Instruments datasheet of the CD isn’t very clear about the maximum current.

However the high on resistance will still prevent you from reaching the adtasheet accuracy. The on resistance will vary over almost anything like supply voltage and temperature. What is possible is to use a current source for each sensor and then only multiplexing the resulting voltage to the ADC input. Yes, you can do that perhaps not with the Cc4051 and perhaps not with that particular 1mA current source, because of voltage compliancehowever the only point of ADC input A0 is to check that the current source is cs4051 outside compliance voltage.


Rds on is huge as much as ohmsand varies with supply voltage, input voltage and temperature. An HC would be better at 5V. If you really want to measure ‘top’ lead resistance for some reason other than break protection for which A0 suffices you can add another analog multiplexer and pick off the voltage at the ‘top’ of the RTD.

Datqsheet can measure the ‘bottom’ lead resistance by measuring relative to ground. Of course, there is no way to measure the resistance of the other ce4051 leads because no significant current passes through them. Realistic problems are going to crop up when you attempt to protect the inputs against the things that can happen off board. You may be better off expending the scratch for genuine analog switches that have a degree of built in protection and adding some external protection around that.

I worked on Bimpelrekkie ‘s as well as Spehro Pefhany’s answers. Since it expands i cant post it as a comment. Neither can datasheeet post as an edit, i guess because it is an answer of sort, a comprehensive one.

For current source, i have added 8 of these, each giving 1mA as per Bimpelrekkie. But they are now activated through a HC IC as below. The remaining circuit consists of routing each of this 1mA to each pt Since no current is flowing, the resistance of mux is not an issue here. Using cr4051 on middle wire of PT and differential measurement of A0 and A1 gives lead wire resistance.

raspberry pi – Multiplexing PT with CD – Electrical Engineering Stack Exchange

The sink of all currents is through a resistor, which i suppose will help in making a noise free measurement. Please correct me if i’m wrong in this. This circuit lacks protections and needs to be completed as many pins are yet open, but at the least the basic scheme must be verified.

If any errors or possible limitations please do let me know. By clicking “Post Your Answer”, you acknowledge that you have read our updated terms of serviceprivacy policy and cookie policyand that your continued use of the website is subject to these policies.

Home Questions Tags Users Unanswered. Thanks a lot for responding so far. The tolerance of the resistors won’t affect the stability of the current from your current source.

It may affect the actual current. You mean once the resistor if soldered fixed, the current from a particular circuit will be stable over time and temperatures? In my edit, i made a mistake. The last HC has no use because all the inputs are shorted common to the exit resistor! I dont know if i should change the edit figure and text or keep it like this. There is no such limit on switch current- 1uA is leakage.


In any case, CD voltage drop is excessive at much over 1mA. Bimpelrekkie One current source for each pt is a good idea, thanks. But this increases components and board size. If some other idea could be evolved, it would be great! Bimpelrekkie thanks for clarifying. Now could i multiplex the input voltages to constant current circuit?

Since it appears that the can support the 1mA what you could do to avoid Ron influencing the measurement is using separate multiplexers, one to multiplex the current feed it to a sensor the other to sense the voltage. The multiplexers would then be switched to the same input at the same time.

No, that is not a problem as long as the current source is working properly. As long as there is enough voltage headroom across the current source circuit so that it is able to actually make that current flow then any extra series resistance doesn’t matter. If you have an ideal current source of 1 mA in series with a resistor, what will the current be? The current will always be 1 mA whatever value the resistor has. Spehro Pefhany k 4 I did not think about voltage compliance with A0.

Instead i though i could make a differential measurement of voltage drop across the first two leads of the PT using A0 and A1 to get lead resistance. Also i did not yet think about protections to the inputs of the board, a very valid point. If i simplify the requirement to 3-wire PT, will it help? Can i use transistors as switchs to route the 1ma instead of CD? Or reed relays could be another way?

I checked with local supplier and they dont have this available. I could order online but it would take weeks and cost a lot import duties to india. I guess i will have to go with the better versions of for now.

I already thank Bimpelrekkie and Spehro Pefhany for helping in such short period. See comment above with idea to replace 8 current sources by one current source and one Finally i didnt go with the above but a modification using Bimpelrekkie ‘s suggesstions.

One can look at the schematic and explanations datashret. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. Post Your Answer Discard By clicking “Post Your Answer”, you acknowledge that you have read our updated terms of serviceprivacy policy and cookie policyand cd40051 your continued use of the website is subject to these policies.