How To Choose A DAQ Card That Wont Let You Down

Nothing is more disheartening to spend weeks building and debugging a program only to discover the hardware won't give you what you want.

Even worst is if you are trusting data that has a flaw in it due to issues in the acquisition. Many of these problems won't throw errors - but they will affect data accuracy.

This article highlights some of the key features of data acquisition (DAQ) cards when designing systems for our customers. This should help you understand how to choose a DAQ card for you.

Speed and Accuracy

OK, you probably thought of this one on your own! But it needs to be right.

Speed tends to be easy. What is the highest frequency you want to capture?  Now multiply by 10, and you have a good DAQ speed. In certain conditions, you can get away with less, but this is the safe bet.

Accuracy is also a headline figure. There are a few elements to this though.

• What is the smallest change you want to detect? This defines the resolution. We find NI'S 16 and 24-bit cards work for the majority of cases, but some cheaper hardware may drop to 10/12 bit which tends to show a lot more in measurements.
• What contribution does noise make? If the resolution is 1mV, but there is 5mV of noise, then you can't use the whole resolution of the card.
• How close to the real value must it be? This is the error of the card, generally given as a percentage. You might find a single value, or it might be split into gain and offset error.
• Can the system be calibrated? DAQ system accuracy drifts over time and with temperature. Does the accuracy specified require manufacturers calibration? Can you adjust anything in software or hardware?

You can find much more detail on what goes into a DAQ cards accuracy here.

Simultaneous Sampling

Some cards will use a single ADC and multiplex between input channels to reduce the price per unit. In many cases, this is perfectly acceptable, but there are a couple of gotchas to be aware of:

1. Phase Error - This means that there is some phase difference between channels. If you are looking to align data between channels at higher speeds, this error can become significant.
2. Ghosting - This is an unusual issue that can have you scratching your head. The multiplexed path can act as a capacitor. This means if it switches between open or high impedance channels you can see the readings from one channel onto the next.

You can solve both of these by using a simultaneous sampling card. A simultaneous sampling card has an ADC per channel meaning there is no multiplexer to cause issues. It does raise the system cost.

If you have ghosting and don't want to pay for a simultaneous sampling card, there is something else you can do. If it is because you have open channels that are being scanned by the DAQ system, you can terminate them. The termination discharges the multiplexer and corrects the readings for the following channels. There are also other options outlined at How Do I Eliminate Ghosting From My Measurements.

Synchronisation

If you measure from multiple channels and want to see the data aligned, then you need to use the right DAQ setup, especially at higher speeds.

 There are two cases to consider:

• If you are using a single DAQ card then the synchronisation can be affected if it is a multiplexed card. See the simultaneous sampling section above as this is the solution.
• If you require multiple cards or multiple functions in a card you need to make sure there is proper synchronisation between them. Synchronisation requires a start trigger, to ensure they begin at the same time, and also a shared clock, to make sure they don't drift apart over time. Without these, the data will be misaligned leading to faulty judgements based on the timing in it.

Quality of Drivers

It is no secret that we are proponents of using NI hardware. We know it inside and out, and after ten years it is extremely rare that it has let us down. A big part of that is the quality and support of the driver.

We commonly hear tales from developers of how they saved £200 on hardware but they ended up doing £800 of extra work to get the driver working.

If you do look at new providers then the things to check for are:

1. Is the driver easy to find, install and use?
2. Is there good support if you run into issues?
3. Does it use a standard protocol? If the communications are Modbus, for example, this significantly reduce the risk as you don't need the vendor's drivers.

Don't get me wrong, we have and will work with other DAQ hardware in the future, but answers to these questions may represent it as a higher risk option.

Control/Stimulus Response

Vendors build most DAQ cards for moving large quantities of data efficiently, often at the cost of latency (the time that it takes to request a change in software and see it in the real world). This latency is a limitation of the drivers and more often the operating system.

If you need to make fast decisions and change outputs, you have two options:

• Some NI hardware can be used in a lower latency hardware-timed single point mode in Real-Time to improve the situation.
• Use an FPGA based card. It is more complicated to program (we can help!) But the response times can now be as fast as the raw IO can support in many cases.

Summary

This article provides a summary of these key concepts. We hope to fill out some more detailed articles on these topics over time. Let us know which are the most interesting to you.

If you want to see what else you need to consider in a DAQ system check out our guide to the measurement chain.

In the meantime, if you have questions or need help with your system. Don't hestitate to contact us.