Introduction
Professional audio engineers typically use the term "balance" to refer to differential signal transmission. This also informs us about the concept of symmetry, and it is also very important in differential systems. In a differential system, the driver has a balanced output, the transmission line has a balanced characteristic, and the receiver has a balanced input.
Typically, two methods are used to process differential signals: electronic and Transformer methods.
1. The electronic method has such characteristics as low cost, small size and weight, excellent low frequency, DC response and so on.
2. The benefits of transformers are excellent common-mode rejection ratios, DC isolation, no power dissipation (almost 100% efficiency), and resistance to harsh EMC environmental disturbances.
This paper focuses on the integrated fully differential amplifiers for differential signal situations. Some basic operations, such as how to convert a single-ended signal to a differential signal and how to build an active antialiasing filter, are discussed here.
What's an integrated, fully differential amplifier?
An integrated, fully differential amplifier is very similar to a standard op amp on a frame.
Figure 1 shows a simplified version of the integrated fully differential amplifier. Q1 and Q2 are input differential pairs. In a standard op amp, the output current is removed only from one side of the input differential pair, and the input current is used to establish a single-ended output voltage. In a fully differential amplifier, the current from both sides of the differential input is used to establish the voltage at a high impedance node formed by the Q3/q5 collector and the Q4/q6 collector. These voltages are then buffered to the differential output out+ and out-.
For first-order approximations, the common-mode voltage sent to the in+ and in-does not change the current flowing through the Q1 or Q2, thus producing no output voltage; it is suppressed. The common-mode output voltage is not controlled by the input. The VCM error amplifier controls the common-mode output voltage by sampling the input, comparing it to the voltage at the VCM, and adjusting the internal feedback.
The two complementary amplifier paths share the same input differential pair, their characteristics are very well matched, and such a framework makes their work points close to each other. As a result, the distortion in the two amplifiers is matched, resulting in symmetrical distortion of the differential signal. Symmetric distortion tends to counteract even harmonics. Laboratory tests have shown that when the signal is measured differentially, the two harmonic components in the differential output THS4141 at 1MHZ are reduced by approximately 5dB compared to the single-ended output at either end. The measured three harmonic components have no change.
Voltage Definitions
To understand how a fully differential amplifier works, it is important to understand the definition of the voltage used to describe the amplifier. Figure 2 shows a block diagram representing a fully differential amplifier and its input-output voltage definition.
The voltage difference between the + and-inputs is the differential input voltage vid. The average of two input voltages is the common-mode input voltage, Vic.
The voltage difference between the + and-outputs is the differential output voltage of the VOD. The common-mode output voltage VOC is the average of two output voltages and is controlled by the voltage at the VCM.
AF is a frequency-dependent amplifier differential gain, so Vod=vid x AF.
Increased noise immunity
This happens in reality when the signal is transmitted from one place to another, and the noise is coupled into the line. In a differential system, keeping the transmission lines as close to each other as possible can cause the noise that is coupled into the inductor to behave as a common-mode voltage. The common noise in the power supply also behaves as a common-mode voltage, and the system is more anti-jamming to external noise. Figure 3 shows the noise immunity of the fully differential amplifier.
Increased dynamic range
Due to the phase change in the differential output, the dynamic range of the differential output increases by twice times (4) compared to the single-ended output in the case of the same voltage swing.
Basic Circuits
To maintain balance in a fully differential amplifier, symmetrical feedback must be taken from both the output on both sides and the inputs on both sides. Symmetric inverting amplifiers are formed on both sides, and the inverting amplifier topology is easier to fit with a fully differential amplifier. Figure 5 shows how to maintain a balanced amplifier by using symmetric feedback, where the feedback resistor RF and the input resistor RG are equal.
It is important to maintain good CMRR performance by maintaining symmetry in the two feedback paths. The CMRR is directly proportional to the resistance matching error. For example, a 0.1% error results in a CMRR of 60dB. For small variations in feedback due to mismatched resistors, the amplifier's differential gain is approximately the average gain on both sides. The output balance is maintained by the VCM error amplifier.
In the past, generating differential signals has been very cumbersome. Once the differential method is used, up to three amplifiers and DC isolation capacitors are required to set the common-mode output voltage. Integrated, fully differential amplifiers provide a better solution. Figure 6 shows an example of converting a single-ended signal to a differential signal.
A major application of a fully differential amplifier is signal processing in the case of an ADC input. Low-pass filters need to be used to remove high-frequency noise to prevent their aliasing into the band of interest. Multilevel Feedback (MFB) is an excellent topology that is easily adapted to a fully differential amplifier. A MFB circuit is used to implement a composite pole pair in a second order low pass filter transfer function. Here is an example 7 shown.
The amount transfer function of the filter circuit is
K Set the pass-band gain, FC is the cutoff frequency of the filter, the FSF is the frequency range factor, q is the quality factor.
Where re is the real part of the compound pole pair, Im is the imaginary part of the compound pole pair. Set R2=R,R3=MR,C1=C,C2=NC, the results are:
Start by determining the ratios, M and N, the desired gain as well as the Q value of the Designed filter, then select C and calculate the R value that requires FC.
The combination of R4, RT and C3 has multiple effects. The R4 will isolate the amplifier output from the input of the ADC. The R4 and RT provide a double interrupt for the transmission line between the amplifier and the ADC, and form a voltage divider. The C3 helps absorb the charge injected by the ADC input port. R4 and C3 form a real pole, which works with the composite poles in the MFB to make a third-order filter, even though it can be simply placed outside the frequency of interest.
Some ADCs with differential inputs provide the appropriate VCM output. Typically, everything that needs to be done here is to provide a bypass capacitor -0.1uf and/or 0.01uF. If VCM is not supplied, the VCM can be generated from the ADC reference voltage shown in 8. The voltage at the summation point is the middle point of the reference voltage and it concentrates the VOC in the intermediate point of the ADC input range.
Each power supply pin should have a tantalum capacitor between 6.8uF to 10uF and 0.01uF to 0.1uF ceramic capacitors close to each other in parallel. Figure 7 shows the 10uF and 0.1uF power bypass capacitors.
Conclusion
The integrated fully differential amplifier is very similar to a standard single-ended op amp, except that the output of the fully differential amp is removed from both sides of the differential input pair to produce a differential output.
When compared to a single-ended system, the differential system provides enhanced external noise immunity, reduced even harmonics, and double dynamic range.
By implementing two symmetrical feedback paths, the inverting amplifier topology is very easy to fit into a fully differential amplifier.
The integrated fully differential amplifier is ideal for driving differential ADC inputs. They provide an easy method for antialiasing filtering, and the desired common-mode voltage can be easily set by the VCM input.
Fully Differential Amplifiers
