Academic Hardware Products (myDAQ, myRIO)

cancel
Showing results for 
Search instead for 
Did you mean: 

Using Attenuators with myDAQ

Solved!
Go to solution

So I am trying to use an attenuator (from mini-circuits, BNC, 50ohm) from an outputted signal on the myDAQ to get the voltages in the microvolts range.  However, instead of attenuating, the signal clips instead. I have tried a unity gain buffer, but the signal still clips. I have also just used an Agilent FG and Oscilloscope, and it worked then. 

 

Does anyone know how I can rectify this issue so I can using these attenuators on the myDAQ?

 

Thank you for your time.

0 Kudos
Message 1 of 10
(7,655 Views)

Hello Mochipoo,

 

What are the characteristics of your output signal?  

Josh B
Applications Engineer
National Instruments
Message 2 of 10
(7,633 Views)

Hi Josh,

 

I am currently using the Function Generator on myDAQ (using NI ELVIXmx), and tested with a 20Hz, 1Vpp and 10Vpp. The 10Vpp had clipping, with the 1Vpp did not seem to clip, but only had ~0.4 dB loss instead of 3.

 

 

This is a picture of the 20, Hz 10Vpp output, with unity gain buffer and 3dB attenuator. 

 with buffer.JPG

 

Thank you for replying!

0 Kudos
Message 3 of 10
(7,628 Views)

Hello Mochipoo,

 

What are you using to read in the signal?  Are you using the myDAQ for that as well?

Josh B
Applications Engineer
National Instruments
0 Kudos
Message 4 of 10
(7,615 Views)

Yes I am! I am attempting to do a full-loop system with myDAQ.

0 Kudos
Message 5 of 10
(7,613 Views)

Hi Mochipoo,

 

Do you see the same behavior without the attenuator?  Can you perform the same acquisition and post the result without it?

Josh B
Applications Engineer
National Instruments
0 Kudos
Message 6 of 10
(7,586 Views)

If I dont use the attenuator, it just looks like a 10Vpp sine wave... right?

0 Kudos
Message 7 of 10
(7,582 Views)

Hi Mochipoo,

 

Hopefully that is what happens.  I just want to make sure that it actually does function properly without the attenuator.

Josh B
Applications Engineer
National Instruments
0 Kudos
Message 8 of 10
(7,567 Views)
Solution
Accepted by topic author Mochipoo

Hello Mochipoo,

 

There are a couple of limitations you should consider before trying to drive that attenuator. A 3dB 50 Ohm attenuator is intended to divide the voltage amplitude from a 50 Ohm source down to 70.7%. That's basically a load resistance of about 150 Ohms.

 

-- Your benchtop FGEN has a 50 Ohm output impedance, but the myDAQ AO is a much lower impedance (about 1 Ohm at DC). The voltage divider you'd get has much less attenuation (150 Ohm / [1 Ohm + 150 Ohm] ~= 99%. 

 

-- Driving 10V into that attenuator requires a lot of current (10V / 200 Ohms = 50 mA in the case of the benchtop; 10V / 150 Ohms = 67 mA in the case of myDAQ AO).

 

-- The myDAQ AO lines have current limits to protect them from overheating and to keep from violating the USB power specifications. The limit is around 2.4 mA, so if you try to drive a 150 Ohm load with it, you'll clamp very low (about 0.3 V - 0.4 V).

 

-- As you described, you can buffer the myDAQ AO line with an external amplifier, but if you are using the myDAQ's +/- 15V rails, you need to be aware of their current limits (again, included to prevent thermal damage or USB spec violation). Our operational spec is +/- 16 mA symmetrically on +/- 15V or 32 mA on one of the rails. They will operate above that, but they have current limiters that will engage around 60-75 mA. I suspect that's why you're seeing your buffered signal clamp. Also, many amplifiers have internal current limits between 20-50 mA so they may not be able to drive that much output anyway.

 

If you are just trying to make smaller output steps, I recommend building your own resistor divider with an impedance that won't engage the myDAQ AO limiters (like 10 kOhm total load). You can make the attenuation factor whatever you want with the ratio. If the resulting output impedance is too high for whatever you are driving, you could rebuffer the divided signal. Since you aren't driving a heavy load, you won't have the power limitation problems.

 

Also, be aware that there are two operational ranges for the myDAQ AO: +/- 10V and +/- 2V. They are both 16-bit resolution, so you can get 5x smaller steps just by using the 2V range.

 

I hope this is helpful. Good luck and let us know if you have further questions.

 

Charles Y.

National Instruments

 

 

Message 9 of 10
(7,540 Views)

Thank you Sir! This was very helpful.

 

I appreciate you taking your time for your reply.

0 Kudos
Message 10 of 10
(7,525 Views)