# Why Is the TB 4302C Less Accurate than the PXIe-4302?

Updated Feb 23, 2018

## Reported In

### Hardware

• PXIe-4302
• TB-4302
• Terminal Block

## Issue Details

Why is the terminal block connector to the PXIe-4302 going to reduce the accuracy of the device? The absolute accuracy calculation of the PXIe-4302 is listed on page 5 of the Device Specifications and works out to 0.04% error. The error specifications lists the TB-4302C as having 0.5% error. Why is the terminal block going to reduce the accuracy by more than 10 times the specifications of the measurement device?

## Solution

For accuracy calculations of the PXIe-4302 we need to use the values for a 50mV(10mA) signal, not the values for a 10V signal. The 50mV signal is what the TB4302C is designed to operate with, and here is the result from an example calculation:

Absolute Accuracy = Reading*Gain Error + Offset Error + Noise Uncertainty
Absolute Accuracy = (50mv)*0.087% + 68uV + Noise Uncertainty = 0.111mV + Noise Uncertainty

Assuming that we are reading a 10mA signal across the 5 ohm shunt resistor our Reading would be 50mV, and this is in the 100mV range in the spec sheet.
Now, noise uncertainty varies with the number of samples taken, so assuming that Noise Uncertainty was 0(which is impossible), we'd have the accuracy as 0.111mV.

0.111mV/50mV = 0.222% error

The discrepancy would be comparing the 0.222% error (best case) from the PXIe-4302 to the 0.5% error (worst case) on the TB-4302C. Realistically, the error of the PXIe-4302 would be closer to 0.5% when taking into account the Noise Uncertainty.