8 RTD (PT100) Input Schematic
8 RTD (PT100) Input Schematic
(OP)
Hi,
I need advice in designing a board for temperature measurement using PT100.
I know that I need a stable current generator (of about 1ma) to the PT100 and then read the voltage drop.
My questiom is: Do I need a current generator for each PT100, or can I use one of all the 8 ?
The current generator is generally feed by a stable voltage reference. This APP note
ww1.microchip.com/downloads/en/AppNotes/00687b.pdf
from microchip shows that kind of circuit. Can I use Vref (2,5V) to drive 8 current generators like the one show on figure 4 ?
Thanks in advance for any help
Joaquim
I need advice in designing a board for temperature measurement using PT100.
I know that I need a stable current generator (of about 1ma) to the PT100 and then read the voltage drop.
My questiom is: Do I need a current generator for each PT100, or can I use one of all the 8 ?
The current generator is generally feed by a stable voltage reference. This APP note
ww1.microchip.com/downloads/en/AppNotes/00687b.pdf
from microchip shows that kind of circuit. Can I use Vref (2,5V) to drive 8 current generators like the one show on figure 4 ?
Thanks in advance for any help
Joaquim





RE: 8 RTD (PT100) Input Schematic
TTFN
RE: 8 RTD (PT100) Input Schematic
But for sure, you should just use one.
Alternatively you can just use the RTDs as the R in an RC circuit and hence use any old voltage reference for the charger.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 8 RTD (PT100) Input Schematic
Is this only for Pt 100 ohm 385 IEC or for other alphas, like the 100/120 ohm nickel or 10 ohm copper bulbs?
RE: 8 RTD (PT100) Input Schematic
Thanks for all the awnsers. My plan is to use a 3 wire layout. Standard PT100, the voltage drop is then measured with an 8 input, 24 bit, serial ADC (LTC2418) from linear.
I already have this working with one channel. But when I add more channels using the same current source I get some erratic readings. On the bench everything works ok, but in the field, with remote PT100's the erratic readings start.
And the problem is not on the location, because other comercial readers (with only one channel) work ok.
Joaquim
RE: 8 RTD (PT100) Input Schematic
Look at: ht
for switching the ref between all the different RTDs.
Also watch the 1mA as that may cause heating of the RTD and strange drifts.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 8 RTD (PT100) Input Schematic
RE: 8 RTD (PT100) Input Schematic
Is it common practice to cycle the current so that it is only ON when actually required (resulting in a very low duty cycle, for the heating, based on the measurement time compared to the desired update cycle)?
RE: 8 RTD (PT100) Input Schematic
Why not use something like an Analog Devices AD7711 for an all-in-one solution?
Analog Devices also have some IC's built to use AC excitation which helps eliminate measuring parasitic thermocouples. An example is the AD7730.
RE: 8 RTD (PT100) Input Schematic
It smoked is correct with the comment that you need one current source per PRT.
The current source has a huge number of error sources. Remember that a 100R PRT has 0.38 ohms per degreeC on a 100R basic. That is 0.4% error gives 1 degree error! You need 0.1% resistors to define the currents. You should then use one measurement channel to measure the 2.5V reference to give a ratio measurement and therefore a more stable measurement.
RE: 8 RTD (PT100) Input Schematic
Three-wire is pretty much the de facto standard. You can compensate for the lead resistance, close to perfectly, with three leads. Four leads are often used when your circuitry needs to be rudimentary, and you don't mind bigger sensor heads and extra cost for the RTDs.
You do not need two current sources for the three wire solution.
1mA continuously is no problem IF you can calibrate your system to offset the self heating.
1mA or even a few mA switched briefly in, during measurement, works very well and is quite common as long as the "switched on" time is rigidly enforced otherwise, you will get horrendous reading variations that are not subject to calibration.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 8 RTD (PT100) Input Schematic
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: 8 RTD (PT100) Input Schematic
RE: 8 RTD (PT100) Input Schematic
I have used this method to great effect many times. Until you work thru the description and math you won't "get it".
Here's the description(at bottom):
http:/
Here's similar:
http://
And here's what I'm telling the OP:
Text: http://w
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 8 RTD (PT100) Input Schematic
To answer one origional question - I don't see any reason why you couldn't use the same Vref to feed 8 current sources. You could use a multi-channel input A/D and by just adding more of the op-amp circuits build the 8-input circuit.
RE: 8 RTD (PT100) Input Schematic
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 8 RTD (PT100) Input Schematic
Electronic Design has often a problem with prefixes. Now it happened again. The 200 milliamps shall read 200 microamps. Just to avoid any confusion.
Gunnar Englund
www.gke.org
RE: 8 RTD (PT100) Input Schematic
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 8 RTD (PT100) Input Schematic
Morcego - I don't fully understand but in your other post did you mean that the single channel version of the circuit is not working in the field? Does this mean there a problem with the basic one channel circuit that needs addressing first? Is this monitor for a motor?