The reference voltage Vref defines {HEAD ~ the DC level} of {TAIL ~ both the virtual ground} and the output node 
The reference voltage Vref defines { HEAD ~ the DC level } of { TAIL ~ both the virtual ground } and the output node
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH