Too low output current with USB-6501

[studerp]

For my measurement I connected an output port (Output High voltage, Pin 28) with a resistor of 10kOhm to ground (Pin 32). For the output drive type active drive I measured a voltage of 3.33V and a current of 0.33mA. I repeated the same measurement with open drain configuration, this leads to 3.27V and 0.33mA. I use NI DAQmx and changed the output drive type with a property node in labview 8.5.1. Output high voltage was generated with labview. In a second measurement I set output high voltage with the control panel in measurement & automation from DAQmx which led to the same result.

For my measurement I would expect a higher output current especially for the active drive mode, since the specification states a voltage between 2.8 and 3.6V for 2mA. I do not understand why the output current is not 0.5mA which would lead to 5V.

If I do the same measurement with  the +5V output source (pin 31) instead of the output port (pin 28), the current is as expected 0.5mA and a voltage of 4.8V can be measured.

For all measurements I used a NI USB-6501 that was previously tested by an national instrument engineer.   

[James_McN]

Hi Studer,

Lets have a look at the two measurements you are doing:

1)  Output Active Drive - In this case you are being limited by the voltage.  The spec states that the output voltage will be between 2.8 and 3.6V.  You are seeing 3.3V which is in the spec however put 3.3V over a 10k resistor and you will only see 0.33mA so this is in the spec of the device.

2) Output Open Collector - In this case the spec does give up to 5V output however this is only up to 0.4mA (nominal).  In this case this limit of current is limiting the voltage, this is why you dont see 5V.

I hope this clarifies the functionality you are seeing and demonstrates that it is working within specifications as we would expect.  If you require more current you can provide up to 7.5mA at 5V by using an external pull-up resistor.  You can find details of this in page 14 on the manual at http://www.ni.com/pdf/manuals/371456h.pdf.

Regards,

[studerp]

Hello James Mc

Thank you for your answer. I remeasured the two output drive types with different resistors between output port and ground.

1) Output Active Drive - For a very low current of 0.06 mA or even if no resistor is betwen output port and ground I measure only a voltage of 3.34V. I would expect a voltage of approximately 5V for a current of 0.06A which would decrease if the current increases.

2) Output Open Drain - The measured current voltage curve corresponds to the specifications.

Why is it not possible to reach 5V for the output pins if they are in the output active drive mode?

Regards

Priska Studer   

[James_McN]

Hi Priska,

Unfortunately this is the way that the cards have been designed.  I suspect that this is to reduce the power requirements of the card so that it can be supported by the USB bus without external power.  This is still inside of TTL specifications so is likely to still be suitable for most purposes.

Regards,

[studerp · Accepted solution]

Hello James Mc,

Thank you very much for the anwer. To reach CMOS specifications (between 3.5 and 5V) I will add an external pull-up resistor and use the output ports in Open drain.

Regards,

Priska Studer

[csoma]

Does a different way of setting it up help?

See http://forums.ni.com/ni/board/message?board.id=70&message.id=9871#M9871