Academic Company Events NI Developer Zone Support Solutions Products & Services Contact NI MyNI
4 ratings:
 3.5 out of 5     Rate this Document

Choosing Pull-Up Resistor Values for R Series Devices Digital Output Lines

Hardware: Reconfigurable I/O (RIO)>>R Series>>PCI-7831R, Reconfigurable I/O (RIO)>>R Series>>PXI-7831R, Reconfigurable I/O (RIO)>>R Series>>PCI-7813R, Reconfigurable I/O (RIO)>>R Series>>PXI-7813R, Reconfigurable I/O (RIO)>>R Series>>PCI-7833R, Reconfigurable I/O (RIO)>>R Series>>PXI-7833R, Reconfigurable I/O (RIO)>>R Series>>PCI-7811R, Reconfigurable I/O (RIO)>>R Series>>PCI-7831R, Reconfigurable I/O (RIO)>>R Series>>PXI-7811R, Reconfigurable I/O (RIO)>>R Series>>PCI-7830R, Reconfigurable I/O (RIO)>>R Series>>PXI-7831R, Reconfigurable I/O (RIO)>>R Series>>PXI-7830R

Problem:
By default, the digital output lines of the R Series devices output a high value of 3.3 V. According to the manual, to create a TTL signal with a 5 V high on the digital output lines, I need to add an external pull-up resistor. What is the output impedance of R Series devices and what value should the pull-up resistor be?

Solution:
The output impedance for the R Series devices is non-linear due to the circuitry used to provide 5V tolerance. The output impedance is low (<50 ohms) for output voltages near 0V. As the output voltage increases to around 3.3V, the impedance rises to approximately 100 ohms. As the voltage rises beyond 3.3V, the impedance rises very quickly to megohms.

Here's an approximation for how to calculate the value of pull-up resistor that you need:
  1. Assume that the R Series device outputs will drive the digital output line quickly between 0V and 3.3V. The pull-up resistor will have to pull the output up from 3.3V to 5V. For a rough approximation, assume that this is a simple RC circuit where Rp is the value of your pull-up resistor, and Ctot is the total capacitance* on the digital output line. The R Series device output will be high-impedance beyond 3.3V, so it can be ignored for this calculation.
  2. Solve for the value of Rp required to pull-up the output based on the timing requirements of your application. Lower values for Rp will pull-up the output faster.
  3. Make sure that the value of Rp that you have chosen will not result in too much current when the 7831R is trying to drive the output low.

For example:
Vt = 4.7V. This is the threshold level that we need to pull-up to.
DeltaT = 1us. This is the time required for the pull-up resistor to pull the digital output line from 3.3V to the 4.7V threshold level
Ctot = 100pF. Ctot represents the total capacitance on the digital output line.

The pull-up resistor, Rp, would then be:
Rp = DeltaT / ( Ctot * ( ln(1 - 3.3/5) - ln(1 - Vt/5) ) )
Rp = 1us / ( 100pF * ( ln(1 - 3.3/5) - ln(1 - 4.7/5) ) ) = 5.765 kOhms

*Note: The onboard capacitance between a DO and GND is about 28pF. The cable adds about 60pF per meter.

Related Links:
Product Manuals: NI 783xR User Manual
LabVIEW FPGA Module 8.2 Help

Attachments:





Report Date: 11/11/2004
Last Updated: 02/07/2007
Document ID: 3FAGPSL1

Your Feedback! poor Poor  |  Excellent excellent   Yes No
 Document Quality? 
 Answered Your Question? 
  1 2 3 4 5
Please Contact NI for all product and support inquiries.submit