# How to quantify the error introduced by CMRR given in dB?

Here is the extract from the manual:

I want to have an idea about how much voltage error occurs when applied 10V signal. And is this CMRR zero if differential ended configuration is used? How can I calculate the Vout error for 10V input? Whats the difference between LLGND and GND in the manual? Is CMRR measures due to LLGND?

Here is the manual: http://www.mccdaq.com/pdfs/manuals/PCI-DAS6034-35-36.pdf

Common mode rejection ratio (CMRR) does not give you an absolute common mode error. What it does tell you is how much common mode signal (which is usually noise or some bias voltage) is attenuated. For example, 85dB CMRR means that signal common to both differential pins attenuated $10^{\frac{85\text{dB}}{20}} \approx 17782\text{ times}$. You can apply that to your common mode signal to find the common mode error. This applies to differential and single ended systems. If you're asking this question, you should be making differential measurements.