my ic is giving constant 330 milivolt signal it does not go below

[hatti1989]

hi together,

i have a problem. i also have connected an ic (AD594). and thermo couple and i am measuring voltage signal through NI USB-6009. i am getting good result for temprature greature than 33 degree centi. but when temprature goes below 33 . my ic is giving constant 330 milivolt signal it does not go below. means am able to measure only minimum temp 33 degree. my room temp is 20 degree . How can I check temp under 33 degrees?

Thanks for help

[johnsold]

Are you using a single power supply or a split supply configuration? What load do you have connected to the output? 

The USB-6009 analog inputs are equivalent to 144000 ohms connected to 1.4 V. That would source 10 uA at ground or about 7.4 uA at 330 mV. Footnote 4 of the AD594 datasheet indicates that the current sink capability in single supply mode is limited to the current to ground through a 50000 ohm resistor. At 330 mV that current is 6.6 uA. 

I think the fine print and the weird input circuit fo the USB-6009 got you. Does the AD594 work OK if you measure the output with a digital multimeter with the USB-6009 disconnected?  If so, you may need a buffer amplifier with a rail to rail input and output capability.

Lynn

[hatti1989]

So the IC is powered by a separate power supply.

Here are pictures of the wiring of the AD594 and the LabView programming

Yes the AD594 worked before with a measuring card "CIO - DAS08" by the company MCC and the output with the digital multimeter is ok too.

[johnsold]

I cannot tell anything from your photographs except that you do nice, neat wiring.

The fact that it works with the DMM and a different DAQ card makes me think that my discussion of the USB-6009 input circuit and the limited drive of the AD594 at low outputs is probably the problem.

You will need some kind of buffer amplifier which can drive the USB-6009 input or you will need to use a different DAQ device.

Lynn

[Suse]

Hi,

is there anything I can do for you or could you solve your problem?

Best regards

Susi

[johnsold]

Susi,

<rant mode = ON>

If you have any influence with Marketing, try to get themto put a label on the USB-6008/9 devices which says in large bold print:

"Warning! The screwy input circuit in this device is only happy if you drive it with an ideal voltage source! It should not be used by inexperienced persons!"

Of course the experienced persons will know the input circuit is screwy and won't use it anyway.

<rant mode = OFF>

Seriously, that input circuit causes a lot of the target market of the USB-6008/9 devices headaches. Because 1. They bought on price without reading the manual. 2. Even if they read the manual they probably do not understand the implications of Figure 7 in the User Guide. 3. The manual makes no mention of the implications of the low input impedance or the internal source connection. The last paragraph on page 18 of the User Guide is flat out wrong if the signal is connected through a source impedance greater than a few hundred ohms.

Lynn

[Suse]

see this:

Explanation of the Analog Input Circuitry on the 6008/6009

http://digital.ni.com/public.nsf/allkb/C7E181E51E4299FC862570A700604141?OpenDocument

How Do I Configure My NI Device to Be Open-Drain (Open Collector) or Push-Pull (Active Drive)?

http://digital.ni.com/public.nsf/allkb/D9B4018C3E540CE4862570F30068C0DB?OpenDocument

[hatti1989 · Accepted solution]

HERE IS THE ANSWER.... 

http://forums.ni.com/t5/Multifunction-DAQ/How-to-read-thermocouple-output-with-NI-USB-6009/td-p/9902...

Thank you guys