myDAQ tutorial: Increase analog output (AO) current drive with op amp voltage follower

[edoering]

The myDAQ analog output (AO) voltage range is ±10 volts with a maximum output current of 2 mA, considerably lower than a typical benchtop function generator capable of driving at least 100mA. A lower-than-expected AO voltage indicates the connected circuit demands more current than myDAQ can source. The following video clip demonstrates the analog output's behavior under increasing loads and then presents the op amp voltage follower as a simple way to increase the effective current drive capability of the myDAQ analog output.

NOTE: The “Set AO Voltage and Show AI Voltage” LabVIEW application shown in the video is linked at the bottom of this document.

[Patrick_L]

Hi,

I'm wondering how you were able to power the op-amp from the +/-15v rail of the myDAQ. I had to use an external power supply for my 741 to make this work. Everytime I tried to power the op-amp from the myDAQ, I got an error about the maximum power that can be supplied being exceeded. Did you use a specific op-amp for this lab?

Thanks already.

[edoering]

I used the TI TL072 op amp, but I would expect any dual-supply op amp to work sucessfully. Normally an op amp only draws significant current from the power supply when the op amp output drives a heavy load. Try connecting the 741 like this: ground the two signal inputs, leave the output disconnected, and then connect the power supply pins to the myDAQ dual supply. If you still see the over-limit error then try another op amp, otherwise proceed to building your original circuit. An over-limit error at this point suggests a wiring problem.

Hope that helps!

[Patrick_L]

Thanks,

Just so you know. I'm new to applied electronics  but I'm trying to learn as much as possible.

It did work wih the 741 once I followed the steps above : first connecting inputs to ground, then the power pins, and finally the output pin. It also got me wondering again. How much current would it be possible to source using the op-amp in the voltage follower configuration. Is there a specific formula?

[edoering]

You can learn a lot about your particular op amp from its datasheet. For example, a Google search on "741 datasheet" turns up http://www.ti.com/lit/ds/symlink/lm741.pdf which describes the output behavior. From this reference you can see an upper limit on output current (the short-circuit current), but that means your output voltage is necessarily zero. The "output voltage swing" specification states the range of output voltages you can achieve for a minimum load resistance RL. Datasheets for other op amps intended for high output current applications (e.g., OPA541) will directly state the maximum output current.

The myDAQ 15-volt power supply is limited to approximately 30mA, consequently any op amp serving as a voltage follower is limited to this value.

[robert.trow]

Hi,

I would like to build the voltage follower circuit, and have recently purchased a couple ACL072 I.C. chips.  My question is, could you post a diagram or direct me to where I can find the circuit diagram of exactly where everything connects in the op-amp voltage follower circuit above.  Basically, I cannot quite make out some of the connections in the video.

Thank you,

Robert

[robert.trow]

I actually meant 072ACP chips.

[juliocveras]

Hi edoering,

I´m trying to do a PID control for temperature with myDAQ AO. Indeed, it works but when the PID control is acting the DAQ AI starts to get crazy. I suspect that the GND from my source may have some influence on AO-GND on myDAQ, is that possible ? How can I fix such problem ?

P.S.: My suspect is because when the source changes the +V to -V it shows a current peak that is bigger than the source supports. That peak could influenciate someway on myDAQ ?

Thanks,

Júlio Véras

[edoering]

Take a look at this excellent article on how to connect the analog inputs to your circuit: http://www.ni.com/white-paper/3344/en/. If you have a single-ended from your sensor then you need to ground the AI- input, however, if you have a differential style output from the sensor you will likely need to use the pull resistors (see the article) in addition to connecting the AI- and AI+ inputs.

Charge injection and dual input channels is another possible source of problems when the signal source is high impedance. When you have both AI channels active the multiplexer rapidly switches back and forth between the two channels and the single analog-to-digital converter, and this "siphons off" some charge on every switching event, effectively lowering the AI input resistance from the rated 4Gohm to a value potentially in the megaohm region. If charge injection is indeed the problem, try enabling only one AI channel and you should see the problem clear up.

Hope there is something there that can help out!

Ed

[juliocveras]

Ok Ed,

Thanks. First, I was treating thermocouple as "Measuring Grounded Signal Sources" but thanks the article you pointed to me I will treat now correctly, as "Measuring Floating Sources".

About the AI problem, I really need to use about three AI channels. Do you think that I could use two different myDAQ, one myDAQ to AO control and another muDAQ to AI reading ?   ....or maybe one myDAQ to one AO and two AI, and another myDAQ to one AO and one AI ?

Thanks a lot!

[edoering]

I don't have personal experience doing this but I know that it is possible to have multiple myDAQs controlled by a single LabVIEW VI.

[Dahlen]

This is one of the circuits I am using in my analog circuits classes to boost the output of AO 0.

[SaraIAU]

Hey

I have a really basic question

I am using AO with 10 V that has a current of 2mA , thevoltage is enough to operate the pump I want to control but the required current is 80 mA 

so how can I increase the 2mA current in order to be able to control it?