A fully integrated differential filter will allow for optimum component matching which will, in turn, minimize these errors in the two signal paths. Gain and phase imbalance in the differential signal path will result in degraded rejection of common-mode noise, spurs, and even order harmonics. Ideally, each of the two single-ended signal paths has the same gain and are 180 degrees out of phase. Others include gain and phase matching between the two single-ended signals. As described above, one benefit of differential design is common-mode rejection. Whether the differential filter is constructed using two (dual) single-ended filters, or one fully differential filter, the ability to integrate the passive components on the same substrate provides some important performance advantages.
There are many benefits to integrated differential filters. Benefits of Integrated Differential Filters Mini-Circuits has developed a family of integrated balun-filter products that combine these functions into one component, simplifying this implementation and enabling a significant reduction in board space.įigure 1: Typical RF transceiver using discrete components. In Figure 1, the balun and the differential filter are shown as two separate discrete surface mount (SMT) components. Since single-ended components are matched to 50Ω, and the differential filter input impedance is matched to 100Ω, a balun is required to transform impedance and split the signal from single-ended to differential. In this case the balun is shown adjacent to the anti-aliasing filter.
It should be noted that the balun can be used in several locations in the transceiver chain to create the differential signal. This is typically done using a balun, as shown in Figure 1. In either case, to go from single-ended domain to differential domain, and provide a matching impedance, an impedance transformation is required. The differential filter can be composed of either 2 single-ended filters, each matched to 50Ω, or a single truly differential filter matched to 100Ω differentially. A notional implementation of a transceiver is shown in Figure 1 based on these assumptions. Mini-Circuits has answered the call by developing a family of dual/differential filter products that enable that miniaturization while supporting the need for differential lines at ADC inputs and DAC outputs.Ĭonsidering the benefits of differential signaling described above, it is desirable to maintain differential signaling as far back in the signal chain as possible for a receiver, and as far forward as possible for a transmitter. The typical board area taken up by surface mount components used in filter construction can be significant, and the industry is driving towards the perpetual miniaturization of all components. Similarly, the output differential filter for a DAC provides filtering of wideband noise and spurs generated in the DAC.įor both the ADC and DAC filters, the preferred design is a differential low pass filter (LPF) to screen out the noise and spurs. An anti-aliasing filter also filters out wideband spurs generated in the receiver that would otherwise alias in the first Nyquist zone of the ADC. An anti-alias filter placed after the amplifier and before the ADC input will limit the amplifier’s bandwidth, and thus its output noise contribution to the ADC noise floor.
If an amplifier drives the ADC with a wide bandwidth, the amplifier’s output noise beyond the first Nyquist region will fold into the first Nyquist region through the ADC sampling process and add to the noise floor of the ADC, which can degrade the overall noise performance of the signal chain. To interface with a differential ADCs or DACs, a single-ended-to-differential, or differential-to-single-ended, a transformer or balun is needed.įor an ADC, anti-aliasing filtering at the input is an important function to filter out wideband thermal noise and prevent it from folding into the first Nyquist region of the ADC (defined in frequency as half of the ADC sample rate).
Discrete transceivers on the other hand are often designed with single-ended, 50Ω matched components such as low noise amplifiers (LNAs), mixers and IF gain amplifiers. Additionally, differential circuits allow for half the voltage swing on each output compared to a single-ended design. Differential circuits provide many advantages over single-ended designs, including common-mode rejection of thermal noise, even order harmonics, and power supply noise and spurs. Today’s analog-to-digital converters (ADC) and digital-to-analog converters (DAC) are typically differential circuit designs. Differential Signal Processing in Analog-to-Digital Converters (ADCs) and Digital-to-Analog Converters (DACs)