Test Procedure of Common-Mode Rejection Ratio for USB-6353

This article outlines the procedure for testing the Common-Mode Rejection Ratio (CMRR) of the USB-6353 device. The CMRR is a critical specification that measures the ability of a device to reject common-mode signals, ensuring accurate differential signal measurements in the presence of EMI.

Preparation

1. Self-Calibration: Perform self-calibration to eliminate any unwanted errors due to time lapse in the field of USB-6353.
2. Using DMM: Use a 6.5-digit digital multimeter (DMM) to measure the accurate AO output voltage level.

Measurement Procedure

1. Short AI+ and AI-: Short AI+ and AI- and connect them to the same source. Instead of connecting from two different channels at the same voltage level, these separate channel sources may have small differences that could affect the measurement.
2. Connect GND Pin: Connect the source to the GND pin of USB-6343/6341.

Measurement

1. Measure Source Voltage: Use the 6.5-digit DMM to measure the accurate source voltage (Vs).
2. Measure Error Offset Voltage: Short AI+/- inputs and connect them to the ground pin to measure the error offset voltage (Vos).
3. Measure Leaked Common Voltage: Disconnect the ground connection and connect to the signal source to measure the leaked common voltage (Vcm).

The formula for calculating CMRR is following:

*The Vos and Vcm in the equation should be measured using the differential mode of AI0+ and AI0- or other combinations.

Measure Source Voltage: Use the 6.5-digit DMM to measure the accurate source voltage (Vs). Vs = 3.000157 V

Measure Error Offset Voltage: Short AI+/- inputs and connect them to the ground pin to measure the error offset voltage (Vos). Vos = -0.00106614 V

Measure Leaked Common Voltage: Disconnect the ground connection and connect to the signal source to measure the leaked common voltage (Vcm). Vcm = -0.00106139 V

CMRR: 116.01 dB