Simplify the design of analog anti-aliasing filters and other functional circuits

Precision analog-to-digital converters (ADCs) serve as the critical bridge between real-world signals and advanced digital signal processing systems. They are widely utilized in high-end test and measurement equipment across industrial applications. However, connecting a sensor or another signal source to an ADC and achieving the full performance promised by the product’s specifications can be quite challenging. In most cases, additional circuitry such as buffering, voltage protection, or other support functions is required to ensure optimal performance. So, how can this challenge be simplified? Let's explore the innovative solutions offered by modern ADCs. One such example is the LTC2358 8-channel ADC from Analog Devices, which features an integrated picoampere input analog buffer. This buffer not only delivers the high performance of the LTC2348 but also enables flexible, arbitrary input measurements with superior accuracy and ease of use. The LTC2358’s buffer allows for more efficient signal conditioning that is tailored to the specific needs of the input signal. Its purely capacitive pA stage inputs can directly interface with low-current sensors, simplifying the design of anti-aliasing filters and other analog circuits. Let’s take a closer look at how buffer inputs can enhance system performance. Many sensors, even those with slow or low-output characteristics, can be connected directly to the LTC2358 without the need for intermediate signal conditioning. Traditional 8-channel ADCs often require multiple op amps for buffering, but the LTC2358 eliminates these components, saving board space and power consumption. For example, a simple thermistor-based circuit can be directly connected to the ADC, where the output voltage depends on the ratio of the thermistor to a fixed resistor. The ADC reference ensures accurate measurements even if the reference drifts. Using high-resistance thermistors requires very high input impedance, which the LTC2358’s capacitive input handles effortlessly. Adding a 20kΩ resistor can further improve accuracy. Moreover, the LTC2358’s high sampling rate and low noise allow for fast switching in parallel with the thermistor. By briefly disconnecting the switch, the thermistor remains at ambient temperature, minimizing self-heating and improving measurement accuracy. In cases where the ADC input exceeds its range—whether due to overloading, faulty sensors, or wiring issues—the LTC2358 is designed to handle such scenarios gracefully. It does not crash or lose accuracy; instead, it reports a saturated value while maintaining normal operation on other channels. Even when inputs exceed the power supply voltage, the ADC remains safe thanks to internal diode clamps and a 10mA current limit. This robustness makes the LTC2358 ideal for systems that must handle unexpected or extreme conditions. For instance, using a series resistor with the input allows handling voltages up to 40V without affecting other ADC channels. Similarly, the device can tolerate inputs below the negative supply, though accuracy may decrease slightly under such conditions. Beyond basic sensing, the LTC2358’s low input current and wide common-mode range make it suitable for specialized applications like photodiode measurements. Its ultra-low input current—typically less than 10 pA—enables direct connection to photodiodes without introducing significant errors. By measuring the voltage across a resistor instead of the op-amp output, the LTC2358 effectively eliminates the impact of op-amp offset and noise, resulting in more accurate readings. This approach simplifies the overall design and enhances reliability in complex systems. With its combination of high performance, built-in buffering, and robust overdrive protection, the LTC2358 stands out as a powerful solution for multichannel systems requiring precision, flexibility, and reliability. Whether you're working with low-current sensors, high-voltage inputs, or demanding environmental conditions, the LTC2358 offers a streamlined, effective way to connect and process signals.

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