Although transformer coupling devices can be built to exhibit improved linearity and a very much smaller coupling capacitance Cc between input and output than optoelectronic couplers (i.e., Cc 0.1 pF versus 1.0 pF), they do not func tion at do or very low frequencies, and have an upper frequency limit of about 1 kHz An Analog Devices model AD204 is an example of a low-cost, miniature isola tion amplifier that is transformer coupled from input to output