amplify digital signal voltage

[rkkallur]

Hi,

I am trying to read a frequency input on my cRIO 9004 unit, using a NI-9425 model. I am aware that the 9425 unit is a static digital input module. However I can read frequency input(from a function generator) ,as my frequency input range is much less than the update frequency (140 kHz).  Now the 9425 recognises anything <5V as Off state and > 10V as on-state. My signal is a 0-5V signal, so the problm is i can never read it on the module. i have looked into other compitable modules, but the sad part is my budget does not permit a new unit now. So i am trying to work around it. I am looking for a a digital signal voltage amplifier that can amplify my 0-5V signal to 0-10V signal, THAT MAINTAINS THE FREQUENCY OF THE SIGNAL INTACT, as that is the very data I want. I have been looking at voltage -translators from TI,NXP etc. who offer digital signal voltage amplification, but I am not sure if it is a good solution for  LOSSLESS DATA ACQUISITION. i am basically a Mechanical Engineer, and have not faced this problem before. Has anyone come across such a situation? What do the electronics gurus suggest? Does NI offer any such voltage amplification for digital data that is also cost effective?

Thank you!

Rajesh 

[ABCPrograms]

Can't you use a secondary voltage supply which can supply the 10V and put an opto-coupler in between?

The opto-coupler acts like a relay, it's only safer because there is no electrical/mechanical connection between the signal side and power side.

Let your 5V switch the opto-coupler from false-true, and on the power side you have the 10V which is going to your DAQ card

Concerning to speed, because an opto-coupler works with light, it's for sure fast enough to handle the 140Hz frequence

[johnsold]

Bjorn,

Some optocouplers are much too slow for 140 kHz.  The speed is related to the internal capacitance of the base emitter junction and the amount of photocurrent developed in the junction, not the speed of the light.

Rajesh,

If there is no need for isolation, and it seems that there is not, then a simple amplifier should work.  One transistor and two resistors.  A 2N7000 FET should work fine. Its switching speed is in the tens of nanoseconds. The signal will be inverted but that will not change the frequency.  

Lynn

[ABCPrograms]

Hi Lynn,

There are optocoupler which are even faster then that.  I've been using them up and untill 200Hz.  Haven't tried farther.  But indeed you're right, I should have expressed myself differently.  After I read my post again I did seem as I was talking that it went that fast because of the light, and that's not true

[rkkallur]

Thanks Bjorn and Lynn for your suggestion.

Bjorn: Do you mean 200Hz or 200 kiloHZ? i need speeds in kiloHz. 

Lynn: that seems simple. and the signal that i get from the sensor is isolated from the power supply to the sensor. So i guess i wont need isolation. What about a summing op-amp? any differneces in performance in the two?

Thanks

Rajesh

[johnsold]

An op amp may work. Some types are too slow while others would be fine. Make sure that the large signal bandwidth is greater than 1.4 MHz (ten times the signal frequency so you do not round the edges two much). Also make sure that the output can exceed the 10 V requirement.  Many op amps with 12 or 15 V power supplies will swing the output to 10 V but not much higher.

Lynn

[rkkallur]

Ok thats exactly what my controls professor suggested me too. a 15V supply and a working frequency 10 times higher. 

Thanks! 

Rajesh

[rkkallur]

Hi,

may I ask one another question. What would you suggest for the  casing for  the PCB if i were to use these IC's, to actually give robust connections and protect from environment(magnetic field, heat, liquids etc). Or are these available with proper casing and stuff. could not find much online. 

[Drew_T]

Rajesh,

Building cases for PCBs is not my area of expertise, but if there is a specific environmental hazard (say heat for example), you could always research some material (in our example, insulative) and build your own.

Good Luck!

Drew T.

[johnsold]

Rajesh,

Your request is rather broad.  Magnetic field specifications: Strength of field? Constant or time varying? Over what frequency range? Heat: What temperature range?  How fast does the temperature change? What kinds of liquids? How many connectors? One or separate for Input, Output, and Power?  What about shock and vibration?

I suspect that you are worrying too much, perhaps because you are outside your area of expertise.  If this thing is to be installed close to the NI-9425, then its packaging can be similar to that of the 9425.  I would guess that a simple die-cast aluminum box with an o-ring seal would be quite adequate.  They are available in several sizes and are quite reasonably priced.

Lynn