Related technologies of differential circuit design in ADI’s communication applications

Successfully capturing signals with sufficient fidelity is a major problem in the design of communication systems. Strict standards and specifications will require the selection of a suitable interface topology. This symposium introduces the advantages of differential design technology and how its performance advantages affect the strict system requirements in today’s high-performance communication systems. In addition, we will review the definition of radio frequency, outline the system budget, and compare different implementation methods.

What are the relevant technologies for differential circuit design in communication applications? First, make a comparison of single-ended” title=”single-ended”>single-ended and differential signals” title=”differential signal”>differential signals, and then briefly introduce some factors that need to be considered in the signal chain and system performance of the receiver, and then You will discover the advantages of differential applications. Comparing with single-ended applications from the perspective of driving the ADC, we will find that it is easier to achieve higher data rates in differential applications. Finally, we will return to the system design level to summarize the benefits of differential applications.

Single-ended and differential signaling

First talked about the concept of single-ended and differential signals, which everyone knows better. Here we use another way to express, we can divide the signal into unbalanced signal or balanced signal, single-ended signal “title=”single-ended signal”> single-ended signal belongs to unbalanced signal, because it is a single-sided signal , So it is relatively speaking, there is no balanced signal pair. Compared with balanced signals, unbalanced signals generally produce higher harmonic distortion.

The differential signal is a balanced signal, and the differential pair generally has a common common mode level and a differential mode level with the same amplitude. When measuring a differential signal or a balanced signal, what we are concerned about is the change in the difference between the positive and negative input signals. The harmonic distortion caused by this balanced signal is relatively small.

System level design

On the other hand, when the communication system is applied, we see the signal chain of a more general superheterodyne receiver. Figure 1 shows the signal chain of a general superheterodyne receiver. A low-noise amplifier is connected after the antenna. , Used to amplify the signal and suppress noise, and then use a two-stage mixer to down-convert the signal to a lower frequency, during which we will add appropriate filters to filter out the noise and harmonics outside the useful signal band, and then drive ADC buffer op amp” title=”op amp”> op amp. This is our main discussion today. The main purpose of this level of op amp is to adjust the signal level range and improve the driving ability, sometimes as Conversion between single-ended differential. Before entering the ADC, we need to add anti-aliasing filters, and finally use the ADC to perform analog-to-digital conversion of the baseband signal. We see if the system wants to achieve a higher dynamic range “title=” dynamic Range”>Dynamic range, in addition to the signal, too much noise and harmonics cannot be introduced.

Figure 1 The signal chain of a general-purpose superheterodyne receiver

Let’s take a look at the more noteworthy performance and indicators in a communication system. Before we compare single-ended signals and differential signals, we need to understand some system-level design considerations.

So, what kind of design is a better RF system design? First of all, the signal sensitivity should be high, which means low noise, and the phase noise introduced by the clock should also be low. The input signal must have sufficient drive capability and related indicators such as high third-order intercept point and 1dB compression point. Then it is whether the performance of each module is good enough, whether the signal and noise can be distinguished well, whether the linearity is good enough, and so on. In addition, it is the consideration of low power consumption and low cost.

We say that differential signal chains have many advantages over single-ended signals. Since it is a differential mode signal, the output is two differential signals. In fact, the amplitude of the output differential mode signal is relatively doubled. From another perspective, under the same output range condition, the working voltage will be lower. In this way, in applications requiring low harmonic distortion, sufficient amplitude margin can be guaranteed. The odd function-like characteristics of the differential system itself can eliminate the even harmonic terms in the system, that is to say, the second, fourth, and sixth harmonics, etc. The harmonics at these frequency points are relatively small compared to the odd harmonics. I can’t even see it. Finally, since the return path of the signal is no longer the ground plane, the signal is not so sensitive to the influence of the ground plane or the power plane, which reduces the introduction of noise coupling and achieves a better anti-electromagnetic interference effect.

As shown in Figure 2, single-ended signals are more sensitive to common-mode noise, power supply noise, and electromagnetic interference, and operational amplifiers will amplify these noises to a certain extent. The differential signal, because the signals on both sides form a current loop, suppresses common mode noise and interference, and only effectively amplifies the differential mode signal.

Through derivation, we can also see the odd-order characteristics of differential amplification. Ideally, we can only see the fundamental and odd-order harmonics on the spectrum. Here we only give a conclusion. It is worth noting that the third harmonic and the third-order intercept point it causes. IP3 is the theoretical input power at the intersection of the fundamental wave and the third-order distortion output curve, which is a measure of the linearity of the amplifier. Important indicators:

In the design of communication systems, driving, extracting and loading useful signals on the ADC input is a critical issue. For high-precision system design, proper selection of devices and interface methods is required. We will give you a few examples, but please understand before that, as shown in Figure 3, what we want to extract is the useful signal in the blue part, its energy is very small and there are surrounding signals and noise interference. In order to catch it, we need to pay attention to noise, dynamic range, and some other ADC-related indicators, which will be explained in detail in the following slides. We see that the main modules for functional implementation include buffer operational amplifiers, anti-aliasing filters and ADCs.

Figure 2 The difference between single-ended and differential signals

Figure 3 Useful signal and noise

Figure 4 is an example of a single-ended input single-ended op amp. You can see the signal gains of the four stages of IF amplifier, anti-aliasing filter, transformer and ADC, input and output 3rd-order intercept power, and the coefficient of noise introduced. And other indicators. The single-ended signal is converted to a differential signal before the ADC using a passive transformer. It should be noted here that the terminal matching impedance of the ADC is assumed to be 200Ω, and since the previous stages are all characteristic impedances of 50Ω, the impedance ratio of the transformer is set to 1:4.

If the transformer is advanced and the signal is converted into a differential signal before the op amp, the single-ended op amp is changed to a differential op amp, which constitutes a fully differential structure. As shown in Figure 5.

Here we will talk about the equivalent calculation of the overall noise figure of the cascade system and the third-order intercept point of the input and output. When considering the overall noise figure, the first stage has the greatest impact; when considering the intercept point index, the last stage has the most obvious impact.

Consider again the relationship between the spurious-free dynamic range and the system’s third-order intercept point. We know that as the input signal energy increases, when the third-order intermodulation distortion and noise floor are just equal, the system reaches the maximum SFDR. At this time, you can use this formula To express: SFDR = (2/3)(IIP3-NF-10log( TERMAL NOISE).

So we can calculate the total signal gain, third-order intercept point, noise figure and spurious-free dynamic range of the two single-ended-to-differential conversion methods just mentioned. The indicators are similar. The overall distortion and noise figure of the differential active drive structure are slightly higher, but the SFDR performance is also higher. Also note that in the single-ended passive conversion structure, if the intermediate frequency amplifier is removed, the full-scale reference input power is 6dBm, and the design of the anti-aliasing filter is an asymmetrical structure. And the whole design needs to add more resistive matching devices, which requires a strong front-end drive capability, that is to say, large current and power consumption. In addition, the even harmonics of single-ended op amps, common mode rejection, and power supply rejection issues will also affect the performance of the overall system to a certain extent.

On the other hand, when transmitting data, it can be transmitted bit by bit, or it can be divided into symbols for transmission, such as two bits per symbol, and then correspond to the 4 phases, and then act on the carrier. To send on. This is a very common modulation mode, namely QPSK.

Usually, we can use constellation diagrams to describe different modulation methods. We know that higher-order modulation can be used in higher data rate transceivers, but at the same time it requires lower local oscillator leakage, better power amplifier linearity, and higher The system bandwidth and demodulator signal-to-noise ratio. On the one hand, ADI is also developing higher-performance products to meet the needs of customers. On the other hand, we must pay attention to the principle of the problem when designing the system, and adopt appropriate methods and techniques to solve it.

In Figure 6, we can see the influence of noise and harmonics in the receiving system on the EVM. In other words, the demodulated signal will be offset from the ideal constellation position. Generally, we use the error vector amplitude to measure it. Excessive error vector amplitude will cause symbol errors and worsen the bit error rate. Especially in the high-order modulation mode, the positions between the symbols are closer, and the requirements for the error vector amplitude are stricter.

Figure 4 Example of single-ended input and single-ended output

Figure 5 Example of a fully differential structure

Figure 6 The influence of noise and harmonics in the receiving system on the EVM of the error vector magnitude

From this we can conclude that higher-order modulation has a higher data rate, and at the same time a better EVM, and a better EVM means a higher spurious-free dynamic range SFDR, and SFDR and signal The noise ratio, intermodulation distortion and each harmonic term are related. Therefore, to improve the above performance indicators, a balanced signal and differential structure can be significantly improved.

Summarize

Finally, for a good radio frequency system, the main concern is how to improve the sensitivity to useful signals, so as to better separate the signal from noise, harmonics and various interferences. The benefits of differential applications are better common-mode rejection, power supply rejection, electromagnetic interference resistance, better linearity, and a larger dynamic range relative to single-ended signals under the same conditions. Undoubtedly, the differential structure has obvious advantages, and it is more suitable for high-performance radio frequency systems.