Get ADC data beyond the datasheet
When selecting a high-resolution ADC for a design, you often need to know about characteristics that may not be published in the data sheet. Take, for example, transition noise and SNR. You may not find these specifications in a data sheet. Fortunately, we designers now have a tool that can analyze ADC for these and other parameters.
ATE system manufacturer LTX-Credence has developed the "Signature Analysis" toolset that can analyze converter products such as the AD7960 class, designed for high-end instrumentation and ATE. The toolset is designed for systems that require careful analysis of the transfer function or direct measurements of the output based on stimulus inputs. The toolet let us characterize the AD7960's noise performance over all codes rather than the typical shorted inputs or at several other distinct converter levels.
When selecting an ADC, you may have to consider the overall ADC efficiency, power, size, and price. In addition, pay close attention to the static and dynamic performance over the Nyquist bandwidth. This article introduces a toolset to help see outside the datasheet which, after all, will help lead to a decision to select a precision ADC for that new system design to be rolled out. We will now demonstrate the performance of the Analog Devices AD7960, an 18-bit PulSAR ADC, using the Signature Analysis Toolset.
The AD7960, and 18-bit, 5-Msample/s differential ADC, shown in Figure 1, uses CAPDAC (capacitive digital-to-analog converter) technology to reduce noise and improve linearity without adding latency or pipeline delay. The AD7960 returns to the acquisition mode about 100ns after the start of conversion and its acquisition time is approximately 50% of the total cycle time. So it has nearly the same acquisition time despite being twice as fast as the next fastest 18 bit SAR ADC. This makes the AD7960 easy to drive and relaxes the burden on the ADC driver’s settling time requirement. It offers the wide bandwidth, high accuracy (INL: ±0.8LSB, SNR: 99dB and THD: -117dB typical), and fast sampling (200ns) required for high end data-acquisition systems while reducing power dissipation and cost in multichannel applications.
Figure 1. AD7960 Functional Block Diagram shows the CAPDAC as part of the SAR (successive approximation register) loop.
The AD7960 series digital interface uses LVDS (low-voltage differential signaling), offering Self-clocked and Echoed-clock modes that provide high-speed data transfer up to 300MHz (CLK± and D±) between the ADC and the digital host. The LVDS interface reduces the number of digital signals and eases signal routing, because several devices can share a common clock. This also reduces power dissipation, which is especially useful in multiplexed applications.
The self-clocked mode simplifies the interface with the host processor, allowing complex timing with a header that synchronizes the data from each conversion. This mode is especially beneficial when using many ADCs per system and where board space, power dissipation and, layout routing constraints are present. A header is required to allow the digital host to acquire the data output because there is no clock output synchronous to the data. The echoed clock mode is useful when using a few ADCs per system without any board space or power consumption constraints. This mode offers robust timing at the expense of an extra differential pair (DCO±).
The AD7960 operates from 1.8-V and 5-V power supplies, dissipating only 39mW at 5Msamples/s when converting in self-clocked mode and 46.5mW at 5Msamples/s when converting in echoed-clock mode. Its power dissipation scales linearly with sample rate, as shown in Figure 2, making it a good fit for low-power applications. The power dissipation at very slow sample rates is dominated by the LVDS static power.
Figure 2. The AD7960's power consumption vs. throughput rate is linear.
The AD7960 series lets you use any of three external reference options: 2.048V, 4.096V, and 5V. An on-chip buffer doubles the 2.048-V reference voltage, so the conversions are referred to 4.096V or 5V.