Design Tips to Reduce ADC Signal-to-Noise Ratio Loss

Publisher:梦想学院Latest update time:2012-05-22 Source: 21IC中国电子网Keywords:ADC Reading articles on mobile phones Scan QR code
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This article will discuss the main factors that affect the SNR loss introduced by signal scaling, how to quantify it, and more importantly: how to minimize this effect.

Many signals generated by sensors or systems are bipolar high-voltage signals (such as the widely used ±10V signals). However, there are many simple ways to pass this signal through the ADC; various integrated high-voltage ADC solutions can also be used: they can handle this large full-scale input signal without sacrificing SNR. These solutions require very high supply voltages to meet the input range requirements, and their power consumption is also considerable (Figure 1). These high-voltage ADCs also narrow the range of signal conditioning (op amp) solutions. If the signal needs to be multiplexed with a combination of high-voltage and low-voltage inputs, the system cost will increase significantly (Figure 2). Figure 1:



High-voltage ADCs can accommodate large input signals, but at the expense of greater power consumption. To implement this solution, ±15V and +5V supplies are usually required.



Figure 2: Multiplexed bipolar high-voltage ADC system.

Input amplifiers can also be used to scale the signal to match the full-scale input range of the low-voltage ADC. This signal conditioning circuit can be connected to a multiplexer input so that all signals fit within the range of the ADC (Figure 3).



Figure 3: High-voltage multiplexer system using a single low-voltage ADC.

[page] When an amplifier is used to scale the signal voltage, the noise is referenced to the amplifier input. In this case, there are two main noise sources: the input-referred noise of the amplifier itself, and the scaled-down input-referred noise of the ADC. These two noise sources combine in a quadratic manner. In addition, the amplifier noise is filtered by the input bandwidth of the ADC and the antialiasing filter between the amplifier and the ADC input, see Figure 4. Figure



4: The scaling amplifier introduces noise, but the noise is filtered by the RC circuit and the input network of the ADC.

The system SNR (at the amplifier input) is calculated as: Where: VnADC is the input RMS noise of the ADC; VnOPA is the input-referred noise of the amplifier (X times the input reference) = single-pole -3dB frequency. Given the full-scale range of the ADC, the input-referred noise of the ADC, and the scale factor of the amplifier, there are two variables that affect the goal of reducing SNR loss: the filter cutoff frequency and the input-referred noise of the amplifier. If the signal source has low frequency components, the filter can be designed so that the amplifier can tolerate a larger input noise (higher input noise is usually associated with lower power consumption and cost). If the ADC limits the bandwidth of the system, the amplifier needs to have a low enough input referred noise to keep the SNR loss within acceptable limits. For example, given a ±10V input signal and a 5VP-P full-scale range ADC with an SNR of 92dB, the scale factor (ratio of input to full-scale range) is 4. The ADC input referred noise in the data sheet is 44.4nV RMS. Assuming the filter cutoff frequency is 10kHz and the amplifier input referred noise is 10nV/(Hz) 1/2, the SNR loss is: SNR(loss) = 0.035dB. If there is no filter and assuming an ADC bandwidth of 10MHz, the required input referred noise to achieve the same SNR loss becomes 0.3nV/(Hz) 1/2, which is a very strict requirement. For an ADC with the same bandwidth of 10MHz, if SNR(loss)=0.5dB is allowed, the noise requirement for the amplifier is 4nV/(Hz) 1/2, which is relatively easy to achieve. Therefore, if the system bandwidth and the allowable SNR loss are given, adding a proportional amplifier to convert the high-voltage signal to a low-voltage ADC with a full-scale range will be a completely feasible solution. When multiple signals with different swings are fed to a multiplexed low-voltage ADC, this solution can achieve a cost-effective system.















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