Fully differential op amps may be intimidating to some designers, But op amps began as fully differential components over 50 years ago. Techniques about how to use the fully differential versions have been almost lost over the decades. However, today's fully differential op amps offer performance advantages unheard of in those first units.
An op amp is used as a closed-loop device. Most designers know how to close the loop on a single-ended op amp:
3.1 Differential Gain Stages
So, how is the loop closed on a fully differential op amp? If there are two outputs, both of them have to be operated closed loop. Therefore, the equivalent way of closing the loop on a fully differential op amp is:
The gain of the differential stage is:
The schematic shown in Figure 4 is a fully differential gain circuit. Fully differential applications, however, are somewhat limited. Very often the fully differential op amp is used to convert a single-ended signal to a differential signal—perhaps to connect to the differential input of an A/D converter.
The gain of the single-ended to differential stage is:
Texas Instruments fully differential op amps have an additional pin, Vocm, which stands for common-mode output voltage (level). The function of this pin can be either an input or an output, because its source is just a voltage divider off the power supply, but it is seldom used as an output. When it is used as an output, it corresponds to the common-mode voltage about which the Vout+ and Vout– outputs swing.
The most common use of the Vocm pin is to set the output common-mode level of the fully differential op amp. This is a very useful function, because it can be used to match the common mode point of a data converter to which the fully differential amplifier is connected. Highprecision/ high-speed data converters often employ differential inputs and provide a reference output.
and 0.7 V above the negative rail.
Fully Differential op amps, although not a new type of component, are new to most designers. Techniques to use these components are similar to the techniques used to design inverting single-ended op amps. The number of outputs and feedback paths is increased to 2, doubling the number of passive components needed to support a fully differential design. The Vocm pin of the device is a new concept, which facilitates connection to a reference output from a data converter.