Accelerometer attenuator

[Stoker]

I am new to this area. I am testing MEMS devices on a drop table (shock impact) so I am using a High-g shock accelerometer that has an output voltage bias of +11v. I already have an SC-2345. I am considering purchasing the PCI-6251 for its high sample rate and the ACC01 accelerometor module to plug into the SC-2345. The only problem is that the ACC01 it has an input range of +/-5v.

 If I place an attenuating Op-Amp on the signal before sending it into the ACC01 will it suffice? Will I have problems with the signal, noise etc. Is there a more efficient option? We are only prototyping at the moment so we have a budget, and a NI sales rep suggested the PCI-4461 which is too expensive. Any suggestions are welcome, as we have to use the +11v output accelerometer. Thanks

[Buechsenschuetz]

I don't see any severe problems using a voltage divider (and an opamp, if necessary). If your amplifier has a low-impedance output (as most amplifiers have) and the board you are using has a high-impedance input you can use just a voltage divider of which the total value of resistors should not exceed 10 p.c. of the board's input impedance.

Of course signal quality will degrade somewhat but as you are already in the high-level area I don't think this will affect measurements too much.

Don't forget to wire your ground/reference lines properly. Wrong grounding of signals can generate more noise than any voltage divider or op amp in the signal path.

[Ian_Ren]

You may also consider PCI 4474, which costs $1995 (+ $45 for a SMB to BNC cable), which is cheaper than PCI 4461.

Since your interest is in impact, I assume that you are not interested in the DC component of the acceleration. You can get rid of the bias voltage simply by using AC couple.

Ian

[Henrik_Volkers]

Just in case you go for the voltage divider: If you don't want to loose to much precision look for a good thermally coupling of the (two) resistors (mount them both horizontal on a PCB and choose the same (low) thermo coefficient). Since it's the ratio of these elements that build your divider the absolute values are not that important if you do a calibration. You can buy them already laser trimmed on one ceramic. However I assume you work (do your testing) in a well temperature controlled lab

[Stoker]

Greatly appreciate the feedback. Should have mentioned that due to impact testing we need high sample rates so the most suitable and inexpensive config seems to be the ACC01 Module for the SC-2345, PCI-6251. I will then proceed with a voltage divider to reduce the +11V max IP from the Accelerometer to a +5V max to meet with the ACC01 Input Voltage. It will work out at under 1200 Euro which is well within the budget.

My main worry was with signal interference if I interrupted the raw signal from the accelerometer but from feedback this does not seem to be a problem.

Again appreciate the help, look forward to using forums in the future.

[Buechsenschuetz]

The only thing I can ad is recommending to place the voltage divider as close to the input of the board as possible, and to use the same grounding point as for the original signal. There is a very comprehensive article about grounding schemes on this site. Refer to the manual of the board you are using for grounding options.

[Logan_K]

Let's step back for a second... there is a very important point that has not been brought up yet - is your accelerometer IEPE compatible or does it require a separate charge-amp? IEPE style accelerometers are what most of NI's accelerometer products are designed to work with. Designing external circuitry around your accelerometer will be very difficult if it is IEPE style because you will need to retain the current excitation that is required on the same two lines that the signal is measured off of. I honestly don't think this would be worth your time...

If you could you provide a link to the datasheet of your accelerometer it might make this a little easier?

The SCC-ACC01, by the way, is designed to handle a maximum working voltage of 12V, which is a combination of the common mode voltage + the signal voltage. So, if the maximum voltage from the accelerometer will never exceed 11V, then it is still within range of the SCC-ACC01.

Logan

[Stoker]

I am using Dytran 3200B series shock accelerometers: http://www.dytran.com/products/3200B.pdf . A rep for Dytran gave me the following information after questioning him:

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Thank you for the information. It's difficult to see if the accelerometers will be compatible, although it looks like the powering of 24V and 4mA will be OK. However, the input seems to be differential, and most accelerometers that I have come across are single-ended. The Dytran series are compatible with what is often called 'ICP' powering, which is sometimes referred to as Isotron, Deltatron, etc, and I'm sure if you went to National and asked them if they are compatible with ICP, they will confirm. Many of the other National Instruments input cards I have used have ICP powering as standard, and these work fine, but I am not familiar with the SCC-ACC01,

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[Logan_K]

ICP and IEPE are synoymous, and the SCC-ACC01 is definitely ICP compatible. It looks like the maximum sensitivity that you can have is 2mV/g, so if you go past 500g at this sensitivity then there is a chance that you will exceed the 12V working voltage range, but otherwise I am certain that your accelerometer will work great with the module.

Regards,
Logan

[Ian_Ren]

Logan

Correct me if I am wrong, but I think ACC01 is (or can be) AC coupled. Therefore, the bias voltage will be filtered out, hence the range is at least +/- 5V.

Ian