Whether 'tis nobler to signal via complementary means, or to take arms against a sea of noise… ah, but I take literary license with Shakespeare. But it does raise a real question… should a designer use differential or single ended methods for carrying analog signals? The answer also contains a question… which is, “it depends on the SNR requirements” or “it depends, what are the system requirements?” to be exact. In high performance systems, noise can be everywhere, but will that warrant the additional complexity of differential signals?
In low speed signaling it is much simpler to use a single (ground referenced) signal. In the singled ended mode there are two options – unipolar and bipolar. The unipolar option swings the signal between V+ and ground. This is common for single supply systems and can actually be configured in a bipolar fashion with biasing (more on this in a minute). Bipolar signaling in classic sense swings the signal between V- and V+ centered at ground. Many operational amplifier circuits are designed with this type of signaling and (NOTE: IMPORTANT INFORMATION FOLLOWS) will not work with a single supply op-amp (e.g. +5 and ground). However, by moving the bias point of the amplifier to V+/2, you can emulate the split supply bipolar system and the standard op-amp circuits will again work (more in an upcoming blog post). In this configuration, the bias voltage (+2.5V in the example) is the new virtual ground reference. All signal references are made to this voltage. If you AC couple the input signal it will be bipolar relative to the Vbias voltage (the virtual ground). In our example “working” inverting amplifier that would look like +/- 2.5V signal input relative to +2.5V bias – output would be inverted relative to the same bias voltage.
As signal speeds increase, the voltage swings are reduced to mitigate capacitive loading. It is not uncommon to see 1 - 2V or +/- 1V levels driving (capacitive) cabling. As frequency content increases, it is often a requirement to move to differential pairs (and controlled impedance transmission lines) which are terminated at both ends. The termination is both to generate a voltage for the receiver or amplifier and to terminate the transmission line to limit reflections. But what if an engineer has a design using a high speed input signal and needs to convert it to a differential signal. This is common for designs using high-speed analog to digital converters such as the ADC14DS105. This ADC has differential inputs… so they can either transformer couple which will degraded low frequency response or use the circuit shown below. This circuit DC couples the bipolar signal resulting in DC accuracy – important for applications such as video… it also uses a unique differential amplifier - the LMH6553 - which has a protection clamp on the output. This prevents large input swings from damaging the ADC - something that could happen if the input is a connector on the outside of the equipment! In addition, the circuit below is terminated for 50 ohm transmission lines.
Bipolar to Differential ADC Front End
Hope this brief discussion enlightens you to the trials and tribulations of single versus differential signaling. I’ll elaborate on this topic and some more op-amp circuits (if you like them) in future posts… till next time!