# Resistor Accuracy of Programmable Resistor Module (PXIe-2727)

Updated Apr 6, 2020

• PXIe-2727
• PXI-2720
• PXIe-2725
• PXI-2722

## Issue Details

• What is accuracy of programmable resistor module?
• The spec sheet of PXIe-2727 shows the accuracy by range(Typical Channel Accuracy). What is the accuracy outside the range?

## Solution

Tolerance of the resistance changes with the resistor accuracy, relay and trace path resistance, and temperature.

The PXIe-2727 is composed of numerous resistors of doubling resistance value starting at 0.25 ohm(0.25, 0.5, 1, 2, 4, 8...ohm) that can be switched together to create various resistance values from 0.25 ohm to 16,320 ohm at 0.25 ohm steps.

Each resistor in the 2727 has an accuracy of .5%. (Refer to NI PXIe-2727 Specifications) Therefore, if you select a particular resistance value, you need to determine how many resistors will be used to create that resistance value, and further calculate the system accuracy from the resistors used to get the overall accuracy.

See the example below :
1. Programmed resistance of 4 ohms : we would divide this by the step size of 0.25 ohms to get 16 for our "binary" value of 10000. The number of 1's in the binary number tells us how many resistors must be used to make this value. In the case of getting a 4 ohm resistance, only 1 resistor is used. Therefore the accuracy will simply be 0.5% of the 4 ohm resistor that is used.
2. Programmed resistance of 8.5 ohms : we would divide by 0.25 to get 34 which has a binary value of 100010, meaning 2 resistors will be used to get this value. To get 8.5 ohms, the 8 and 0.5 ohm resistors are used. To calculate the accuracy at this resistance, we would calculate the system accuracy for these two resistors together which will be below formula.
• • Therefore, our accuracy will be 8.5 ± 0.04 ohm.