How to Design a High-precision ADC?

1. What is ADC?

Essentially, the main difference between analog chips and digital chips is the difference in signal processing. As the name suggests, analog chips process analog signals, while digital chips process digital signals. The analog signal changes continuously over time, such as temperature, humidity, sound, speed, and so on. Their biggest feature is that there are countless different values within a certain time range.

In contrast, a digital signal is a bunch of discrete values, such as the binary 0101 used in computers. Since the transistor has two states of on and off, it can naturally represent the values of 0 and 1. There is no way for a transistor to achieve a state like 10% on or 31.5% off, so it is a digital signal.

In order to connect the two independent fields of analog and digital, it is necessary to use two kinds of chips as a bridge, one is the analog-to-digital conversion chip ADC, and the other is the digital-to-analog conversion chip DAC.

As the name implies, the analog-to-digital conversion chip ADC is used to convert analog signals into digital signals, and the analog-to-digital conversion DAC is just the other way round, converting digital signals into analog signals. However, in practical applications, ADC accounts for a higher proportion. Data shows that 80% of the applications where analog-to-digital conversion is applied are ADCs. Especially in the digital society, almost everything has been digitized, which is convenient for subsequent processing, transmission, and storage.

Many people may think that it is not difficult that the analog signal to be converted into a digital signal. In fact, the ADC is the most difficult analog chip. At the top conference ISSCC in the field of semiconductors and integrated circuits, that is, the International Solid-State Circuits Conference, there are quite a few articles on ADC design.

2. How to design and optimize ADC?

So, how exactly is the ADC implemented? Simply put, several processes such as sampling, quantization, and encoding are required. In other words, we first need to sample this signal and record the voltage value of the signal at regular intervals. The collected value will be quantized, converted into the corresponding digital signal value, and finally expressed by some kind of code, such as complement code, gray code, and so on.

ADC has many parameter indicators, of which there are two common parameters, one is ADC sampling rate or data rate, and the other is resolution. The sampling rate is well understood, that is, how many samplings can be done per unit time, and the more sampling points, the better the original signal can be restored.

Resolution is defined as the smallest change in the value of the input signal. This smallest change in value will change a code value of the ADC digital output value. In the case that the ADC has the same input range, the higher the resolution, the smaller the minimum change represented by a code value. If an ADC has 3 bits, then the entire voltage range can be divided into 2^3=8 parts. If the voltage range is 0-10V, then each part represents 1.25V. In other words, if the voltage change is less than this value, then the ADC cannot capture this small change. The important thing to note is that ADC resolution and ADC accuracy are two completely different concepts.

ADC code

There are many specific implementation forms of ADC, the common ones include successive approximation ADC (SAR), and there is also a Delta-Sigma ADC. For example, a common successive approximation ADC mainly integrates a voltage comparator, a register, a DAC, and some control circuits in the circuit. Its essence is to use binary search to determine the digital signal corresponding to the analog voltage. That is, in the beginning, the input voltage is compared with half of the reference voltage. If the input voltage is larger, then compare with three-quarters of the reference voltage. On the contrary, if the input voltage is smaller, it is compared with a quarter of the reference voltage. And so on, until the comparison is complete.

3. ADC drive design

However, even the most basic ADC is not simple in actual engineering applications, so supporting resource is particularly important. For example, ADCs often cannot work independently. They need to cooperate with other external circuits to function. One of the most important external circuits is the drive circuit.

As mentioned earlier, the ADC needs to sample, quantize and encode the input signal, and output an N-bit digital signal. These operations are usually completed in one cycle of the digital clock. This means that during the sampling process, the input signal should remain unchanged. This is a bit similar to the hold time of a clock in a digital signal.

Inside the ADC, its input actually contains a switch and a capacitor array, which is usually equivalent to a switch and a sampling capacitor. When the switch is closed, the capacitor is charged. After it is charged, the switch is opened, and the comparator and the DAC cooperate with each other to complete the sampling and quantization operation of the ADC at this time.

There are some problems. First of all, if the performance requirement of the ADC are relatively high. For example, if its sampling frequency is required to be high, then the time to charge the capacitor inside the ADC will be very short. If the sampling frequency is 1 million samples per second, then the charging time, that is, the capture time (TACQ) may only be 300 nanoseconds. If no circuit is added to the input as a driver, it will basically not be able to meet the demand for such a high sampling frequency. So in response to this problem, we usually add an operational amplifier to the ADC front end as a driver, so that sufficient charge can be provided to the sampling capacitor within a short sampling time.

This is not over yet. Although we can directly connect the op amp and ADC directly, we rarely design this way in practical applications. Because when the sampling frequency is very high, a very high bandwidth op amp is required if it is directly connected to the op amp. In addition, as can be seen from the simulation, the initial conversion may generate a large transient current when switching, and the driver circuit needs to be able to charge the ADC's internal sampling capacitor within a short ADC capture time (TACQ).

ADS7042 circuit

In order to meet these conditions and avoid large instantaneous currents, we can add an RC circuit before the ADC. We have learned in the university that the RC circuit is used for filtering, but its main function here is to use this extra capacitor to achieve faster charging. The op amp can fill this capacitor, and then when the ADC internal switch is closed, charge the ADC internal capacitor through this capacitor. Of course, in addition to this capacitor, part of the charge also comes from the front-end op amp. This RC circuit is also called a charge bucket filter circuit, which can effectively reduce the bandwidth requirements of the front-end op amp, so we can choose a lower bandwidth and lower cost ordinary op amp to meet the design needs. At the same time, it also eliminates the initial instantaneous current and greatly improves the stability of the circuit.

Here comes another question. How to determine the specific size and indicators of these amplifiers and RC circuits? There are two methods here, one is to derive through theoretical formulas, this is a very detailed derivation process on the Internet. According to the ADC indicators, such as resolution, sampling rate, reference voltage, etc., you can derive the required RC circuit and op-amp parameter data step by step.

Of course, there is another method, which is to perform simulation calculations through ready-made design tools and simulation tools. For example, Texas Instruments TI provides a series of related tools to simplify all the above calculation processes. In the beginning, you can select the corresponding device according to the performance indicators of the ADC, and then use the ADC SAR Drive tool to directly calculate the value of the resistance and capacitance, and get the corresponding performance indicators.

In order to further simplify the design process, TI not only provides design tools, but also a complete ecosystem to integrate these tools.

TI circuit cookbook

Taking SAR drive design as an example, TI provides many classic ADC circuit design solutions. For example, this "high-voltage battery monitor circuit" teaches us step by step from design description goals, to how to choose suitable devices, and how to model and simulate, and get the ideal performance index.

*This article was translated from: https://mp.weixin.qq.com/s/4r69kgCY5p1gOZnlEAd8CA