Abstract

The input of a traditional successive approximation register (SAR) analog-to-digital converter (ADC) has a switched capacitor sample and hold circuit. The process of sampling creates large transient currents (milliamps) that need to settle in a short time (nanoseconds). One of the fundamental challenges for traditional SAR applications is finding an amplifier that is capable of providing that fast-transient current to sample and hold circuit. Generally, the bandwidth of the amplifier needs to be far greater than the sampling rate requirements. The reference input for the traditional SAR ADC has a similar switched capacitor circuit, so frequently a wide bandwidth reference buffer is required. Furthermore, because the traditional SAR ADC input requires a wide bandwidth input amplifier, the output filter of the SAR ADC needs to be adjusted to a specific cutoff to eliminate transients. The selection of the filter components is a significant design challenge and the resulting filter bandwidth is generally not optimal for antialiasing and noise reduction.

The new ADS9218 SAR ADC uses an internal, wide-bandwidth, linear amplifier to drive the ADC input and reference input. The input impedance of the amplifier is in the giga-ohms, so the bandwidth requirements for the application are only limited by the required bandwidth of the signal to be converted. For example, using a traditional SAR ADC, a 1-MSPS SAR ADC typically needs an amplifier with at least 20-MHz bandwidth to respond to the input transients. Using the ADS9218, the amplifier bandwidth is only set by the signal chain frequency and distortion requirements, and does not need to respond to ADC transients because the transients are handled internally. This simplifies the design requirements and allows for many more amplifier options. Furthermore, the amplifier can be followed by a simple RC antialiasing filter, which can be tuned to the required cutoff frequency without impacting settling performance. Finally, the reference input of ADS9218 does not require any special reference buffer to achieve good reference settling as the ADC reference input has an integrated buffer.