As we continually beat ourselves silly to defy Moore’s Law and achieve smaller geometries and IC processes, the supply or bias rail tends to follow suit. Most devices use a single-sided supply to power the component, but most signals still come to us in a bipolar fashion. To establish a new “zero” point or center code for an amplifier and converter, there needs to be a common-mode (CM) voltage defined between the two. If not, your design basically won’t work.
The common ground between two devices can be elusive at times. For both amplifiers and analog-to-digital converters (ADCs), these requirements often can be overlooked when you’re specifying a new signal-chain application, usually because the datasheet stinks! Both devices will certainly have limitations to accommodate input/output range and supply. So be wary, and for what it’s worth, read the datasheet.
Common Mode Defined
Now that you’ve chosen the amp and the high-speed ADC, what could go wrong? You’ve done the extensive noise analysis and found the design will meet your specifications for the application at hand. Have you thought about the CM voltage spec between the two devices being connected? Gotcha!
Tech support gets questions from customers when they don’t understand this parameter, which specifies what the two devices can handle. If your design is ac coupled and you don’t need any dc content, you can stop reading right now. Otherwise, for ac-coupled applications, use a coupling capacitor between the amplifier/converter to break the CM mismatches. This allows the design to optimize the CM bias of both the amplifier’s outputs and the converter’s inputs.
If your application is dc coupled, you need to preserve the dc content of the signal. CM is very important here so your signals can be ground down accurately into the digital bits, codes, and least significant bits (LSBs).
A CM voltage is simply the center point around which the signals move (Fig. 1). You can also think of it as the new center point or zero code if you’re a converter nerd. As an amplifier, CM is established on the outputs, usually through a VOCM pin or something similar. Be careful, though, as these pins have certain current and voltage range requirements too.
It might be best to review the amplifier datasheet and/or use a robust bias point that doesn’t load down any adjacent circuitry or reference point within your circuit. Don’t simply tap off a converter’s voltage reference pin (VREF), which is usually half the converter’s full scale, since it may not be able to provide enough bias with good accuracy.
It also would be prudent to review the pin specifications on the converter’s datasheet. Usually something like a simple voltage divider with 1% resistor tolerances and/or a buffer driver will do to set this CM bias properly for an amplifier.
The converter’s side needs an established CM bias on the analog inputs to establish this reference above ground. But before we plug along, ask yourself if you’re using a buffered or unbuffered converter. If it’s an unbuffered converter (a.k.a., a switched-capacitor type), there is a requirement for providing an external CM bias on the analog inputs.
Typically set by the converter’s internal buffer, buffered converters have self-biased analog inputs. This level is usually half of the supply plus a diode drop above (AVDD/2 + 0.7 V), whereas an unbuffered converter doesn’t have an internal buffer and self bias and requires a CM bias of AVDD/2 or half the analog supply. Therefore, designers must provide this CM bias externally, which can be done in a variety of ways.
Some converters have a VCM or CML pin that allows designers to provide bias through a couple of termination resistors tied to the analog inputs. Designers also can use a transformer’s center tap or simply provide a couple of resistor dividers on each leg of the analog inputs tying them between AVDD and ground.
Again, stay away from the VREF pin or check the datasheet. Most pins with this type of annotation aren’t equipped to supply a CM bias unless they’re buffered through an amplifier externally. Keep in mind that the VREF pin sets up all the internal reference bias within the converter itself. It’s also a function of the input full-scale of the converter.
If the VREF pin is used improperly—i.e., loaded down—you may actually be shifting the input full-scale range of the converter unintentionally. Therefore, you could limit the total dynamic range of the system. Or, even worse, you could bring down the converter. Figure 2 provides some proper circuit examples.
Common Mode Broken
If the CM bias isn’t provided or maintained, the converter will have gain and offset errors that degrade to the overall measurement being acquired. Your converter output will look like Figure 3 or some variation of it. The output spectrum will resemble an overloaded full-scale input. This means the “zero” point of the converter is off center and not optimum.
The designer may find the converter will “clip” early or not reach full scale. Recently, this problem has gotten worse since converters are now using 1.8-V supplies. This means the CM bias for the analog inputs is 0.9 V or AVDD/2. Not all single-supply amplifiers can support such a low CM voltage while maintaining relatively good performance.
However, many new amplifiers have accommodated this voltage and are out on the market today. Therefore, review which amplifiers can be used in your new design. Not just any old amplifier will work because the headroom may become very constrained and the internal transistors may start to cave in.
If a dual supply is used with an amplifier, there should be sufficient headroom in most cases to achieve the proper CM bias. The downside is an extra supply, which means more parts, more money, and more power burned in the design. Simple inverter circuits will work to help with this, but designers and their managers need to agree upon the tradeoff.
CM bias is especially important when connecting a preceding stage to the converter, such as an amplifier. Check the datasheet specifications to make sure the amplifier can meet the converter’s input swing and CM supply requirements.
Rob Reeder is a senior converter applications engineer working in the high-speed signal-processing group at Analog Devices Inc. He received his MSEE and BSEE from Northern Illinois University in DeKalb, Ill.