Selection of Appropriate Acquisition Cards

[kcop]

I am looking for analogue acquisition cards for the following test requirements and could not find any cards from your stock which can meet these requirements. Number of Channels: 64 Acquisition Mode: Differential Minimum Sampling Frequency: 1kHz (no need to be simultaneous sampling) Minimum Voltage Resolution: approx. 2.5µV Maximum input voltage: 16V (no need to read 16V, the important thing is that no damage occurs if all channels are at 16V) The application is testing of automotive switches, whose function it is to make or break a contact which will activate control circuitry. The switch is attached to a specified load which includes both resistive and capacitive components. The load seen by the switch will vary throughout the test, however it will reach a minimum of 5mA. The switch contact resistance will normally be approximately 10mΩ, therefore giving the criteria for the minimum resolution (10mΩ x 5mA = 50µV) from the 50µV typical voltage I calculated that I would need approx 20 steps, therefore giving the 2.5µV requirement. Karl

[Michelle_N]

Hi Karl,

Can you elaborate on what your application is - you mentioned switches but the card you want is for precise analog voltage acquisition. Are you looking at switch debounce? That resolution is quite small...2.5uV.

I found this:

NI 9205
http://sine.ni.com/nips/cds/view/p/lang/en/nid/208800

It has overvoltage protection to +/-30VDC, and you could get 4 with a cDAQ chassis to meet your 64 Diff channel requirement, but the accuracy is not what you want. What have you looked at so far?

[F._Schubert]

Take a look at the 628x high precision DAQ M-Series DAQ cards. I think the have the resolution you need (depends on the range you need to read). I'm not sure on the overvolt requirements, you need to dig in the extended specs (pdf version).

As an alternative, you could use a simple amplifier in front of the card. Have a look on the signal conditioning options. (ok, you will need a multiplexer or relais card then as well)

Felix

[kcop]

Thanks for the recommendations,

I have already tried the USB 6289 and it does have the required resolution, however the overvoltage protection is only on 8 pins. This is not good enough since there will be times when all 16 differential channels of the DAQ will be at +16V

The same issue goes for the NI-9025 here the protection is for 1 pin only.

I have also tried an amplifier circuit using instrumentation amplifiers, however given the high channel count and very small voltages involved calibration of the individual gains is impossible.

Previously I used to use the PCI-6033E, this had overvoltage protection on all pins. Does such a card with an 18-bit resolution exist?

Karl

[F._Schubert]

You could also make your own overvolt protection using zehner diodes.

Felix

[kcop]

I have also tried this, the result is that the zener diode protects the switch from this voltage also.

I then tried putting in a series resistor with the zener and both across the switch, however there then was too much noise introduced with this system.

[Michelle_N]

Hi again,

So, the more I look at your specs it looks like your best bet is finding a card with the highest channel count that includes all other requirments and getting multiple cards. There are just too many tradeoffs between having high channel count, overvoltage protection and high precision for a single card.

[kcop]

Good morning and thanks,

Yes, I agree with your evaluation. However, what cards are there with overvoltage protection on all pins and still has 18-bit resolution?

Karl

[Michelle_N]

What if you built a clamping circuit to handle the overvoltage protection? Then you could use the 6289 with the extra circuitry?

Essentially, I don't think you will find a card that has both the overvoltage protection on all pins (more than 8 pins) and the 18-bit resolution you're looking for.

[Michelle_N]

If you had a much slower sample rate you could get a DMM and a switch.