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High-precision ADC Reference Designs

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High-precision ADC Reference Designs

Analog-to-digital converters are virtually everywhere, where there is a need to convert an analog signal to digital, thus helping us to interface with the analog world around us. It acts as an intermediate device to convert the signals from analog to digital form, displaying output on a display such as LCD with the help of a microcontroller. For instance, a DSP system requires an ADC if the input is analog. A telephone modem has an ADC to convert incoming audio to a digital signal which a computer can understand.

The way an ADC works is fairly complex. It periodically samples analog input at discrete intervals in time resulting in a stream of 0s and 1s. The original analog signal can be reconstructed properly provided that the input is sampled above the Nyquist rate, defined as twice the highest frequency of interest. For economy, signals are often sampled at the minimum required rate. For instance, audio signals use 44 kHz, 22 kHz and 11 kHz sampling rates. Using 44 kHz sampling rate means the converter is sampling the analog audio signal at 44000 times per second. Higher sampling frequency leads to lower distortion and better sound quality. Sometimes, a 20-bit ADC can be made to act as a 24-bit ADC if oversampling is done. This means that the signal is sampled at a rate much higher than the Nyquist rate and then filtered, which results in increased accuracy. For an ADC, the “Resolution” states the number of levels into which an analog input range is divided. An n-bit ADC has the resolution of 1 / 2^n. For example, the Resolution of a 16-bit ADC is 1 / 65536, since 2^16 = 65536. This means that an input analog voltage range of 10 V can be resolved into 10 V / 65536 or 0.153 mV precision.

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An ADC can be implemented in many ways. A direct-conversion ADC or flash ADC samples the input using a bank of comparators in parallel, where each comparator generates a code for a different voltage range. This type of ADC calls for an expensive circuit and high power dissipation, and can provide only up to 8 bits of resolution. Another type of ADC is a successive-approximation ADC, which uses a very special counter circuit known as a successive-approximation register (SAR). This register uses a “trial-and-fit” method of decimal-to-binary conversion. It counts by trying all values of bits starting with the most-significant bit and finishing at the least-significant bit. Throughout the count process, the register monitors the comparator’s output (0 or 1) to see if it is less than or greater than the analog signal input, adjusting the bit values accordingly.

A sigma-delta (ΔΣ) ADC is the converter of choice for modern high-resolution precision measurements such as process control, temperature measurements and weighing scales. In practice, a delta-sigma has a comparator which senses a difference (Δ) between the integrator output and zero volts, and an integrator that sums (Σ) the comparator’s output with the analog input signal. To describe this, a sigma-delta converter feeds the analog signal to an integrator’s input, producing a slope at the output or ramping voltage corresponding to input signal’s magnitude. A comparator then compares this output to ground potential (0 volts). The comparator output has one bit, either 1 or 0, depending on whether the integrator output is positive or negative. This 1-bit output is then latched through a D-type flip-flop clocked at a high frequency, and fed back to another input channel on the integrator, to drive the integrator in the direction of a 0 volt output.To get very high sampling rates, designers sometimes use two to four ADCs and combine their outputs to get an interleaved ADC. For instance, 4 ADCs with 50 Msps (Megasamples per second) sampling rate can be combined to produce an interleaved ADC with 200 Msps sampling rate. A time-interleaved ADC uses N parallel ADCs where each ADC samples data every Nth cycle of the clock. This results in increased sample rate by N times compared to what each individual ADC can manage.

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Modern analog-to-digital converters are monolithic CMOS devices offering high speed and accuracy, minimal temperature dependence, excellent accuracy, minimal power consumption and low conversion time. They integrate many devices onto one chip including programmable gain amplifiers (PGAs), driver amplifiers, multiplexers to handle many analog inputs, voltage references, reference buffers, and interfaces such as SPI (Serial peripheral interface).

Let’s walk through some reference designs which discuss implementation of different types of ADCs along with complete documentation.

  • Simple delta sigma ADC: This is a reference design to implement a sigma-delta analog-to-digital converter in a Lattice CPLD or FPGA. This reference design converts an analog input signal into digital by utilizing a combination of internal and external components: analog comparator, low-pass RC network, sampling element, accumulator and digital low-pass-filter. In devices with LVDS I/O support, only an external RC network needs to be implemented externally, reducing parts count and cost. The design eliminates the need for expensive and dedicated ADCs, power-supply monitors and transducers. More on this Reference Design
  • Ultra-high Sampling rate 12-Bit ADC for Radar: This reference design demonstrates how to attain a ultra-high sample rate (2.0 GSPS) and high dynamic range ADC by interleaving four low-power 12-bit, 500 MSPS analog-to-digital converters ISLA112P50 from Intersil. The design finds applications in high-performance data acquisition in radar, electronic/Signal Intelligence and broadband communications. The design is implemented using Intersil’s ADC technology and SP Devices interleaving algorithms. The solution provides a better SNR and SFDR than a stand-alone ADC. More on this Reference Design
  • High-Speed Delta-sigma ADC: This design is an evaluation platform for a 24-bit high-speed, high-precision delta-sigma analog-to-digital converter (ADC) with up to 4 MSPS (mega samples per second) sampling rate. This is a complete PC based solution for the evaluation of the ADC ADS1675 using the ADCPro evaluation software. The design allows evaluation of all aspects of the ADS1675 device. The board offers a program for collecting, recording, and analyzing data from ADC evaluation boards. The software comes with built in analysis tools including scope, FFT and Histogram displays. More on this Reference Design

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