The use of attenuators with myDAQ

So I try to use an Attenuator (from mini-circuits, BNC 50 ohm) output on the myDAQ for tensions in the microvolt range.  However, rather than mitigate it, the signal of clips instead. I tried a unity gain buffer, but the signal still clips. I used just an Agilent FG and the Oscilloscope, and it worked then.

Anyone know how I can fix this problem so that I can use these attenuators on the myDAQ?

Thank you for your time.

Hello Mochipoo,

There are some limitations that you should consider before trying to drive this attenuator. A 50 Ohm 3dB Attenuator is designed to divide the amplitude of the voltage from a source of 50 ohms to 70.7%. It is basically a load of about 150 Ohms resistance.

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

-10V in this attenuator driving requires a lot of current (10V / 200 Ohms = 50 mA, in the case of the bench; 10V / my 150 Ohms = 67 in the case of AO myDAQ).

-AO myDAQ lines have current limits to protect against overheating and prevent violate the USB power specification. The limit is approximately 2.4 mA, so if you try to drive a 150 ohm load with her, you'll shake very low (approximately 0.3 V - 0.4 V).

-As you describe, you can buffer the AO myDAQ with an external amplifier line, but if you use the myDAQ +/-15V rails, you should be aware of their current limits (once again, included to avoid thermal damage or technical violation USB). Our specifications operational is +/-16 my symmetrically on +/-15V or 32 my on one of the rails. They work above that, but they have current limiters which will be deliver around 60-75 my. I suspect that's why you see your clip of signals in the buffer. In addition, several amplifiers have current limits internal between 20-50 my if they may not be able to drive much output in any case that.

If you just try to take small steps to exit, I recommend you build your own divider of resistance with impedance that will not engage the limiters of AO myDAQ (such as the total of 10 kOhm load). You can do the mitigation of the factor what you want with the ratio. If the output impedance that results is too high for what you're driving, you may refresh the split signal. Since you are not driving a heavy load, you won't have the problems of limitation of power.

In addition, be aware that there are two beaches of operation for the AO myDAQ: ± 10 v and ± 2V. They are the two 16-bit resolution, so you can get 5 x more small steps only by using the range 2V.

I hope it's useful. Good luck and let us know if you have any other questions.

Charles Y.

National Instruments

Tags: NI Products

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