Successive-approximation ADC delivers 20 bits at 1 Msps

May 28, 2013 // By Graham Prophet
The LTC2378-20 is a true 20-bit precision, 1Msample/sec, no latency, successive approximation register (SAR) analogue-to-digital converter with exceptionally low 0.5ppm (typ) and 2ppm (max) Integral Non-Linearity (INL) error.

Linear Technology says that the best-in-class 0.5ppm INL of the LTC2378-20 enables a new generation of precision systems with true 20-bit accuracy, particularly precision applications such as seismic monitoring and semiconductor fabrication. The architecture leads to an extremely stable 2ppm (max) INL and -114dB THD (max) over the device’s entire operating range of -40°C to 85°C.

INL is a key specification in precision applications, such as seismic monitoring and semiconductor fabrication, indicating how much the ADC transfer function deviates from its ideal. Since linearity effectively cannot be calibrated at the system level, the ADC’s INL specification often sets the overall system accuracy.

The LTC2378-20 achieves the industry’s highest signal-to-noise ratio (SNR) of 104dB for a 1Msps no latency ADC. The wide dynamic range and true 20-bit resolution reduce or eliminate the need for additional gain in the signal chain, thereby improving total system accuracy and noise. Fast 1Msps throughput with no latency enables the use of multiplexers to replace costly, independent ADCs with slower throughputs, thus reducing system cost and complexity.

The LTC2378-20 heads a pin- and software-compatible family of 20/18/16-bit SAR ADCs with speeds ranging from 250 ksps up to 2 Msps, with serial SPI interfaces. The 20-bit 250ksps (LTC2376-20) and 500ksps (LTC2377-20) versions will be released in May 2013. The family operates from a 2.5V supply, with a ±5V fully differential input range using an external 5V reference. At 1 Msps, the device achieves low power dissipation of 21 mW, which scales linearly with sample rate. A shutdown mode further reduces power dissipation to 2.5 μW when idle. The LTC2378-20 family offers a digital gain compression (DGC) feature, eliminating the need for a negative ADC driver supply and preserving the full resolution of the ADC, to achieve lower total power consumption of the signal chain with only a small reduction in SNR performance.

The DC1925A evaluation kit demonstrates the 0.5ppm INL and 104dB SNR performance of the LTC2378-20.