Single-ended signals are very common, but increasingly signal paths are being converted to differential signals as part of the signal chain. The benefits of differential signaling are particularly appealing with low supply voltage systems and for driving analog-to-digital converters (ADCs). There are many advantages to using a fully differential amplifier (FDA) to perform the single-ended to differential conversion, even though transformers traditionally have been used for this function. First, let us consider some examples where using a transformer has serious drawbacks, then we will develop a circuit using an FDA.
Transformers are simple, passive and inexpensive, so in many applications they would be the first choice for converting a single-ended to differential signal or back again. The main limitation of transformers is the lack of low-frequency coupling, but there are several other limitations. When the DC voltage of a signal contains information, or when the signal frequency is very low, transformers cannot be used. Additionally, transformers provide no reverse isolation and can present problems, if the source and load impedances are not compatible.
Another reason not to use a transformer is that the core can be influenced by external magnetic fields, such is in an electrical substation or near an magnetic resonance imagery (MRI) machine, or conversely, that the magnetic fields will influence adjacent circuit elements, especially other transformers. Cross-coupling of adjacent transformers can result in poor channel-to-channel isolation, unless the transformers are spaced far apart. Being a passive device, a transformer will have loss. Finally, an FDA is often much smaller than the comparable transformer, particularly in height.
In comparison, a transformer will have from four to six terminals, all of them signal paths. An FDA usually has only four signal path terminals and, in addition, requires power and, optionally, a common-mode control and a power down. Some FDAs also require external resistors to set gain. Figure 1 shows examples of the most common FDA and transformer circuits used to convert single-ended signals to differential signals. An ADC is shown as a generic differential device, but other differential applications might be a balance line driver or the input of a mixer or field programmable gate array (FPGA).
Figure 1A shows an FDA with on-chip feedback and gain-setting resistors. Depending on the source resistance and the FDA configuration, an external resistor may not be required. In most applications the single-ended source will be 50 Ohms. In order to provide a balanced circuit, the external resistor shown would also be 50 Ohms (check the datasheet as not all amplifiers need external matching).
Figure 1B shows an FDA in a more traditional configuration using external resistors for both feedback and setting the gain. In this case resistor values should be described in the amplifier datasheet. Note that termination resistors are not shown.
In both amplifier circuits the amplifier output voltage with no input signal (common-mode voltage) is controlled by the ADC. This is also the case for the flux-coupled transformer (Figure 1C ). However, for the transmission line transformer, the common-mode voltage needs to be set by external resistors and the ADC needs to be protected from ground by a capacitor. Also, unless the signal source and the ADC have the same common-mode voltage, there needs to be another capacitor on the transformer input.
Again, referring to Figure 1D , it looks possible to couple DC information through a transmission line transformer. However, only one ADC input pin is DC-coupled to the VIN source. This is not true DC-coupling because there is a 6 dB gain loss for frequencies below the transformer cutoff frequency. And, as already stated, the other ADC input needs to be electrically isolated from ground.
In conclusion, when DC-coupling, power gain and small size are system requirements, consider an FDA for converting single-ended to differential signals. An amplifier can also give better reverse isolation and impedance matching as well.
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