4-20mA Application: need some advice, please

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Hi all,
I need to monitor six 4-20mA sensors at once, with a resolution of 16 bits.
It is half a hobby project and half a favour to my grandfather, although
I'd also like to put it in my curriculum vitae, for further advantage. I
am not an engineer, I am a self-taught programmer, but the more stuff I add
to my resume, the better, ain't it? ;) So, I think it's now time to start:
I'd like to use one single 12V or 24V battery to power all 6 sensors (one
at a time), convert the 4-20mA signal to a voltage, channel it (through a
multiplexer) into an ADC and read and log the data via a microcontroller.

Could you give me some advice if I'm doing right or wrong? And why?

a) For the battery part, I thought about using one or two 12V sealed lead
battery or two PP3 9V lithium 1200mA/H batteries (a friend told me they
exist and look like normal 9V batteries, although they have x 10 mAH!).

b) For the input stages, I'm thinking about using a scheme like this:

   Battery +24V or +18V (not yet decided)
     |
    RES (to limit max current in case of Sensor's short circuit)
     |
===Sensor (4-20mA type)
     |
   MOSFET-- on/off control (would a relay be better? why?)
     |
     *------*--- to Analog MUX / ADC (Sensor's 0..20mA becomes e.g. 0..5V)
     |      |
 5V Zener  RES (to convert current to voltage)
     |      |
     *------*--- ground
 
I saw the RCV420 by Texas Instruments/Burr Brown, but it costs $22 here
in Italy! And anyway I'm not sure it would really help in my application.

The MOSFET is there to "isolate" every other sensor, so that only one is
getting current at a time. However, I'm not really expert of high-side,
P-Channel MOSFETs and at first I thought about a N-Channel solution, but
on the bottom, between ground and Zener/Resistor. The N-Channel solution
would cause a measuring error though, unless I can take it into account
on the ADC. I think the P-Channel solution may be more "proper" anyway.

The purpose of the bottom Resistor is easy to understand, it will convert
the current into a voltage. The Zener is there to make sure that voltage
doesn't go too much beyond 5V even in case of a Sensor short circuit (the
current will be limited anyway by the top Resistor).

Would a Transil diode instead of the Zener be really useful as protection?
I am thinking about some hundreds of meters of cable which, due to its own
induction, could kick very hard when the MOSFET turns on. Of course, if a
cheaper Zener solution is as good in practice, why waste a Transil there?
A reason may be electrostatic interference or nearby lighting strikes,
would a Transil be a more valid choice than a Zener then (I know that if
the lighting strikes right on the cable, nothing can be saved anyway!).

Now I (think) I could feed directly an ADC input (should I buffer it with
an op-amp? Why? The impedance is already pretty low, and while the ADC
input may draw bias current, I think I can calibrate the whole system at
the digital output, cannot I? Or the ADC input bias current will vary, thus
introducing an error in the measure? I'd like to get near 16bit resolution).

I saw some interesting ADC's from Texas Instruments which contain also an
analog multiplexer. For example the ADS1243 IC, which contains a 8-MUX, a
buffer and even a programmable gain amplifier, and it's even 24 full bits!

Thank you very much for any useful advice,
--
Andrea


Re: 4-20mA Application: need some advice, please

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NB: 4-20mA devices are rarely designed to be powered on/off
    like that.  You'll may have to wait a while (up to a second
    or two perhaps) for the device to power up and stabilize
    before you take a reading.

--
Grant Edwards                   grante             Yow!  I like the way ONLY
                                  at               their mouths move... They
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Re: 4-20mA Application: need some advice, please
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Take a look at using a polyswitch instead.

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Right idea for simple overvoltage protection but I prefer.

      *------*--- Res2 ----*--------*
      |      |             |        |
     ???    RES         5V Zener   Cap
      |      |             |        |
      *------*--- ground --*--------*

Res2 and cap then form your anti-aliasing filter (or part of it).  Res2
and the zener perform overvoltage protection with the resistor current
limiting to protect the zener.  ??? can be replaced by a high value
transorb/transil etc to limit the voltage across RES.  It can the be a
higher voltage and power and its breakdown need not be as precise as the
5V zener.

And as Grant suggested make sure your sensors are going to be happy
power cycling.  It may end up being easier to budget for the extra power
consumption.

Robert


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Re: 4-20mA Application: need some advice, please
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There 2 types of 4-20mA transmitters: 2 wire and 3 wire. The 2 wire, whic
is what you’ve got (from your description) use the current flowing throug
them to power the unit. If you were to multiplex it, it would turn o
every time and there would be a series of electronic bumps and thumps a
is comes into service during which you could not take a reading. Secondl
there is a thermal stabilization time for the unit to meet it
specification. I have seen this time rated as long as 15 minutes so you
accuracy would become suspect. I would highly recommend that you powe
each sensor continuously into its own sense resistor and then feed th
developed voltage into a multiplexer.

The current flowing through   the device is often used to develop
voltage internally. This means there will be a minimum voltage across th
sensor, even though it is a current source. Often the sensor data shee
will state what this voltage is, but it normally will state th
“compliance”, that is the voltage it can drive (the voltage across you
sense resistor). It will be rated at a given supply voltage. The voltag
minimum voltage across the sensor would be the difference between the two
In some applications the sense resistors of several receivers are connecte
in series.

The voltage across your sense resistor is determined by Ohm’s Law. Give
that you want 5V max, the resistance would be 250R. The some of all th
volt drops in the current loop (including across the resistance of th
cabling) must be less than your supply voltage. If you introduce a curren
limiting resistor it will constitute a further volt drop, so take care t
stay within the limit. Robert’s idea of a polyswitch is a good one. Mos
industrial units will be rated for a supply of 24V with a range aroun
that, but the lower the supply voltage, the lower the compliance gets.

As far as a zener or transil for protection against sensor short circuit
I don’t believe that either serves a purpose in that event. You can make
case for the transil for surge suppression. If the sensor does shor
circuit, depending on the other current limits in the circuit, you coul
over-power the zener/transil device and blow it. Added to that, th
tolerance of either device is at best 5% and so you would have to find
device whose minimum turn on (0.95 of the rating) was greater than 5V. A
the other end of the spectrum, the maximum (1.05 of the rating) must b
less than Vss+0.6V.

It is far easier to use a series resistor between the sense resistor an
the multiplexer (as shown by Robert) and then clamp the input to the mu
to Vdd with a diode. A Schottky would be better because of the 0.3V drop
but can have high leakage, and so could cause problems, depending on you
circuitry.

You may want to consider implications of earth loops between the common o
the sense resistors and the common of the A/D.One approach could be t
measure the input voltage differentially using an instrumentatio
amplifier (IA). Although the current loop is inherently noise resistan
with the correct wiring, the IA can also improve the noise rejection

Incidentally you will get 16 bit resolution from your choice of A/D, bu
consider yourself lucky if you get 12 bit accuracy! Have you any idea o
the accuracy of your sensors? And just wait till the temperature changes.

-Aubrey


Re: 4-20mA Application: need some advice, please
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Be careful about that clamp to Vdd.  If the Vdd supply is not designed
to sink current (and few are) you will need to make sure the clamp
current never exceeds the current drawn by the circuit on Vdd,
otherwise Vdd will rise possibly frying your circuit!

I have to admidt that I am pretty ignorant of 4-20 mA loop circuits.
The only current loops I have ever seen are on TTY circuits which are
digital circuits.  In fact, that is what I would do instead of using
the circuit you are planning.  I would design a digital circuit to put
on each sensor to read the value of the sensor locally, then ship the
result back digitally.  This helps to eliminate a lot of issues that
will affect the accuracy of your design.  I expect that if you are
thoughtful about how you design your common 4-20 mA loop, you can use
it to power all of the sensors, all of the remote converters and use
an addressing scheme to poll one sensor at a time on a common wire
without having to use a multiplexer.  But that might get a bit
complex.

Certainly you could operate all the units with a simple mux, but use a
digital controller to return a digital signal on the current loop.



Re: 4-20mA Application: need some advice, please
On 06 Apr 2007 15:23:51 GMT, the renowned snipped-for-privacy@NOSPAMPLEASE.com

1. You don't need an analog mux at all, half a dozen 1N4148s will do
at least as well.

2. A single chip will work to switch power to the transmitters, eg. a
Micrel 2981

3. Pay attention to the specs as Grant Edwards suggests, some
   transmitters may be spec'd with 30 minute warm-up.

4. A constant-current circuit would be better than a series resistor
   or polyswitch (eg. a 25mA 2-wire constant current circuit using
   an LM317 and a resistor. It will need a few volts to work, so it
   affects the compliance. You will need to check the specs to
   determine the minimum supply voltage for your application (
   the compliance range of the transmitter, the drop on your sense
   resistor, the drop in the driver circuit, the drop in the current
   regulator or whatever, and a bit of margin, at say 25mA).  

5. It's best to add some series impedance from your sense resistor,
   clamp that to some fixed voltage (eg. Vdd) and perhaps divide
   that down a bit and buffer it to go into your ADC. You can
   Low-pass filter it a bit too. Take care that too much current
   doesn't flow into Vdd such that it causes Vdd to increase
   (regulators generally don't sink current, only source).


Best regards,
Spehro Pefhany
--
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Re: 4-20mA Application: need some advice, please
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This approach works when you are power cycling each unit- even if you ca
live with a 30 minute warm up, keep in mind that the voltage you measur
is a diode drop below the actual voltage and if that wasn't bad enough i
changes by -2.3mV/degreeC. Your resolution is 5/6553676%uV, so for on
degree temperature change your reading will be out by a count of 30 i.e.
bits for only one degree temperature change.


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The network that is proposed here is two resistors in parallel with th
sense resistor, so you are going to have to calculate the effectiv
resistance. I suspect that if there is any input current to the A/D i
will also add a non-linearity. It will also add the inaccuracy of th
thermal performance of a resistor, but compared to the other inaccuracie
being proposed, it is probably irrelevant.

Re: 4-20mA Application: need some advice, please
says...
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Yes and no, the second resistor is in series with the A/D input, its
return path is through the A/D so its impedance will be quite high.  
That without considering that the resistor used will be a quite high
resistance in any case.  

Now if you add a divider then there will be some effect but given the
ratios it will be small.  The sense resistor is usually a few hundred
Ohms and the series divider 10's of k, so on the order of 1%.  Enough
that you will need to calibrate out or account for.  Of course the diver
precision would need to be accounted for anyway.

Robert

--

Re: 4-20mA Application: need some advice, please
On Sat, 07 Apr 2007 10:05:13 -0500, the renowned "antedeluvian"

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Unh-unh. The transmitters supply a constant current so the diode drop
is of absolutely no consequence provided the compliance range is not
exceeded. The only error is from leakage in the diodes, which is going
to be negligible for a 1N4148 (25nA guaranteed maximum at 25°C/20V,
down around the 20 bit level even at that relatively high reverse
voltage)

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Nope. because the buffer has very high input impedance at DC, and it
is in *series* with the series LPF resistance. 1G + 10K is very high
and shunting say 100 ohms or 250 ohms, it's of no importance. As
Robert says, if you have a second divider it's just in parallel, the
effect is small, and can easily be calibrated out when you calibrate
out the effects of reference inaccuracy and resistor inaccuracy.
Making a clamp that does not affect the signal to an accuracy of 16
bits is not so easy- certainly a zener diode won't work very well
unless the difference between the clamp voltage and operating voltage
is relatively large.

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Hence, the suggested buffer, eh? Some ADCs have a buffer amplifier or
PGA built in.


Best regards,
Spehro Pefhany
--
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Re: 4-20mA Application: need some advice, please
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I misunderstood the proposed connection- my apologies.

-Aubrey

Re: 4-20mA Application: need some advice, please
says...
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I'm curious as to your reasoning on this point.  I can see it might
reduce power losses on a two wire (loop-powered) transmitter but I don't
see the advantage otherwise.

OTOH getting a small enough polyswitch could be a problem.  That might
be reason enough.

Robert


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Re: 4-20mA Application: need some advice, please
On Sat, 7 Apr 2007 13:36:04 -0400, the renowned Robert Adsett

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I'm concerned that the brief surge while the polyswitch breaks might
be high enough to cause a shift in resistance of the precision load
resistor, particularly if it's a small SMT 0.1% thin-film part and
some power supply capacitance is being discharged. I've also seen such
low value metal film precision resistors fail open due to induced
currents from lightning when connected to the outside world directly,
so something right in series with the input might be good.

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They do work on (micro) heating. Heck, he could use both, a constant
current circuit in series with the input (which would also protect
against modest outside voltages, up to +/-50V say) and one or 6
polyswitches in series with the sensor energization to protect the
power supply in case of a short to ground. And a big whacking tranzorb
right across the input if you want to be really safe (if the leakage
current isn't too high).


Best regards,
Spehro Pefhany
--
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Re: 4-20mA Application: need some advice, please
says...
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Ah, I'm used to rather large resistors being used for this.  At least
1/2W often 1W.  Just to deal with worst case faults.

Robert

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