Table of contents
Explanation of technical terms
1. The impact of removing the buffer stage on the signal chain
2. The relationship between SNR and gain
overview
提示：这里可以添加技术概要
An example of signal chain building block parameters is the LTC2387-18 (2-stage AFE), which is a 15MSPS precision SAR converter with buffer stages and digital filters. The signal frequency of the sensor level is 1MHz, the amplitude is 1V, and the gain of 8.2 times will amplify the input signal from 1V to 8.2V peak-to-peak value. The noise bandwidth of the gain stage is 4MHz*π/2, the RTI noise spectral density is 2.3nV/rtHz, and the RTL noise spectral density is 19nV/rtHz. The buffer stage noise bandwidth is 70MHz*π/2 and the noise spectral density is 2nV/rtHz, going to the ADC Nyquist frequency gives a noise bandwidth of 7.5MHz and a noise spectral density of 16.8nV/rtHz
overall architecture process
提示：这里可以添加技术整体架构
It can be seen that the gain stage has a higher noise spectral density but a narrower noise bandwidth; the ADC has a lower noise spectral density but a wider bandwidth than the gain stage; and the buffer has a considerably lower noise spectral density but a wider bandwidth. The pie chart on the right shows the noise ratio, the blue is the noise of the gain stage, the green is the edge noise of the buffer stage, which is about 24uV, and the orange is the ADC noise, which is about 46uV. Overall, the gain and buffer stages are well balanced, the total noise is 68uVRMS, and the SNR of the entire signal chain is 92.6dB, which is an ideal SNR.
Explanation of technical terms
This section is mainly an example.
technical details
提示：这里可以添加技术细节
1. The impact of removing the buffer stage on the signal chain
In the signal chain design, the case shown in the figure below may appear, that is, cancel the buffer stage in the signal chain and directly drive the ADC through the gain stage. Is this design desirable or needs to be avoided?
If the buffer stage is removed, the gain stage filter noise is referred to the ADC input, as shown in the right part of Figure 6. Gain stage noise has a high noise spectral density, and the gain stage will amplify all signals from small amplitudes to large amplitudes, which greatly expands the noise spectral density of the amplifier. The gain stage noise itself already dominates at lower bandwidths, not to mention the gain stage bandwidth being increased to higher frequencies. The noise spectral density of the gain stage is dominant relative to the ADC, and removing the buffer stage greatly increases the noise spectral density.
The right design cancels the buffer stage. Still taking the LTC2387-18 as the design example, the signal with a frequency of 1MHz is amplified from 1V to 8.2V through a gain of 8.2 times. If the buffer stage is cancelled, the bandwidth of the gain stage filter must be increased from four times the signal frequency to the buffer bandwidth, which is the necessary bandwidth to establish the kickback during sampling, so the gain stage noise bandwidth is extended by 4MHz*π/2 It is 75MHz*π/2, an increase of about 19 times. It can be seen intuitively from the noise spectrum that the noise of the gain stage plays a decisive role, the total effective noise is 207μVRMS, and the total SNR is 82.9dB. As can be seen from the pie chart, the SNR drops by 10dB after removing the buffer stage. Therefore, from a noise and SNR standpoint, it is not recommended that engineers remove the buffer stage from a practical design
2. The relationship between SNR and gain
In signal chain design, especially in small signal design, the gain value provided by the gain stage is what we need to consider. Figure 9 shows the relationship between SNR and input signal amplitude.
If a signal with a variable amplitude from 100mV to 3.5V is scaled up to the full scale of the ADC, several conclusions can be drawn:
• SNR close to 77dB when the signal amplitude is 100mV;
• The SNR of the gain stage varies between 77~105dB. As the signal increases, the SNR will increase, and the increase depends on the signal amplitude;
• Noise and signal are amplified by the same factor, so the SNR at the output of the gain stage is independent of the gain factor,
• On the right is an example 15MSPS LTC2387-18 converter, the dashed ADC SNR is 96dB, the AFE SNR is in blue, and the entire signal chain SNR is in orange. It can be seen from the figure that the SNR below 900mV is dominated by AFE, and it is dominated by ADC when it exceeds this range.
In the example shown in the figure, the signal amplitude is 200mV, the bandwidth is 1MHz, the noise spectral density of the gain stage is about 2nV/rtHz, and the noise is 5uVRMS. The blue total SNR increases exponentially, and finally converges to 83dB, which is about a gain of 9, and the AFE noise is about 2~3 times that of the ADC. Higher gains are theoretically possible, but practically limited, and are usually determined by the amplifier bandwidth.
summary
提示：这里可以添加总结
To generate a larger gain, such as a gain of 41 or 42, multiple gain stages may be required. Increasing the gain stage will increase power consumption on the one hand, and may introduce distortion on the other hand. The higher the closed-loop gain, the greater the distortion. To avoid distortion, this is why too much gain is not placed. In addition, large signal swings also produce amplifier distortion, so even if the amplifier gain is quite low, an output signal swing that is too large will cause additional distortion to the signal. Generally speaking, the ADC needs to amplify the signal to the best operating point to achieve ideal linearity while having the best distortion, and -1dB is the recommended sweet spot.