Changing offset of analog Output channel of 4461

[Alok Damle]

Hi All,

I have this slight issue. I am using 4461 card. I would want to operate the card between +/-1 V range by setting 20db as the gain. however I actually want the voltage to vary between 6V to 8V with 24 bit resolution.

As I understand I can acheve this by having the output coupled with 7 V. I have read that the AO of 4461 does provide option to define DC coupling mode. 

I want to know how to specify this value of 7 volts as output offset for the AO channel using DAQmx in DOT NET. or is it that i need to have this voltage applied using external voltage source?

Look forward to your response,

Alok

[Neal M]

Hello Alok,

The DC coupling specification just means that you can have a DC offset in the signal.  This does not mean that it applies one without expanding the voltage range.  In order to do what you are talking about, your range would have to contain the full amplitude of the signal. You would then add your offset to your generation values directly. You could also do this with external circuitry.

It appears that the reason you are looking for this is because you want to output a very accurate signal.  When it comes to absolute accuracy, our DSA devices are not as good as many of our Multifunction DAQ devices, even though it has a higher resolution DAC.  This is because DSA (in general) is more focused on linearity, alias rejection, and dynamic range than accuracy. Please take a look at the following KnowledgeBase on this:

Absolute Accuracy of Dynamic Signal Acquisition Devices

If you want to provide this offset like you talk about in your post, you could use one of our 628x devices.  This would allow you to provide an offset voltage on a APFI line that is added to your signal. 

[Alok Damle]

Hi Neal,

Thanks for your reply. Well this is a discovery for me. It seems your sales engineer are not fully aware of this fact.

I wanted a card with analog output of 18 bit or higher resolution and equivelant accuracy. We did come across 628x. however since the outputs are only 16 bit resolution we rejected that. and the sales engineer stressed me on purchasing 4461 for the purpose. The NI sales engineer told me that the accuracy of this card is in few nano volts and that it can sense the change of voltage within few nanovolts accuratly. also it can generate signal with similar accuracy and resolution.

I also told them my requirement of being able to generate signal between 6 to 8 v with minimum 18 to 20 bit accuracy. That time the engineer replied that it can be done as it provides DC coupling and hence you can generate the signal in the said range with this card (I made him aware that i am not from electronic background and that by DC coupling does he mean that i can set the offset voltage internally? to which he replied yes).

For last two days i was trying to figure out the reasons of huge noise (+/- few mV) that i was sensing at the input and i was doing all possible checks with my other circuits. I never knew that this was due to the worse accuracy of the cards that i bought.

Now please put urself in my place and advice me what to do? I am finding myself helpless after burning about 9000 USD for those cards.

Alok.

[preston_johnson]

Thanks for your reply,

Can you tell us more about the application?  Are you performing some form of audio test?

You can improve the input accuracy by oversampling and averaging.  In other words if your signal is a low frequency signal say 10Hz, you can sample at 1kHz and average every 100 points to produce the 10Hz measurement.  This can be with an RMS average or a Median.  I have seen several customers use this method to get better results than the NI 16bit boards.  If your signal is 1kHz or higher, use the AC coupling feature which will remove the DC offset in your signal and allow you to make accurate dynamic measurements. 

We would like to hear more about the application.  In this way we can best advise you of the right tools. 

[Neal M]

Hi Alok,

For this situation, I think it would be best if we discussed this directly and take it offline. There are a couple of ways we can get into contact:

1. If you are ok with posting an email address on this forum I can contact you through that.
2. You can give our web support team permission to give me the contact information in your profile (by posting here that it is ok) and I can contact you using that information.

Please let me know which would be best for you.

[Alok Damle]

Hi

I was about to call Preston. My contact Info is as follows:

email: alok_damle@yahoo.com

Phone +91 98220 56543

Regards,

Alok Damle

PS: I will delete this post after we get in touch. This is just to make sure internet crawlers dont sniff this address and add me in their list of recipients of Spams!

Message Edited by Alok Damle on 04-08-2008 01:28 PM

Message Edited by Alok Damle on 04-08-2008 01:29 PM