In part 3 of this series we started looking at the multiplexer. Let's continue by considering whether the multiplexer should be integrated with the ADC.
Integrated and Discrete Multiplexed Solutions
There are two types of solutions available for multiplexed applications in the market today depending on the customer’s need -- integrated vs. discrete multiplexed solutions. The benefit of a discrete multiplexed solution appears in terms of flexibility in the selection of appropriate signal conditioning components based on their performance requirements. The users still have to worry about complex design issues related to the channel switching, sequencing and settling time.
One could also argue that users have the flexibility when they can switch the multiplexer input channels and do the external calibration to calibrate out errors, but it will likely increase their boards' sizes and costs at the expense of performance and flexibility. Some customers also prefer to do their own custom digital filtering on the FPGAs rather than use those which are available on-chip for their flexibly.
If the customers use integrated multiplexed based solutions, they don’t need to worry about channel switching, sequencing, and settling time issues. In addition, this approach could offer per-channel configurations, with different input ranges and error calibration options. In this case, the customers have less flexibility on the signal conditioning, but this approach could simplify their designs and save them area and bill-of-materials costs, while offering adequate performance.
Some of the highly integrated SAR and Δ-Σ ADCs available today alleviate many of the challenges associated with designing a precision data-acquisition system. These ICs eliminate the necessity to buffer, level shift, amplify, attenuate, or otherwise condition the input signal. Also eliminated: the concerns regarding common-mode rejection, noise, channel switching, sequencing, and settling time.
Figure 1 shows the AD7682/7689 series of devices: 4- or 8-channel multiplexer + SAR ADC DAS. It's in a very small package and low power, so well suited for battery applications.
An integrated multiplexed SAR ADC DAS.
Figure 2 shows the ADAS3022. It's similar: 8-channel multiplexer, programmable gain amplifier, and SAR ADC DAS.
Another integrated multiplexed SAR ADC DAS.
Figure 3 shows the AD7176-2. It is a highly integrated 24-bit precision Δ-Σ ADC DAS. It offers per-channel configuration with offset and gain calibration. It rejects 50/60Hz line frequency. It has high impedance inputs, so it can directly connect to the high impedance sensors used in process control and other multichannel data acquisition systems.
An integrated multiplexed Δ-Σ ADC DAS.
Figure 4 shows the AD7190. It's one of the industry’s lowest noise 24-bit Δ-Σ ADCs. Again, it has an integrated PGA with high precision and low noise at output data rates from 4.7Hz to 4.8kHz.
Another integrated multiplexed Δ-Σ ADC DAS.
In the next (and last) part of this series, we'll look at the performance you can expect to get out of a DAS system like the ones we've been looking at.