The AD chip is a bridge connecting digital and analog. The selection of AD chip directly affects the final performance of the entire link. AD chips have many indicators, we need to choose according to our own needs. Also for your reference.

1. Sampling rate

The AD chip is used for sampling, so first of all, it is necessary to consider whether the sampling rate is suitable or not. Because the analog signal generally has a certain bandwidth, the sampling rate of our AD chip is greater than 2 times the bandwidth of the analog signal, and it is generally set to 2.5 times in engineering. For example, our analog signal bandwidth is 50Hz, then the sampling rate of our AD chip is at least 100SPS. If the sampling rate does not meet the specification, the signal spectrum will be aliased.

2. Number of digits

AD chips have digit indicators, common 12-bit, 16-bit, 24-bit and other indicators. Theoretically, the higher the number of digits, the better, but not all the digits of the AD chip are valid. Generally, there will be a certain number of effective digits in the AD chip, because the AD chip will definitely not be ideal, and there will be a certain number of digits. Quantization noise, and also caused harmonic distortion, so the number of digits marked on the AD chip is not necessarily the effective number of digits, so pay attention when choosing.

3. Maximum input and minimum resolution

These two can be said to determine the dynamics of AD, each AD chip will have a dynamic input range, exceeding the maximum input value or less than the minimum input will cause signal distortion. The minimum resolution is to check whether our AD chip can be quantized by 1/2^n when the analog signal is minimum input, where n is the number of bits. The maximum input is to ensure that the signal does not exceed the maximum detectable value, and at the same time, the input of the analog signal should be able to fill all the bits of our AD as much as possible. Generally, we will push to the AD chip saturation input point in the middle frequency band, so that the maximum input of the analog signal can just pass the full-scale output of the AD chip, otherwise a part of the dynamics will be lost, that is, the maximum input of the analog signal will never reach the AD chip. Full scale, then the effective number of digits of AD may be reduced again.

4. Frequency Response

Although our sampling rate is only related to the bandwidth of the analog signal, a problem that cannot be avoided by any chip is the frequency response. If we consider the sampling center frequency of 500MH and the bandwidth of 100MHz, and the sampling center frequency of 10GHz, the bandwidth is For a 100MHz signal, although the bandwidth is the same, but the center frequency is different, the same AD may not be selected. The frequency response of the AD affects the sampling error. If the frequency response of the chip itself is not good in the high frequency band, even if other indicators are satisfied, it is impossible. in use.

5. Is there a difference

Differential signals are more common in AD chips. If the test is a single-ended analog signal, then you can choose a single-ended AD chip, but how the analog signal is at a higher frequency, then for anti-interference, a differential input AD chip is generally selected. At this time, it may be necessary to add an AD chip to the front end of the AD. Baluns are used to convert single-ended to differential.

6. Other indicators

AD chips also have many other indicators, such as multi-channel, temperature response, communication interface, power consumption, cost and so on. These are all factors that need to be considered comprehensively, and specific problems will be analyzed in detail.