Application Advantages of Differential Signals and Applications in Analog-to-Digital Converters

Today’s world is a world filled with massive amounts of data. People’s lives have benefited a lot, but system designers are under increasing pressure to select the right driver for an analog-to-digital converter (ADC) is an important issue. As an important bridge between the real world and the data world, ADCs often need to sample at frequencies of hundreds of megahertz and resolutions as high as 16 bits. In this way, choosing the right drive to reach its full potential becomes critical. High bandwidth, high spurious free dynamic range, low noise and low distortion have become important indicators for selecting ADC drivers.

Today’s world is a world filled with massive amounts of data. People’s lives have benefited a lot, but system designers are under increasing pressure to select the right driver for an analog-to-digital converter (ADC) is an important issue. As an important bridge between the real world and the data world, ADCs often need to sample at frequencies of hundreds of megahertz and resolutions as high as 16 bits. In this way, choosing the right drive to reach its full potential becomes critical. High bandwidth, high spurious free dynamic range, low noise and low distortion have become important indicators for selecting ADC drivers.

Advantages of Differential Signaling

At present, there are two schemes used to drive the ADC, one is to use a transformer, and the second is to use a differential amplifier. However, before introducing differential amplifiers, let’s first understand what differential signaling is.

Simply put, a differential signal is the difference between two related signals. This article introduces the voltage differential signal, which has been widely used in audio, data transmission and telephony. Although more complex than a single-ended input signal system, the advantages of a differential signal system are obvious. , the differential signal is highly immune to external electromagnetic interference (EMI). An aggressor affects each end of a differential signal pair equally. Because the signal is determined by the voltage difference, the interference on both sides cancels out, and the signal will not change significantly. Second, differential signals are useful for identifying tiny signals. In the differential signal system, the reference point is determined by the user, and the average signal of the two input terminals can be selected as the reference point, which reduces the swing range of the signal. Third, the signal of the single-ended input system depends on the virtual ground, while the differential signal does not need such a virtual ground, which increases the fidelity and stability of the bipolar signal. Fourth, timing positioning of differential signals. Differential signals are less affected by process and ambient temperature, which can reduce timing errors. The currently popular LVDS is a small-amplitude differential signaling technology.

A differential amplifier is a device that accepts and outputs differential signals. Like an operational amplifier, it can receive double-ended input, but it has a dual output, unlike an operational amplifier that only has a single port. In a differential amplifier, its output common-mode voltage (VOCM) can be independently controlled by the differential voltage. Figure 1 is a simplified schematic of a differential amplifier.

 Application Advantages of Differential Signals and Applications in Analog-to-Digital Converters

Figure 1 Standard differential amplifier

New Differential Amplifier

Differential amplifiers have several advantages. It is the anti-noise ability, which has been mentioned in the introduction of differential signals. The second advantage is increased differential output voltage swing (see Figure 2). The reason for this is not complicated. The two voltages at the output are out of phase, and the difference is of course twice that of the single-ended output. The third advantage is the reduction of even-order signal distortion. To explain this, we express the voltage at the output as the sum of the multi-order functions at the input.

Vout+ = k1Vin + k2Vin2 + k3Vin3 + … , (1)

Vout- = k1(-Vin) + k2(-Vin)2 + k3(-Vin)3+…(2)

Vod = Vout – Vout – = 2k1Vin + 2k3Vin3 + … (3)

It can be seen from equation (3) that the even order is eliminated.

In order to adapt to the development of the market, various companies have launched their own differential amplifier products, such as ADI’s AD4937/4938, TI’s THS4520, MAXIM’s MAX4198/MAX4199 and Linear’s LTC6400. With the advancement of technology, the technical parameters of these products have reached a very high level. Taking AD4937 as an example, its input voltage noise is 2.2nV/√Hz; -3dB bandwidth at 1.6GHz, gain G=1; slew rate is 5000V/μS; 0.1dB gain flatness bandwidth at 125MHz; can drive from DC to 100MHz ADC.

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