sudden change current drive for AO signal

[yusof]

Hi,

When I'm stop generate analog signal, suddenly the current drive for AO increase or decrease sharply in the range -25 mA to 35 mA.

It happen to me many times. Suppose when the signal stop generate, the current drive became 0 mA and when the signal start generate, the current drive became ~5 mA.

How I'm going to tackle this problem?

[Jim_S]

Hello yusof,

Could you provide which module or device is giving you the problem? Also when you stop generating the signal, are you setting the current output to 0mA or are you stopping the task?

When you start generating, do you notice the current change in the same matter?

[yusof]

I'm using NI PCI 6024E

I'm just generate some voltage output and this problem occur when I'm stopping the task.

Every time I'm start generating signal, the current is around 5mA so there is no problem here.

But whem I'm stopping the task, sometimes the current do not become zero but it can go around -25 to 35mA.

This could jeopardize my circuit that connected to DAQ card.

[Jim_S]

Hello yusof,

From looking at the cards specs, it states that it outputs 10V at 5mAs. What voltage are you outputing to your circuit? Does this voltage change when the task stop? From my understanding it shouldn't change when you stop the task but continue to output that voltage. Knowing this, I believe something might be changing in your circuit to cause this voltage change. Is your VI controlling your circuit through any other hardware?

Also how are you testing the current? I am assuming the following below configuration with a DMM. Is this correct?

I would also run a test without your circuit. For example, I would use the DMM in series with a 10kohm resistor between AO and Ground. From here I would output 10V until it's outputing a constant 5mA and turn off the task. Do you notice the current change?

Finally could you explain what your circuit is?

[yusof]

Thanks. After further investigation I found that  I'm apply a wrong configuration and also a misunderstood problem.

Actually I want to generate AC signal as an external modulation of my laser driver.

The problem now, when I stop generate AC signal, there appear DC signal that follow the value of the amplitude of AC signal.

Could I make it by stopping the AC signal , the DC signal also stop (zero value)?

This is very important as to protect my DFB laser from overdrive by the DC value.

Hope to hear from you soon.

[Jim_S · Accepted solution]

Hello yusof,

When you are stopping the AC signal, do you mean that you are stopping the task? If you are only stopping the task, the last value will hold constant until the value is changed or reset. I would suggest writing 0Vs to the card to not output any signal.

[yusof]

Thanks. Now I got the picture.

Cheers

[M_Vrinkle]

Hi Jim,

I have a dumb question -- if the card has a maximum voltage output of 10V, and a current drive of 5mA, does that mean if I try to drive a 1000 Ohm resistor at 10V, I would still only get a maximum current of 5 mA? Or would there really be 10 mA through the resistor? I haven't tried it out yet because I don't want to damage my card... Just checking!!

Thanks!

[AnalogKid2DigitalMan]

Current will probably top off at ~5mA or maybe drop to zero.

The 6024E has short circuit protection to ground so you shouldn't damage them with the 1000 ohm load at 10V.

-AK2DM

[M_Vrinkle]

Thanks for the fast reply! Okay, that's good to know. Would you know if that's the case for a PCI 6703?