Introduction: In "Amplifier Circuit Design, What do you care most about?" In the discussion of the topic, ADI senior experts joined hands with dozens of engineers to discuss the big collision of views and stimulate the spark of wisdom.
  In April, at ADI's "Amplifier Circuit Design, What do you care about most?" During the topic discussion, ADI senior experts joined hands with dozens of engineers to discuss the big collision of views and stimulate the spark of wisdom. Here, in particular to share which is quite a reference point of view, most of the engineers actual design experience, can be described as real dry goods, but also welcome readers to share your feelings.
    Watzmann: We are concerned about the common mode voltage, 0 IF architecture requires ADC and the operation amplifier to adopt DC coupling, the appropriate common mode voltage can avoid the additional power consumption caused by the conversion voltage.
    SunRF: Why does the low-frequency amplifier circuit not consider the maximum power output?
    ADI experts reply to this: in low-frequency circuits, the reverse traveling wave effect is not obvious, and the signal voltage and current are mainly considered. In intermediate frequency and radio frequency applications, due to the presence of reflection, to achieve maximum power transmission, but also consider the problem of impedance matching, otherwise the guiding electromagnetic wave will produce a large reflection.
    What indicators to pay attention to, depending on where the application is, I am more concerned about the gain, bandwidth and noise. Because the ADC I choose is generally used for the front-end drive of ADC, in order to ensure that the noise and accuracy of the op amp do not affect the SNR and other indicators of ADC, the op amp indicators need to be determined according to the ADC input signal bandwidth, significant bits and other indicators, so it is required to select a low noise and high-precision broadband amplifier, such as the ADA4930, which is a good choice. anyway, there is no best, only the most suitable, different applications, the focus of attention will be different.
    fangjinbiao: Adjust the voltage because it broke my heart
    I am most concerned about the offset voltage bandwidth and the input and output voltage range, select these parameters, a circuit can basically work normally. As for other parameters: the common mode rejection ratio of the general operation amplifier is at least 60dB, and the power supply rejection ratio is generally 100dB; If portable low-voltage operation is required, the input and output rail to rail operation amplifier should be selected; If in the high-speed circuit, the first concern is the bandwidth and voltage swing rate; There are also parameters such as input offset current output drive current supply voltage range to choose the appropriate chip according to the actual application.
    Netizen Li Zhenjiang: I think the first thing is the influence of the power supply, and the poor treatment of the power supply part of the board directly affects the effect of the entire work.
    Summer: In the past, when doing precision instruments, the factors considered in the selection are: offset voltage and its temperature drift coefficient, input current, noise spectrum, power supply range, input range, output driving capacity, power consumption. The most used are OP177 and AD8629.
    For the AD8629 such operation also have to consider the input bandwidth. I had been having doubts about SETTLING TIME for the op amp. I found that the load RC selection values of the op amp would affect the ADC's accuracy. When changing the sampling time, the tiny signal oscilloscope would not be able to observe it, and the ADC (24BIT) would read differently if it sampled early. Now farewell to the field of small signal measurement, engaged in audio, video related analog design, the whole signal link and signal amplifier circuit is connected, such as CODEC is the function of DAC and ADC, power amplifier is the operation amplifier, LINE TRANSCEIVER is the instrument amplifier. Now I no longer consider the DESIGN elements of OFFSET and linearity, but instead pay more attention to the dynamic indicators of SNR, Sinabi and THD of the device. I am learning now, and I have written the ADIMTTUTORIAL and LINEAR CIRCUIT DESIGN HANDBOOK well.
    jiangxi0929: Is now doing I-V small signal pre-amplification concern
    1) is the power supply mode, 2) bandwidth, 3) bias current, 4) noise, etc
    Netizen 1512787260: I design more concerned about gain, noise temperature, bandwidth. In amplifier applications, they are basically used in the amplification of front-end RF signals, and the gain has a little requirement, especially the noise temperature, which often requires a good noise temperature coefficient. Of course, in the design, the power supply of the amplifier is also very important, and the incompatibility of the power supply design brings many uncertain problems to the signal amplification.
    TLZme: As a pre-amplification part, individuals first consider the two requirements of gain and noise level. After the pre-stage amplification, the output noise should be as small as possible if the amplification factor is sufficient.
    kkfjenui: Relatively speaking, I am still more concerned about the power rejection ratio (PSRR) problem: with the upgrading of the company's products, cost savings, the increase in the magnitude of the equipment motherboard power supply current requirements. End users want to extend battery life (i.e. efficient DC/DC conversion process, use efficient regulators). Then in practice, switching regulators produce more ripple in the power line than linear regulators
    Netizen Fang Hongmin: I am more concerned about the offset voltage and common mode rejection ratio, the offset voltage is still very important for the occasion of high sampling accuracy requirements, and the error is still very large.
    User changaibin: I am most concerned about the amplifier index is the noise level, because digital circuits, switching power supplies, etc., often bring different degrees of noise, thus interfering with the analog circuit part (that is, taking measures such as filtering can not be completely eliminated), resulting in the distortion of the amplifier output (especially when the input end of the amplifier is a weak small signal).
    Input common model circumference in the late development of product application environment, temperature, and external emi has a related relationship.
    caobz: I am most concerned about the input impedance, offset voltage, and swing rate. Especially in the sampling of high voltage, the resistance value of the partial voltage resistor also needs to use a large resistance value, in order to prevent the input resistance of the op amp itself on the amplification factor, the input impedance must be considered. The offset voltage directly affects the deviation of the output signal, especially after the AC signal is processed, resulting in the amplitude of positive half-cycle and negative half-cycle signal is not equal; The voltage swing rate directly affects the parameters of the signal phase lag, which is extremely important for the real-time detection signal single-chip microcomputer. If the signal is amplified by the operation amplifier, the phase lag of the output signal is too large, which is undoubtedly a very serious problem for the real-time detection system, especially in the closed-loop regulation system, the phase lag is likely to cause the oscillation phenomenon of the regulation system.
    ADI Expert: Thumbs up for your opinion! Very real share! Personal talk about how to choose the right chip in many amplifiers: first of all, the selection range can be reduced by the supply voltage; Bandwidth, input/output range, initial offset voltage, initial bias current, input noise and other parameters can further reduce the type of chip. Of course, in the actual project, a very important point is the cost performance.
   User Zeng Yonglong: I am most concerned about the power rejection ratio and offset voltage, as far as the general amplifier circuit is concerned, the power supply is not handled well, it is easy to introduce noise, making the output waveform very miscellaneous, so the power rejection ratio is very important, as for the offset voltage, it is more important for small signal amplification.
   dingranshiwo: Hello! I am taking the course "Sensor Principles" this semester, so for the recent course design, the most important thing for me is the common mode rejection ratio. The reasons are as follows: When the sensor measurement is relatively small, the high common-mode rejection ratio can suppress the sensor output common-mode voltage, greatly reducing the interference. In particular, the measurement circuit of half bridge and full bridge is cleverly matched to make the circuit measurement more accurate. thank you.
   sunny_adi: I am concerned about AC-PSRR and SR. In the current design, switching power supply is used for its power supply, so AC-PSRR will affect its offset voltage. For high-speed applications, SR is an important parameter that determines the maximum input voltage parameter.
    Net friend green007: If the output amplitude is required to be straight, look at the rail to rail, if the pressure swing rate is a precision class, look at the noise, the imbalance only knows so much.
    Netizen hustei_ly: In the recent project to use a probe inside a certain application of the acquisition of images, a probe needs a lot of auxiliary voltage, so the operation amplifier is used to generate the DC bias required by the probe. When designing, the first consideration is the noise level power supply rejection ratio, and then the offset voltage, bandwidth (of course, it is hoped to be as small as possible). Because the whole board is a Boost boost circuit power supply, although the LDO is added behind, but in order to ensure that the generated bias character meets the probe needs, the power supply output noise (that is, the power supply voltage of the op amp) must be strictly controlled, and the higher PSRR can have a good suppression of the noise on the op amp power supply, and then consider the imbalance voltage. Because the probe bias is generated by the operation amplifier amplification, the resistance is selected with 1% accuracy, so that the amplification multiple is more accurate, so that the output voltage meets the requirements, if the imbalance is too large, it affects the output accuracy, so the offset voltage of the operation amplifier also has a great requirement. Finally, we should pay attention to the bandwidth of the op amp, because here the amplification of the DC level, so the bandwidth is not particularly large requirements, should be selected as small as possible bandwidth of the op amp, high bandwidth of the op amp is easier to introduce noise. There is no great requirement on the operation of the compression and swing rate, CMRR, R2R, etc.
   ADI expert adi_peng: Mainly depends on the application field, if it is small signal amplification, more concerned about input impedance, bias, noise, misalignment, CMRR, PSRR. But personally, the biggest concern is the gain bandwidth product.
    ADI expert ADI_Wei: Yes, what netizens say is very comprehensive, different application areas of concern are different, for high-precision applications, noise, offset voltage may be the most concerned; For high-speed applications, it is necessary to pay attention to bandwidth, piezoswing rate and output current. For weak current detection applications, the input bias current may be the most important, for example, the AD549 bias is relatively small; For high temperature applications, such as oil drilling, high temperature is necessary, such as AD8634; And automotive electronics, you need automotives... No matter what the occasion, the voltage input and output range of the signal, the source of error, the package, the price, the sample supply, etc., are all we have to consider. Therefore, it is often more difficult to choose a suitable op amp than an ADC.
   User wsdymg: For different designs need to consider the parameters are not the same, different applications may consider the focus is not the same, low voltage applications, more attention to rail to rail (R2R), in the accurate measurement of the offset voltage, in high-speed applications more attention to bandwidth, such as the battery management system I am doing now on the common mode voltage is more concerned.
   zachfly: It's really related to the application, I mainly do video related, for analog signals, CMRR and PSRR are too important, light interference has streaks, serious "snow" fluttering. If the chip suppression is difficult to achieve, the designer must analyze the external circuit, plus the necessary peripheral circuit filtering and bypass, as for the noise level, now the chip suppression ratio has been done very well, as long as the input and output design is properly designed, and PCB design should pay special attention to not easily generate public impedance with other digital signals. It's not that hard. As an application engineer, choose the right chip is the most important, in fact, the above mentioned parameters, the chip has been integrated as best as possible, the basic understanding of a concept can be, do not know this is not a sad.
   jianping: The noise level ah, the audio amplifier requirements are relatively high, there is a self-excited oscillation, very angry.
    ADI Expert adi_susan: For high-precision systems, the temperature drift of noise and offset voltage is the most important, because the temperature drift coefficient is difficult to calibrate, so it must be considered. The temperature drift and noise of the amplifier can be equivalent to the signal-to-noise ratio, which can be converted into the corresponding resolution bits, so that the resolution of the ADC can be determined. If the performance specifications of the amplifier are not met, even selecting a 24-bit ADC in the system is useless. In addition, for applications with slightly higher signal frequencies, small signal bandwidth and flutter rate are considered separately.
   Netizen: Why can't I do power amplification with PA02? All according to the circuit diagram given in the chip manual, but it still doesn't work.
    ADI Expert ADI_Wei: Hello! Not very clear PA02, if it is an op amplifier, the op amplifier power amplification needs to see whether the output current supports your power.
   Netizen Shan Zengyu: What I am most concerned about is that you introduce these contents, in which books have easy to understand explanations?
   ADI Expert ADI_Wei: Hello! These indicators are found in the basic books of general operational amplifiers. However, some indicators may require actual design to be more deeply appreciated.
   Netizen Liu Wenlong: When the precision requirements are high, the offset voltage is more important.
   BruceLone: Gain, signal-to-noise ratio, power supply, bandwidth, swing rate, etc.
   Netizen lark1001:
   1. Common mode rejection ratio. Open-loop gain and common-mode rejection ratio are equally important when designing a voltage follower. The common-mode component of the output signal of a differential amplifier is not only related to the common-mode rejection ratio of the operational amplifier used, but also to the resistance matching and closed-loop gain.
   2. Frequency and gain. At DC and low frequencies, the open-loop gain of the op amp is constant. The open-loop gain decreases with the increase of frequency. If the frequency is too high, the actual closed-loop gain will be lower than the ideal value, resulting in a certain error.
   3. Input impedance. In the applications of integrators, microcurrent amplifiers and peak detectors, the influence of input impedance cannot be ignored. At high frequencies, the input capacitance of the amplifier will make the input impedance very small, and its effect on the gain should be considered.
   4. Output impedance. The output impedance of an amplifier determines its load performance. The closed-loop output impedance is always much smaller than the load resistance of the amplifier, so the effect of the output impedance on the gain is completely negligible.
   5, bias current and input impedance. For most op-amps, there is a close relationship between bias current and input impedance, so if the bias current is low enough, the input impedance (differential mode and common mode) must be high enough.
   6. Conversion rate. When the closed-loop gain changes, the conversion rate will change greatly, and it will also change with the supply voltage and temperature changes (temperature increases, the conversion rate tends to decrease). In operational amplifiers using external compensation, the conversion rate can be about 20 times higher than that of standard compensation by external compensation. The conversion rate is large and the response is fast.
   Net friend zhangjsh: I mainly do DC and low frequency systems so I am more concerned about the common mode rejection ratio (CMRR), which directly affects the stability of the system. Rail-to-rail (R2R) is conducive to small signals, allowing for higher peak-to-peak voltage amplitude. The damping coefficient and offset voltage are less considered.
   First of all, the operation amplifier should be concerned about its working voltage, which