Know about Fastest 18-Bit Successive Approximation Register ADC

Due to the evaluation of integrated technology, data acquisition and processing becomes faster and easier with the use of advanced analog to digital converter and ultra-fast ADCs from various analog sensors. These ADCs give best sampling rates with low power consumption, excellent Signal-to-Noise Ratio (SNR), precise throughput, high linearity, and so on. ADCs that are classified based on performance, cost, power, and size vary from 6-bit to 24-bit rates. We can find this converter in a wide range of applications such as instrumentation and measurement, data acquisition by sensors system, energy, medical systems, and communication systems, and so on.

Analog to Digital Converter

Almost all environmental measurable parameters like pressure, sound, temperature, light, etc. are in analog form. Suppose in case of a temperature monitoring system, processing and storing the analog temperature data is not possible with digital computers and processors. Such a system needs a transducer or intermediate device to convert the analog data into digital data to communicate with the digital processors like microprocessors and microcontrollers.

Analog to Digital Converter (ADC) is an electronic integrated circuit that converts the signals from analog to digital or binary form.

The figure given below depicts how analog to digital conversion takes place, wherein the bit rate or digital bit number decides the resolution of the signal. The Figure.b shows a 3- bit ADC which samples the analog signal at some instants to produce the digitized signal. The two main consideration for converting analog signals to digital signals are sampling rate and quantization levels or sampling precision.

Assume that we have one volt signal to be converted from digital format by using 3-bit ADC such that a total of 2^3=8 divisions are available for 1V. This means 1/8=0.125V is called as quantization level represented for each division as 000 for 0V, 001 for 0.125, and likewise upto 111 for 1V. To get a better precision of the signal, the bit rates can be increased further like 6, 8, 12, 14, 16, and so on. Thus, quantization gives the smallest change in the analog signal that results from a change in the digital signal.

Another parameter is the sampling rate because when the signal is sampled at a different frequency level – other than the desired one – then the output signal will be completely misrepresented. Therefore, the accrued signal reconstruction introduces distortion if the sampling rate is smaller than the Niquist rate. Niquist theorem states that unless you sample at twice the rate of the largest frequency content of the signal, the reconstruction of the signal at ADC output is not possible as you can observe in the diagram. In practical terms, this rate is 5-10 times the maximum frequency of the signal.

The types of analog to digital converters include:

• Dual Slope A/D Converter • Successive Approximation A/D Converter • Flash A/D Converter • Delta –Sigma A/D converter

The Dual slope type A/D is a slow speed and relatively medium cost device. The flash type is a high-speed and high-cost device. The delta-sigma converter is a slow-speed and low-cost device, whereas the successive type is a medium to fast-speed, low-cost converter device.

Selecting a suitable A/D converter for specific application is based on the specifications like precision, type, linearity, etc. It is always important to design a system to allow more bits than initially required. If an application needs 10 bits of accuracy, then it is better to choose a 12-bit converter.

One of the major benefits of a SAR converter is that at high data acquisition rate it is able to get connected to multiplexed inputs. As the input is sampled and held in an internal capacitor, the charge on the capacitor is converted to a digital output code by the successive approximation routine. Hence, this charge is held throughout the conversion time, and only a fast-changing input is responsible for the initial sample and hold period or acquisition time. This particular characteristic makes this SAR converter an ideal choice for many real-time applications, including touch-screen sensing, medical, motor speed control system and other data acquisition systems.

AD7641 Fastest 18-Bit SAR ADC

AD7641 is the fastest ADC available in the market from the analog devices family with some typical features and INL performance of +/-2 LSB’s including 18 bits of resolution. This ADC offers features like an internal reference of 2.5V, signal to noise ratio of 93DB , low operational power of 2.5V, and a threshold limit of -100db. A combination of AC and DC performance fascinates customers’ needs – conversion of fast moving signals to high level of frequency. High speed data acquisition and automatic test equipment, digital signal processing and communications are a few applications that take advantage of AD7641 representing a rare combination of speed and accuracy.

16-bit and 18-bit Successive Approximation register ADCs are still maintaining higher values due to their high cost and overall performance. These devices raise the top speeds of 18-bit SAR ADCs from 800 KSPS to 2MSPS. AD7641 is available in both 48 lead chip scale packages and 48 lead LQFP. These packages operate at a specified temperature range of – 40 degrees centigrade to +85 degrees centigrade. A functional block diagram of an 18-Bit SAR ADC is given below.

The AD7641 is an 18-bit, 2 MSPS, charge redistribution Successive Approximation. ADC is a fully differential, analog-to-digital converter (ADC) that operates from a single power supply of 2.5V. This part consists of a high-speed 18-bit sampling ADC, a conversion clock, an internal reference known as buffer, error correction circuits, serial and parallel system interface ports. This SAR ADC sorts into two very-high sampling rate modes, i.e., Wide band Warp and Warp mode and a faster mode (normal) for asynchronous rate applications. In addition to the more outdated DC parameters such as gain and linearity, the AD7641 is a hardware factory-regulated and tested to ensure AC parameters, such as signal-to-noise ratio.

Modes of Operation

AD7641 SAR ADC features have three modes of operations: wide band warp, warp mode and normal mode. Each of these modes is suitable for specific applications.

When WARP = high, NORMAL = high – it gives wide band warp mode, and if WARP = high, NORMAL = low – it gives warp mode. Both these modes allow fastest conversion rate, which is approximately 2 MSPS. However, if the conversion time is more than 1ms, then these modes full specified accuracy is not guaranteed. After powering- up the device, the first conversion result is ignored if two consecutive conversions times are longer than 1 ms. With these modes, the AD7641 is best suited for the applications that requires high accuracy and faster sample rates. Compared with the warp mode, the wide band warp mode offers slightly improved THD and linearity.

When NORMAL = low, WARP = low, then it is considered as a Normal mode, which is the fastest mode with conversion rates up to 1.5 MSPS without having any limitation on time between two consecutive conversions. This mode is suitable for asynchronous applications wherein faster sampling rates and high accuracy are required like data acquisition.

This is all about the 18- bit fastest SAR analog to digital converter with the basic concept of ADC. These fastest ADCs are predominant in process industries for acquiring the real-time analog values from various sensing devices. You can also write some of the applications of this type of ADC in the comment section below so that other readers can get motivated to read about this ADC – and also please leave your suggestions as well. You can download its data sheet from a link attached to it.

Photo Credits

• Analog to Digital Converter by allaboutcircuits
• ADC Resolution by upenn
• Different Sampling Rates of ADC by calrec