Carissa Sipp, Systems Engineer, brings us this session.
Fully differential amplifiers (FDAs) are versatile tools to use in a signal chain, and they offer a variety of benefits. Fully differential signal processing provided by FDAs gives the circuit designer increased immunity to external noise, two times the dynamic range, and reduced even-order harmonics over traditional amplifiers with single-ended outputs. Two popular application uses for differential amplifiers that we will discuss today are: driving differential signal chains, and enabling single-ended to differential conversion in place of transformers.
First let’s review the fully differential amplifier architecture. An FDA has both differential inputs and outputs, as well as an added VOCM pin used to set the output common-mode voltage. This is different than in a traditional operational amplifier (op amp) with a single-ended output, where the output common-mode voltage and the single-ended output are inherently the same signal. In an FDA, the output is differential, and the output common-mode voltage can be independent and is controlled by sampling the input to the VOCM pin.
Typically, if the VOCM pin is left floating, the typical bias point will be VCC /2 (or midway between supplies) through an internal voltage divider. In addition to the added VOCM control versus a traditional op amp, an FDA has multiple feedback paths. FDA architecture can be very helpful in designing active filtering used to create second-order and higher filters using architectures such as Bessel, Chebyshev, and Butterworth. These filters are useful in systems driving differential analog-to-digital converters (ADCs).
Inherent in their differential architecture, FDAs also help improve system dynamic range. System noise can accumulate and impact dynamic range as signals travel across the PCB as well as cables and wiring through the signal and ground paths. FDA noise immunity is inherent in the differential structure enabling noise to be rejected at the inputs, power supplies, and outputs that appear as a common mode voltage.
Single-ended components cannot reject ground noise, as each part has a different reference point, and, despite diligent design efforts to ground high frequency ground currents, issues may arise in which differential signaling can improve performance. Along with greater noise immunity from the common-mode rejection property of the fully differential amplifier, the phase difference between the outputs enables the output voltage swing to increase by a factor of two (6 dB) over a single-ended output with the same voltage swing (see Figure 1). This increases the amplifier's headroom using the same power supply and can allow for use of a lower power supply and dissipation for the same signal swing.
Another advantage of FDAs and differential signal chains in general is the inherent cancelation of even order harmonics. Using power series expansion, given a sine wave input and ignoring the DC component, Figure 3 showcases second-order harmonic cancelation in non-linear differential devices such as amplifiers (FDAs). While full cancelation cannot be achieved in real devices, these products will see a benefit from balanced design (within error) over a single-ended configuration (see Figure 2).
In addition to the differential signal chain benefits, a fully differential amplifier can be useful in converting single-ended inputs to differential conversion. These amplifiers can enable the signal chain to preserve the DC components. Unlike baluns (transformers), they are smaller in size, lower cost, wider bandwidth, and provide extra gain desired and/or needed to drive something like a differential ADC full scale.
In using an FDA the input impedance matching to the source impedance is critical and beneficial, as this prevents high-frequency reflections. Also, level shifting and common-mode voltage limits are carefully considered to align to ADC and amplifier signals.
These all can be accomplished with the proper choice and circuit configuration of an FDA. One example using an FDA, for example the LMH6554, to produce a balanced (differential) signal to drive a 16-bit ADC from a single-ended source is shown in Figure 4. A similar figure, equations, and further information on SE-DE drive using this low-power, 2.8 GHz, ultra-linear FDA can be found here: LMH6554.
Please join us next time when we will discuss creating clock trees with multiple devices.
- AN-2177 Using the LMH6554 as an ADC Driver , Application Report (SNOA565A), Texas Instruments, April 2013
- Download the LMH6554 datasheet
- Razavi, Behzad. RF Microelectronics. Upper Saddle River, NJ: Prentice Hall, 1998
- Gray, Paul R., and Robert G. Meyer. Analysis and Design of Analog Integrated Circuits Fourth Edition. New York: Wiley, 2001