This step change represents how much the input signal has changed in that time. At the end of the conversion time, the capacitive DAC, which holds the previous sample charge, is switched back to the input. The conversion time, t CONV (Figure 2), is when the capacitive DAC is disconnected from the input and is performing bit trials to generate the digital code. To calculate the size of the step, we need to know the input signal frequency, amplitude, and ADC conversion time. The minimum RC value needed to settle the ADC input while also optimally band limiting the noise can be calculated assuming exponential settling of a step input. Too much band limiting will increase the settling time and distort the input signal, however. The RC network limits the bandwidth of the input signal and reduces the amount of noise fed to the ADC by the amplifier and upstream circuitry. Its 10-MHz input bandwidth means that low-noise is needed over a wide bandwidth to get a good signal-to-noise ratio (SNR). The ADC input presents a switched-cap load to the driving circuitry. To select a suitable RC filter, we must calculate the RC bandwidth for single-channel or multiplexed applications, then select values for R and C.ДЇigure 1 shows a typical amplifier, single-pole RC filter, and ADC. Major considerations include input frequency, throughput, and input multiplexing. Looking at the various application factors that influence amplifier and RC choice, we provide design guidelines that lead to the best solution. The RC filter limits the amount of out-of-band noise arriving at the ADC input and helps to attenuate the kick from the switched capacitors in the ADCвЂ™s input.Д¬hoosing the right amplifier and RC filter for a SAR ADC can be a challenge, especially when the application needs to differ from the routine data sheet usage of the ADC. The amplifier conditions the input signal-as well as acting as a low-impedance buffer between the signal source and the ADC input. The front end consists of two parts: the driving amplifier and the RC filter. Useful information on the other areas, which are device- and system-specific, can be found in data sheets-and in this articleвЂ™s references. This article focuses on the circuit requirements and trade-offs in designing the front end. The three principal areas to consider are the front end, which interfaces the analog input signal to the ADC, the voltage reference, and the digital interface. Once a particular precision SAR ADC has been chosen, system designers must determine the support circuitry needed to obtain the best results. Successive-approximation (SAR) ADCs offer high resolution, excellent accuracy, and low power consumption. Front-End Amplifier and RC Filter Design for a Precision SAR Analog-to-Digital Converter

Rc filter designer