The idea of fully-differential op-amps is not new. The first commercial op-amp, the K2-W, utilized two dual section tubes (4 active circuit elements) to implement an op-amp with differential inputs and outputs. It required a 300 Vdc power supply, dissipating 4.5 W of power, had a corner frequency of 1 Hz, and a gain bandwidth product of 1 MHz(1).
In an era of discrete tube or transistor op-amp modules, any potential advantage to be gained from fully-differential circuitry was masked by primitive op-amp module performance. Fully-differential output op-amps were abandoned in favor of single ended op-amps. Fully-differential op-amps were all but forgotten, even when IC technology was developed. The main reason appears to be the simplicity of using single ended op-amps. The number of passive components required to support a fully-differential circuit is approximately double that of a single-ended circuit. The thinking may have been "Why double the number of passive components when there is nothing to be gained?"
Almost 50 years later, IC processing has matured to the point that fully-differential op-amps are possible that offer significant advantage over their single-ended cousins. The advantages of differential logic have been exploited for 2 decades. More recently, advanced high-speed A/D converters have adopted differential inputs. Single-ended op-amps require a problematic transformer to interface to these differential input A/D converters. This is the application that spurred the development of fully-differential op-amps. An op-amp with differential outputs, however, has far more uses than one application.
2 BASIC CIRCUITS
The easiest way to construct fully-differential circuits is to think of the inverting op-amp feedback topology. In fully-differential op-amp circuits, there are two inverting feedback paths:
• Inverting input to noninverting output
• Noninverting input to inverting output
Both feedback paths must be closed in order for the fully-differential op-amp to operate properly.
When a gain is specified in the following sections, it is a differential gain – that is the gain at VOUT+ with a return of VOUT-. Another way of thinking of differential outputs is that each signal is the return path for the other.
2.1 A New Pin
Fully-differential op-amps have an extra input pin (VOCM). The purpose of this pin is to provide a place to input a potentially noisy signal that will appear simultaneously on both inputs – i.e. common mode noise. The fully-differential op-amp can then reject the common mode noise.
The VOCM pin can be connected to a data converter reference voltage pin to achieve tight tracking between the op-amp common mode voltage and the data converter common mode voltage. In this application, the data converter also provides a free dc level conversion for single supply circuits. The common mode voltage of the data converter is also the dc operating point of the single-supply circuit. The designer should take care, however, that the dc operating point of the circuit is within the common mode range of the op-amp + and – inputs. This can most easily be achieved by summing a dc level into the inputs equal or close to the common mode voltage.
A gain stage is a basic op-amp circuit. Nothing has really changed from the single-ended design, except that two feedback pathways have been closed. The differential gain is still Rf /Rin a familiar concept to analog designers.
This circuit can be converted to a single-ended input by connecting either of the signal inputs to ground. The gain equation remains unchanged, because the gain is the differential gain.
An instrumentation amplifier can be constructed from two single-ended amplifiers and a fully-differential amplifier as shown in Figure 2. Both polarities of the output signal are available, of course, and there is no ground dependence.