Multiplexer with 4-20ma inputs

Question lacks detail. What are you trying to do?

...Jim Thompson

Sure. However, it probably won't work properly unless it's been designed to work with that kind of signal, and it might destroy the mux if it's not. You might also have to consider the settling times of the transmitters. Perhaps easier to convert to voltage and mux that, but you'll most likely have a larger error term that varies from channel to channel.

Best regards, Spehro Pefhany

Since we're talking actual electronics today, here's an on-topic, on-thread (sorta) question for multiplexor gurus.

The SN74CBT3251 is described by TI as a digital demux. However, a little digging on the TI website shows that it has a fairly linear transfer function from 0V to about 3.5V.

Any reason this part should not be used as an analog switch to select one of eight voltages to feed to a PIC A/D input?

So the only way is to convert each signal to in example 0-5volts.

I asked about the inverse solution because in that way I would have used only 1 current to voltage converter.

But if it's impossible I'll use a current to voltage converter for every input signal.

Many Thanks Marco

"Marco T." schreef in bericht news:ea5dda$iur$ snipped-for-privacy@nnrp.ngi.it...

Yep, big save, 1 resistor instead of 4.

Yep, 1 resistor for each.

I'm developing a system that should acquire analog signal and convert to digital.

So I thought to use Texas Instruments RCV420, but for 16 channel it's expensive.

Are you sure I could use a simple resistor?

Many Thanks Marco Toschi I thought to use Texas Instruments RCV420.

A 4-20mA signal is designed usually for 1K load, so the voltage swing could be around 20-24V. As long on your load (on the multiplexer end) does not exceed a voltage drop bigger than the analog multiplexer input you can do it. With other words you can't switch a 4-20mA through a standard analogic voltage multiplexer, unless you're converting it (with a resistor) to an acceptable potential. Switching 4-20mA at high voltage is also possible with custom current multiplexers.

greetings, Vasile Surducan senior engineer

If I were interested in same scaling between sources I'd use a current-steering multiplexer so that all sources operated through the same sense resistor.

...Jim Thompson

"Marco T." schreef in bericht news:ea5f66$jn3$ snipped-for-privacy@nnrp.ngi.it...

used

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Sure, why not? A simple 250 ohm resistor would convert your 4-20mA to 1-5V. That is not 0-5V, but you can deal with that in software.

Well, there's no reason to depend on limited compliance of the current source to limit the input voltage; the current can simply be steered into a clamp, provided each clamp can handle the maximum current (sometimes >30mA) at the voltage, and has low enough leakage to not affect the signal current excessively. A relatively high voltage zener might be good enough, but then the mux will have to handle the voltage too. Low voltage zeners have nasty soft "knees".

Best regards, Spehro Pefhany

Less might be better.

It's certainly not impossible, just maybe not optimal. Many muxes can't handle 20mA+ currents without excessive voltage drop, and you have to deal clamping the voltage in some cases etc. 0.1% resistors are pretty cheap these days, or use stable 1% resistors and correct for scale errors in your software or firmware.

Best regards, Spehro Pefhany

"Spehro Pefhany" schreef in bericht news: snipped-for-privacy@4ax.com...

Would you consider that? This is typically all wired to outside world nasty environment long cable stuff with serious unknown behaviour.

Sure, assuming it's configured properly* there's abso-frick'n-lutely no problem in this sitation.

• I mean using 2-wire xmtrs with a common + bus or 4-wire xmtrs with either isolated supply or common - bus.

Best regards, Spehro Pefhany

Hello Spehro,

Unless there was a birthday in the electricians shop, somebody brought in some booze and then one of them goes off to connect the last dozen sensors. POOF ... oops, guess that was the primary side terminals.

Spehro, What does a typical 4-to-20mA receiver look like?

...Jim Thompson

If the inputs of the multiplexer are designed to take 4-20 mA inputs, then yes, of course.

Otherwise we need more information.

Good Luck! ich

Usually just a precision resistor followed by some signal conditioning. TI nee BB will be happy to sell you an expensive differential chip that allows the inputs to float around dozens of volts. But it's seldom a requirement-- even their application circuits are single-ended. HART protocol current loop systems (hybrid analog-digital) use FSK (Bell 202) on top of the 4~20mA to allow bidirectional digital signalling, so that kind of receiver would also usually pick off or inject some AC before the signal conditioning.

Best regards, Spehro Pefhany

Spehro, don't forget the current is carrying the information, any leackage in the current value will means errors. As long he did'nt mention if the multiplexor will be on the transmitter end or to the receiver end, I can't say more.

best regards, Vasile

That's why I said "low enough leakage to not affect the signal current excessively". Only the selected channel matters. 16uA would represent an error of 0.1% of span. It could be done with BJTs and diodes too.

Receiver, so that his system can read one of many inputs at a time.

Best regards, Spehro Pefhany