Currently, most high-speed analog-to-digital converters (ADCs) in the world have differential inputs. These ADCs are widely used in a variety of terminal applications, but are not limited to communication wireless infrastructure and backhaul, as well as test and measurement oscilloscopes and spectrum analyzers. To support this input architecture, engineers must design a signal chain for differential connection with the ADC.
In order to obtain the best performance, the user must select a balun (balanced-unbalanced transformer) in the signal chain, although this may cause coupling problems in some applications. However, coupling problems do not always occur, especially in certain test and measurement applications that require DC components. Fully Differential Amplifier (FDA) is a multi-purpose tool, it can replace balun (or use with it) at the same time, and provide a variety of advantages. Compared with traditional amplifiers that use single-ended output, circuit designers can increase the circuit’s immunity to external noise when using a fully differential signal processing spectrum analyzer implemented by the FDA, thereby doubling the dynamic range and reducing the number of incidents. Subharmonics.
Let us first review the basic knowledge of the fully differential amplifier (FDA), the important technical specifications of the FDA, and the meaning of these technical specifications. Then sit down and we will talk to you about how to use a balun-type FDA to connect the signal chain with additional performance.
What is FDA?
Imagine if you don't use advanced devices-FDA integrated circuits to drive differential ADCs. In addition to balun, a solution requires two operational amplifiers to provide differential signals. One operational amplifier provides a positive (VIN+) input signal, and the other provides a negative (VIN-) input signal. If you want to establish proper gain outside the operational amplifier (op amp), you will need to use a total of 8 resistors, which will be very complicated to design. Now, engineers only need half the number of resistors and an IC, and can use an FDA to provide a single-ended-to-differential interface and a differential-to-differential interface of the ADC. At the same time, this IC can turn on the DC component without balun, which is different from the balun that provides DC isolation. The key point of this is that excellent frequency response of DC and low frequency is required in many applications.
So, what exactly is the FDA? Basically, FDA is a device with two amplifiers. The main differential amplifier (from VIN to VOUT) consists of multiple feedback paths and Vocm error amplifiers, and Vocm error amplifiers are more commonly referred to as common-mode output amplifiers.
Let's discuss the Vocm error amplifier first. The Vocm amplifier internally samples the differential voltage (VOUT+ and VOUT–) and compares this voltage with the voltage applied to the VOCM pin. Through an internal feedback loop, the Vocm amplifier drives the "error" voltage (the voltage between the input pins) of the Vocm error amplifier to 0. In this case, VOUT_cm (Figure 1) = Vocm. If the VOCM pin remains in the floating state, an internal voltage divider usually sets the default value of the bias point to VCC/2 (the middle position between the power supplies). The Vocm setting on the (VOCM) pin will affect the overall output swing (discussed later). These characteristics are different from traditional operational amplifiers with single-ended outputs. In a traditional operational amplifier, the output common-mode voltage and single-ended output are actually the same signal that affects the dynamic range of the operational amplifier.
In addition to the Vocm error amplifier, the main differential amplifier in the FDA has VOUT+ and VOUT- outputs and multiple feedback paths. When analyzing this amplifier, it is best to think of it as including two reverse feedback paths. One path is a reverse input to a non-reverse output, and the other path is a non-reverse input to a reverse output (Figure 1). In order for the FDA to function properly, both paths must be closed. And, in order to maintain balance, the feedback paths should also be kept equal. The analysis of these two paths is very complicated. In order to help introduce the basic knowledge of FDA and their role in assisted design, the analysis of them in this article is relatively simple. To understand the basic FDA input and output voltage definitions, please see Figure 1.
In Figure 2, we have added an external resistor to the basic FDA diagram to set the gain. It is precisely because of this that the analysis of the internal differential amplifier has become complicated. For the purpose of simplification, we designate β1 and β2 as feedback terms.
Through these two equations, let's take a look at the VOUT(diff) equation composed of feedback, VIN+, VIN– and Vocm.
Equation (3) shows that when the feedback terms are not equal, the differential output voltage depends on Vocm. From this we can see that the feedback terms should be equal, or as close as possible, which is very important, because the Vocm term will produce offset and noise. The feedback term should be equal to the equation, which can be simplified to:
Advantages of FDA
Due to the inherent properties of the differential architecture, the FDA can also help improve the dynamic range of the system. When the signal passes through the printed circuit board (PCB), cables and wiring, and through the signal and ground paths, system noise will accumulate and affect the dynamic range.
The noise immunity of FDA is the inherent property of the differential structure. It can suppress coupling noise on the input. It is usually expressed as the power supply and output of the common-mode voltage in a typical operational amplifier. According to equation (4), a balance is achieved in the FDA, Vocm is eliminated, or the value is too low to provide this advantage. Because each component has a different reference point, single-ended components cannot suppress ground noise. Although a lot of design work is used to ground high-frequency ground currents, problems still arise where differential signaling improves performance. The noise accumulated in a typical operational amplifier will degrade the signal-to-noise ratio (SNR) performance, thereby affecting system design.
In addition to the greater noise immunity caused by the FDA's common-mode rejection properties, the phase difference between the outputs makes the output voltage swing twice (6dB) that of a single-ended output with the same voltage swing (Figure 3). In this case, using the same power supply increases the headroom of the amplifier, and for the same signal swing, allowing the use of a lower power power supply, thereby reducing dissipation.
The advantage of FDA and differential signal chain is that even harmonics are fundamentally eliminated. Use the power series expansion, specify a sine wave input, and ignore the DC component. Figure 5 shows the second-order harmonic elimination in the amplifier and other (FDA) nonlinear differential devices. Although it cannot be completely eliminated in an ideal device, the balanced design (within the margin of error) used in these products needs to be better than a single-ended configuration (Figure 4).
Another advantage of FDA is the ability to provide excellent output equalization, which is critical to the driving of differential ADCs. The phase and amplitude components of the signal entering the ADC are ideally matched to achieve the best performance. When one of the amplitude and phase is unbalanced, or both are unbalanced, a common mode component will appear on the output, thereby degrading the SNR performance. In order to achieve phase balance, an ideal FDA can provide a 180° phase difference between the VOUT+ and VOUT- signals. Since the internal common-mode feedback circuit forces the output common-mode voltage to be equal to the common-mode voltage applied to Vocm, the balance error is minimized. Please see the equation (5) showing the performance, which gives the calculation method of the balance error:
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