A differential circuit operates using a differential input voltage defined the difference between two input voltage (referred to herein as Vin1 and Vin2).  Ideally, for some differential circuits, the circuit should operate based on the difference in the input voltages (Vin1−Vin2) regardless of the common mode level of the input voltages.  The “common mode voltage” (potentially also referred to as “VCM” hereinafter) is defined as the average at any given time of the input voltages (i.e., (Vin1+Vin2)/2).  The differential mode voltage for each of the inputs is defined as the difference between the input voltage and the common mode voltage (e.g., Vin1−VCM, or VCM−Vin2). 
As a practical matter, however, the operation of a typical differential circuit will depend on the common mode voltage.  If the common mode voltage were to vary outside of a given range, the differential circuit might not operate at all.  Such circuits may obtain more stable performance, therefore, by keeping the common mode voltage stable.  However, this is not always practical either.  In fact, in some operational environments, common mode voltage may vary by several orders of magnitude more than the differential mode voltage. 
Accordingly, if the common mode voltage varies unpredictably, the differential circuit may consequently have some unpredictable operational component.  The variation of common mode voltage is often termed “common mode noise”.  The ability of a circuit to adjust for common mode variations without affecting the circuit's operation is often measured in terms of a “common mode rejection ratio”. 
To increase a circuit's common mode rejection ratio, some circuits have components that compensate for common mode voltage.  One typical way to do this is to use a feedback loop.  However, the feedback loop typically has limited bandwidth.  If the frequency of the common mode voltage is above a certain threshold, the common mode rejection may become significantly weakened.