NRSE & RSE for a Grounded Signal source

[vki]

Dear members

For NRSE  type AI connection for Grounded signal source ,  where AI GND of DAQ 6229 is be connected?. Say if a requirement is to measure power supply o/p voltage , supply + would go to AI CH input, supply negative is taken to  AI Sense then where AI GND of DAQ 6229 ( M series) to be taken. Can AI Sense & AI GND can be tied together. 

Can you also pl. explain whether AI sense has any role to play in GRSE type AI connection. I have faced problem earlier that DAQ unable to read unless it is connected to AI GND. If I use a GRSE type AI connection for a grounded signal source ( inputs > 2V), Is it correct to connect the DAQ AI GND ( AI GND of all channels tied together) & connected to analog source ground.

 Can M DAQ 6229  be used only as one AI type of differential / NRSE / RSE or can be used as

differential as well as NRSE / RSE for different types of inputs.

Thanks Vinod

[Abhinav T]

Hi Vinod,

I would highly recommend you read the Field Wiring and Noise Considerations article here: http://zone.ni.com/devzone/cda/tut/p/id/3344

Table 1 shows you what connections are neccesary for NRSE and RSE type measurements.

In RSE mode you will connect your positive signal to any of the AI X channels and the negative to AI GND - this will ground the negative signal
IN NRSE mode you will connect your positive signal to AI X just like before and the negative signal to AI SENSE - the ADC will calculate the difference between AIX and AI SENSE in this case and give you a voltage.

Connecting AI SENSE to AI GND should be done only using bias resistors (as shown in Table 1 earlier), however this is only required when you have a floating voltage source.

Please let me know if this helps answer your questioins.

[vki]

Hi Abhinav

Thanks for reply. I have further query to ask given hereunder.

1. For NRSE mode , it is clear that -ve of measured point has to go to AI Sense , what I would like

to know where do I connect the AI GND of DAQ , can this be left open or else connected to AC

earth. If I am just measuring o/p voltage of a power supply ( Grounded source ) , there are 2

measument points +ve which goes to channel i/p ( AI i/p) , -ve to AI sense , where to connect

the AI Ground of DAQ. 

2. For a grounded type source , if I use RSE scheme  then is there any need for connecting

AI sense to AI GND ?. I have gone through the ni application note on field wiring & ground scheme,

it says if yr measument voltages are above 1 V , then RSE type scheme can be used for

grounded system.

Thanks Vinod

[Abhinav T]

Vinod,

1) AI GND is automatically connected to the board's ground - you do not need to connect anything to it for NRSE unless you have a floating signal - then you will have to connect bias resistors as shown in the bottom left diagram below:



2) Connecting a grounded source to RSE is not recommended - it can lead to ground loops and potenitally damaging your hardware - please use NRSE or Differential for grounded sources instead.

Message Edited by Abhinav T on 08-24-2007 06:37 AM

[vki]

Hello Abhinav

Thanks for yr reply.

I have few queries on differential mode measurement & use of scaling resistors.

1. What is the allowed o/p source impedance of UUT that would not affect the measurement in differential mode , I am going to use NI USB M6229 DAQ.Ideally source impedance of measured point ( e.g power supply o/p) should be zero .

Say I have to measure 24 V in a grounded source & I want to use differential mode. I would require scaler to

 lower the voltage . If I use a scaler combination 100K / 33K ( 1/4 scaling ) to lower the voltage to 6v , now I

am measuring voltage across 33K , one end to ai + , & other end of 33K ( which is also PSU -ve) to ai- ,

Whether this can be measured correctly by DAQ.

  2. When I use a scaler as mentioned in above , the source impedance for ai+  looking from DAQ side is 100K||33K

but for other ai- it is none as connected straight to -ve of PSU ; Can this create any problem in measurement

imbalance from point of view of differential measurement.

3. In the above case do I need to connect any series resistor between one measurement point ( 33K one end)

 and ai+ point for protection. Similarly other end of 33K ( which is -ve of power supply ) to ai - can be connected

straight away or series resistor  is required.

( I am attaching the schematic)

Thanks  Vinod

[Abhinav T]

Hi Vinod,

You should be able use the voltage divider you have designed above. The input impedence of the DAQ card according to the 6229 specifications is 1G Ohms. This will have a small effect on the voltage divider but it will not be noticable on your DAQ card. The one thing that I would caution you agianst is trying to measure an AC signal using this setup. You are liable to see reflections using this setup and this can possibly change the measured voltage.

[vki]

Hello Abhinav

Thanks for yr reply.

I have a query regarding value of resistors to be chosen for differential measurement involving a floating source , between AI ( analog input channel) and AIGND at DAQ

. Application note recommends value between 10K to 100K , some members in forum have said

that upto 1M can be used. At some place in an NI article it says 100 times the source impedance.

Can you please clarify whether I can use a value above 100K. I have now selected 33K || 10 K scaler ,

I have tried using 100K between AI to AIGND but it slightly changes the actual voltage about 5% to 8%.

Thanks Vinod

[hoffiz]

Hi vki

Something that might be changing the voltage slightly is the precision of the resistors.  But I will like to ask you are you still doing the “voltage divider” that you have attached. The voltage that you are going to be measuring is the drop across the 33K resistor so this resistor, I would recommend to select a 1% precision resistor.  But as far as the connection it seems that you have everything connected correctly.

[vki]

Hello Jaime

I would like to state

1. Error in voltage is not linked to precision of resistors , when I remove the connections from DAQ

inputs & measure with multimeter it gives me correct voltage measurement , however when I

connect these to DAQ inputs in a differential config. ( 100 K resistors connected from AI to AIGND ) voltage changes giving roughly 4-5 % error.

2. NI application note recommends that resistor between differential input to AIGND should be

10K to 100K or else 100 times the input impedance. For this measurement I changed the

resistor scaler from 100K || 33K to 10K || 3K which gave me better results & error reduced.

3. Other then above , I have a requirement to generate a sine wave to feed a RC network & measure

voltage ; I am currently using Labview 7.1 , can you recommend a vi or some application note

which can help me in this.

Thanks

Vinod

[Matthew_W]

 Hi Vinod,

 I'd like to jump in here and offer a few comments.

 1. Looking at Table 1 in the Field Wiring and Noise Considerations for Analog Signals document, the Differential input to the DAQ device has two possible connection scenarios:
      a. Floating Signal Source
      b. Grounded Signal Source

     In scenario a. Floating Signal Source, there is a 10-100k ohm pulldown to AI GND for AI+ and AI-
     In scenario b. Grounded signal source, there is no pulldown.

    From reading the earlier posts it sounds like your signal source is a power supply. This is typically a grounded signal source, in which case the pulldowns are not necessary. They maybe
    loading the power supply a little bit causing the voltage drop.

2. For the requirement of generating a sine wave I would immediately look to the NI Example Finder.
     a. Run Labview
     b. Click Help
     c. Click NI Example Finder
     d. Browse according to Type, Hardware Input&Output -> DAQmx -> Analog Generation -> Voltage - > Cont. Gen Volt. Wfm - Int Clk.VI

   This example can generate a sine/triangle/square waveform, load it into your DAQ card buffer and generate the waveform to a analog output channel.

Hope this helps, please do post back with specific questions if still experiencing problems.

Best regards,

 MatthewW
 Applications Engineer
 National Instruments