Any physical signal, such as a sound, or a light, or a voltage, is considered an analog signal. The thing about analog signals is that they are infinite in nature. There are infinite numbers of sounds we can hear, there are infinite colors to perceive, and there are infinite numbers between the 1 and 2.
Digital on the other hand deals with discrete or finite signals or finite. As you can imagine, dealing with analog signals is very difficult.
They are difficult to store and even more difficult to process. This is where analog to digital converters come into play. An analog to digital converter ADC , converts any analog signal into quantifiable data, which makes it easier to process and store, as well as more accurate and reliable by minimizing errors. Any digital processor, like a computer, needs digital input for processing, transporting and storing data. Once something is converted into a number, there is a lot that can be done with it.
Calculations, manipulations, translations, transmissions, encoding, encrypting, etc. However, the reverse also needs to be done. The output is required in analog form and for that a DAC digital analog converter. It converts digital output into analog output so that it is meaningful for the user. There are many instruments that require analog to digital converters. Radars for example, pick up signal strengths and convert it into digital values for further processing.
This relationship is shown in the following equations. In this example, the required Additionally the Nyquist criterion states that the sampling rate, fs, should be at least twice the maximum incoming frequency, fin, so a 20 kSPS ADC would suffice. Having a set of search criteria on hand, there are many ways to find the ADC that can fit the requirements.
By entering resolution and sample rate, a number of choices are suggested. Many bit ADCs specify If you would like to have better noise performance, use oversampling to push the ENOB up to 16 bits n-bit improvement is obtained from 4 n oversampling.
With oversampling, one could use a lower resolution ADC: a bit ADC oversampled by 4 4 oversampling will yield bit noise performance. In our example, this means a bit ADC with 5. Or, a bit ADC oversampled by 4 2 ; or 1. The 2. Knowing the input voltage range of the ADC will help us in designing the gain block. This means that we can design our gain blocks to have a gain of 10 for the example on hand. Although the AD is easy to drive, the driver amplifier needs to meet certain requirements.
For example, the noise generated by the driver amplifier needs to be kept as low as possible to preserve the SNR and transition noise performance of the AD, but remember that the gain block amplifies both signal and noise together. To keep the noise at the same level before and after the gain block, we need to select an amplifier and components that have much lower noise. The noise coming from the amplifier can be further filtered by an external filter.
How much noise is allowed at the input of the opamp? We should design a gain block that has much lower noise floor, say by a factor of 10 since we gain up by This will ensure that noise from amplifier is much less than the noise floor of the sensor.
To calculate the noise margin, we can roughly assume that the noise at the input of the op amp is the total noise of the op amp plus the noise of the ADC. The first order of op amp selection after knowing the input signal bandwidth is to pick an op amp that has an acceptable gain-bandwidth product GBWP and that can process this signal with minimum amount of dc and ac errors. To get the best gain bandwidth product, the signal bandwidth, noise gain, and gain error are required. These terms are all defined below.
As a guide, pick an amplifier that has gain bandwidth greater than times the input signal BW if you want to keep the gain error below 0. Additionally, we need an amplifier that settles quickly and has good drive capability.
Remember that our noise budget requires the overall noise at the input of the op amp to be less than The AD low power, precision JFET input amplifiers feature extremely low input bias current and rail-to-rail output that can operate with supplies of 5 V to 26 V. Its relevant specs are stated in the table below.
We can configure the op amp in a noninverting configuration with the component values shown in the table. All active and passive components generate noise of their own, so it is important to choose components that do not degrade performance. As an example, it is wasteful to buy a low noise op amp and surround it with large resistors. It is extremely important to have a good understanding of all the error sources in the designed circuit.
In order to achieve the best SNR, we need to write out the overall noise equation for the above solution.
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