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비디오 시리즈

Precision labs 시리즈: 완전 차동 증폭기

완전 차동 증폭기(FDA)에 대한 이 시리즈에서는 표준 단일 종단 신호를 통한 차동 신호의 이점을 알아봅니다. 단일 종단 신호를 완전 차동 신호로 변환하는 완전 차동 증폭기라는 새로운 통합 증폭기 아키텍처를 소개합니다. 또한 통합 아키텍처가 개별 단일 종단 연산 증폭기를 사용하여 구성된 차동 증폭기보다 뛰어난 방법을 소개합니다. 이 비디오에서는 입력 신호, 전압, 전류 제어, 전압 및 전류 출력 사이의 관계를 분석할 수 있도록 준비합니다. FDA의 게인 구성과 입력 및 출력 범위 호환은 아날로그-디지털 컨버터(ADC)와 인터페이싱할 때 중요합니다. 또한 TINA-TI 매크로 모델을 사용하여 FDA를 적절히 보상 및 안정화하는 방법과 SPICE에서 증폭기 위상 여유를 확인하는 방법도 알아봅니다.

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      발표자

      Hello, and welcome to the next installment of TI Precision Labs. In this series, we will study a new amplifier architecture called the fully-differential amplifier, or FDA for short. Before we get into the architecture and features of a fully-differential amplifier, let's first understand what a fully-differential signal is.

      A differential signal has two complimentary outputs that are 180 degrees out of phase, as shown here. Each output is level shifted by an identical DC common-mode voltage. The final differential output is obtained by taking the difference between each single-ended output.

      The differential output, therefore, has twice the magnitude of each single-ended output signal. Differential signaling has been commonly used in audio, data transmission, telephone systems, and high speed data acquisition systems because of its inherent resistance to external noise sources. More recently, differential signaling is becoming increasingly popular in high-precision acquisition systems, as more [? Sar ?] and delta sigma ADCs adopt the differential input architecture.

      A differential signal has several advantages over its single-ended counterpart. Since a differential signal is a difference of two single-ended signals that are out of phase with each other, any common-mode perturbation will affect both signals equally and will Ideally be canceled at the differential output. This rejection of common-mode disturbances is one of the main advantages of the fully-differential architecture.

      Even order harmonic distortion enjoys similar benefits to the common-mode cancellation discussed previously. Hence, differential signaling tends to have improved DHT performance compared to its single-ended counterparts. Yet another advantage of differential signaling is the increased dynamic range. As shown here, while each single-ended signal has a two volt peak-to-peak swing, the differential output swings by four volts peak-to-peak, thereby doubling the signal swing.

      This figure shows the standard pin connections on a fully-differential amplifier, or FDA. There are two power supply pins as well as two input pins, like the standard single-ended [? op-amp. ?] An FDA has two differential output pins, which produce the complementary outputs.

      The VOCM pin controls a secondary loop internal to the amplifier and sets the DC output common-mode voltage of the FDA. An FDA may be configured to convert a single-ended input signal to a differential output, as shown here in the lower left figure. This configuration is commonly used as the interface between a single-ended op-amp or a sensor, and a differential ADC.

      An FDA is a symmetrical device, which implies that the source and ground connections shown here can be reversed without affecting the amplifier's functionality and performance. An FDA can also be configured as a differential in differential out circuit, as shown in the lower right figure. The differential gain of the amplifier is set by the ratio of the feedback resistor, RF, and the gain resistor, RG, while the common-mode gain is set by a combination of VOCM and the feedback network. An FDA will be configured in a negative feedback loop as long as the feedback is between the input and output pins of opposite polarity.

      An FDA typically refers to an integrated amplifier. However, a fully-differential amplifier can also be realized using discrete, single-ended op-amps. Two different methods are presented here. The first method shown in this figure has the benefit of presenting a high input impedance to the previous driving stage.

      Amplifier one is configured as a buffer and drives both the non-inverting output as well as amplifier two. Amplifier two, which is configured as an inverting amplifier, in turn, drives the inverting output. There are a few disadvantages to this architecture. Since amplifier one drives amplifier two, there is an inherent part difference between out plus and out minus.

      The phase difference between the two parts will result in a balance error that dilutes the benefits of the fully-differential architecture, namely, common-mode rejection and improved even order harmonic distortion. In differential signaling, any common-mode differences between the individual single-ended outputs will not be rejected and is converted into a differential output. In addition to the part difference, the two discrete amplifiers have inferior matching and symmetry compared to an integrated fully-differential amplifier.

      Other things remaining the same, better matching and symmetry will always achieve superior common-mode rejection and distortion performance. The output noise from the two amplifiers is given by the equation shown here. An integrated fully-differential amplifier typically has better noise performance for the same quiescent power consumed by the two discrete amplifier architectures shown in this design.

      Another disadvantage is the lack of common-mode voltage control. The common-mode gain is fixed at one [? volt per ?] volt in this configuration. A second method of realizing a fully-differential amplifier using discrete single-ended op-amps is shown here.

      This amplifier configuration, unlike the previous one, can be used to convert both a single-ended input or a differential input to a differential output like a typical FDA. The amplifier has two independent feedback loops-- a differential gain loop and a secondary common-mode feedback loop. The forward part differential gain loop is realized by the two input amplifiers shown here, while the third amplifier is part of the common-mode feedback loop, which allows the system to maintain the desired common-mode output voltage, similar to an integrated FDA.

      Both discrete amplifier architectures have significant drawbacks compared to the integrated solution-- namely, [? Purim ?] matching, higher quiescent power per megahertz of bandwidth, and higher noise. Now that we have seen how a fully-differential amplifier is implemented using discrete components, let's study the integrated design. Shown here is the internal block diagram of the THD4551, a high-precision FDA commonly used to drive Sar and delta sigma ADCs.

      At the core of the device is a differential high open loop gain forward path amplifier. Also integral to the design is a secondary error amplifier that controls the FDA's output common-mode voltage. The error amplifier is typically a wide bandwidth, low noise, single-ended op-amp that works by detecting the average voltage at the forward amplifier's differential output and comparing it against the voltage at the VOCM pin.

      The integrated 5.2 kilo-ohm resistors detect the feed-forward amplifier's average output voltage, or in other words, it's common-mode voltage. The error amplifier, then, compares this average voltage against the DC voltage at its non-inverting pin and adjust its output voltage. The error amplifier output subsequently drives an internal bias node in the feed-forward amplifier.

      An op-amp in a negative feedback loop will ideally have both inputs tracking each other. The negative feedback of the error amplifier, therefore, acts as a server loop, adjusting the feed-forward amplifier's internal bias node until the voltage at its inverting pin is equal to VOCM. When the VOCM pin is left floating, the voltage at the error amplifier's non-inverting input defaults to mid-supply because of the internal 300 kilo-ohm resistive divider between the supplies.

      If the VOCM pin is left floating, a capacitor should be connected externally from the VOCM pin to ground to reduce the high frequency thermal noise contribution from the 300 kilo-ohm resistors. In case an application requires an output common-mode that is different from mid supply, drive the VOCM pin externally with a low output impedance DC source. A precision DAC, the [? VRETH ?] of an ADC, is typically used for this purpose.

      The operation of a fully-differential amplifier can be analyzed by following three golden rules. The first rule-- the two input pins of an FDA track each other identically. This is analogous to the virtual ground concept of a single-ended op-amp. Note that both the DC and AC signal components will be equal.

      The next two rules are related to the FDA's output pins. Rule two-- the two single-ended outputs of the FDA are identical to each other in frequency and magnitude but are 180 degrees out of phase, as shown here. The third and final rule is that the two single-ended outputs share the same DC common mode.

      Or in other words, the instantaneous average of the two output signals is always equal to VOCM. For example, if VOCM is set to 2.5 volts and the voltage at V out plus is at 3.5 volts, then the voltage at V out minus should be 1.5 volts to satisfy rules two and three of the FDA.

      This concludes the introduction to fully-differential amplifiers. Thank you for your time and attention. Please take the quiz to test your knowledge on the basic operating principles of FDAs.

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      Precision labs 시리즈: 완전 차동 증폭기