One ADC Per Channel (It is Simpler)

OpenDAQ specifies that each channel in an OpenDAQ system will utilize a distinct analog to digital converter (ADC). There are several benefits that are realized with this approach, and actually simplify OpenDAQ designs.

In the early days of data acquisition, the ADC was one of, if not the, most expensive components in a DAQ product. In addition to the cost, the ADC was also one of the components that consumed the greatest amount of power. Due to these limitations, a multi-channel DAQ system almost always utilized a single ADC and some switching mechanism in order to allow more than one analog channel to be digitized.

Switching mechanisms have inherent flaws.

There are limitations on how quickly any switching system can change from one channel to the next. Mechanical technology, such as reed relays, have the least impact on the overall quality of the switched signal. But they are also the slowest, requiring several milliseconds to engage and then disengage a specific analog input channel.

Solid state switches are much faster than reed relays. However, these switches have an annoying side-effect called charge-injection, which momentarily change the input signal. This effect is not easy to compensate for, and can have a large impact on input signals with a high impedance, or signals with which we are trying to measure with high accuracy.

In a switched system, each channel must be digitized serially. This means that each signal will be digitized at a different time. In some applications it is necessary to digitize each signal at the same time. This can be achieved in a switched system by adding sample and hold amplifiers to each channel. These amplifiers allow the capture of an analog signal at a specific moment in time. But these amplifiers add cost, and will begin to lose the captured signal the longer the signal is being held.

 

In addition to the problems directly attributed to signal switching mechanisms, indirect problems are also present in a switching DAQ system.

 

Any circuitry beyond the switching circuitry in a switching DAQ system must be able to swing from the maximum measured voltage to the minimum measured voltage (or vice versa), at a rate that is typically several times more quickly than the maximum acquisition rate of the digitizing ADC.

For example, a DAQ system that can digitize at a rate of 100 KHZ may need the common circuitry after the switching system to swing and settle at a rate of 1 MHZ in order to accurately measure signals from different channels that are at the extreme ends of the measurable voltage range. Even though such a system is limited to a maximum measurable signal frequency of 50 KHZ, the common circuitry must be 20 times faster.

Anti-aliasing filters are necessary in any sampled DAQ system. In order for filters to work correctly, these filters must be present in front of the switching mechanism needed for a switching DAQ system. So these filters have to be replicated on all channels. This increases cost, power consumption, and consumes circuit board space.

 

Both the cost and the power consumption of analog to digital converters has come down considerably. In addition, ADC chips have gotten smaller, and use fewer pins as they use serial interfaces. These benefits can now effectively mitigate the drawbacks associated with switched DAQ systems.

 

One final benefit with using individual ADCs is that doing so makes it possible to sample different analog channels at different rates. This capability is useful in minimizing the amount of data collected when sampling signals with different rates of change.

For example, it makes little sense to measure a thermocouple at 100 KHZ when a thermocouple signal changes at much lower rates. When measuring signals from phenomena with varying rates of change, the size of the acquired data can be greatly reduced.

The area of dynamic signal analysis can also greatly benefit from having varying channels measure at different rates. This allows greater resolution at fundamental frequencies, as well as harmonic frequencies, and allows greater resolution at different fundamental frequencies found in gearbox systems, for example.

 

Overall throughput on systems using individual ADCs will depend on the ability for a micro-controller to move the data, typically through a serial interface, between each ADC chip and the micro-controller. Subsequent correction (offset and gain correction) as well as post-processing (calculating RMS AC voltage, performing FFT analysis, etc.) will also contribute to the speed at which a single micro-controller can mange multiple analog channels.

Therefore, there will be a practical limit in any Open DAQ system as to how many channels a single micro-controller can manage. Since OpenDAQ allows, and actually encourages the use of multiple micro-controllers to divide up the work, OpenDAQ is inherently more flexible in creating systems with few or many data acquisition channels in a predictable and modular fashion.

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