6501 digital input seems to be soucing too much current for my driving circuit to handle.

[vanderkin]

The NI-6501 digital input seems to be soucing too much current for my driving circuit to handle. 
The driving circuit is an open collector with a 68k pull up to 5V. 
The digital output schematic of the the 6501 shows that it has a 4.7k pull-up. 
Is this pull-up connected even when the pin is configured as an input? 
Is there a way to configure the pull up to be removed?
Or can I remove the 4.7k pull-up physically from the 6501 for the 4 inputs that are not able to sink much current?

[Brian F.]

Hello vanderkin.
 
Thank you for contacting National Instruments.
 
Because the 6501 has an internal pull-up resistor, the resistor is connected even when the pin is configured as an input.  There is no way to configure the pull up to be removed and National Instruments does not recommend that you physically remove the pull-up resistor.  Doing so would void any type of warranty on the device. 
 
In order to further assist you, could you tell me a bit more about your application.  Additionally, what current value do you desire to be output and how much current is being output. 
 
Have a great day!
 
Brian F
Applications Engineer
National Instruments

[Brian F.]

Hello vanderkin.
 
The USB-6501 can be configured to either sink or source current if you are generating a Digital Output Task.  This functionality can be configured in a DAQmx channel property node by selecting Digital Output>>Output Drive Type.  If you change the property node to write, you can control whether or not the device is in "Open Collector" or "Active Drive" mode which effectively configures the device to sink or source current. 
 
If you have any other questions, we will be happy to answer them!
 
 
Brian F
Applications Engineer
National Instruments

[Brian F.]

Hi Vanderkin. 
 
Sorry for that mixup.  It seems that the 4.7kohm resistor in the 6501 is providing a path of less resistance for your current and thus is being routed back into your circuit after the digital input is measured.  In order to stop this, I would recommend routing the signal to be input into the 6501 to a buffer amplifier first.  The buffer amplifier, with unity gain, will not affect the level of your signal, but it will restrict the current flow into the USB-6501.  For more information on buffer amplifiers, please refer to the following website:
 
Buffer Amplifier
<a href="http://en.wikipedia.org/wiki/Buffer_amplifier" target="_blank">http://en.wikipedia.org/wiki/Buffer_amplifier</a>
&nbsp;
Another way to improve the performance of your application is to upgrade your DIO card to one of our M series cards which offer more input impedance than the 6501.&nbsp; This higher input impedance will reduce the current and voltage that are being routed back through the circuit.&nbsp;
&nbsp;
Good luck on your application!
&nbsp;
Brian F
Applications Engineer
National Instruments

[Brian F.]

Hello vanderkin.&nbsp;
&nbsp;
I am currently talking to other engineers to explore where those pull up resistors reside as well as verifying the safe operation of the rest of the board without those pull up resistors.&nbsp; I will let you know when I have an update for you.&nbsp;&nbsp;However, I would still recommend that you try using a buffered amplifier since this is certainly safer for your board than removing onboard components.&nbsp;
&nbsp;
Have a great day!
&nbsp;
Brian F
Applications Engineer
National Instruments