INA223: 0.1Hz to 10Hz Voltage Noise Measurement

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Replies: 15

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Part Number: INA223

Hi Sir

 

 

For correlating figure 28 “0.1Hz to 10Hz Voltage Noise”

We builds up active filter as TI verified design mentioned in document #SLAU522

And take OPA827 as DUT, briefly connecting and wiring without shielding can.

 

The resulting waveform is approximate and satisfied as the attachment (PK-Pk = 24.2mV) but not INA223 while using as DUT instead

 

OPA827

 


 

 

 

According to figure 28 of INA223, the estimation should be

 


(1) Gain = 20

VOUTPK-PK = 200nV/ Div x 6 Div x 20 (Gain) X 100 (Filter Gain) = 2.4mV

 

(2) Gain = 300

VOUTPK-PK = 200nV/ Div x 1 Div x 300 (Gain) X 100 (Filter Gain) = 6mV

 

The resulting is 10 times more than expectation; we have no idea what dominate the measurement, and how to identify

 

We simply set VCM about 1V and VIN+ - VIN- is about 5mV via voltage divider (200 ohm and 1 ohm), tie VIN – to GND.


Thanks for recommendation and measurement guide!

Regards

Ben

 


15 Replies

  • In reply to Ben Huang1:

    Hi Ben,

    I would connect the enclosure to the signal ground near the DUT.

    To shield cell phone radiation the shield must not have even the smallest gap or break!

    One way to improve the shielding effectiveness is to use several shields following the onion-shell principle. So, you could have your big metal enclosure AND an additional shield close to the DUT itself. The inner shield should be connected to signal ground as close as possible to the DUT.

    A good noise measurement can be really tricky...

    Kai
  • In reply to kai klaas69:

    Hi Kai


    Thanks for recommendation!

    Would you make sure the shielding effective before making noise measurement?
    (I simply check the connection of cellular phone, is it necessary to confirm the wireless connection failed? how about yours?)

    Regards
    Ben
  • In reply to Ben Huang1:

    Hi Ben,

    the best indication that the shield is working as intended is when the noise level goes down. :-)

    During the noise measurement I would just forbid the use of any cellphones, ipads, etc. near the setup. Also, turn-off any device which is not used for the measurement. Turn-off all fluorescent lamps. These are very noisy! Do also pull out all irrelevant mains plugs. Keep all mains transformers well away from the setup.

    Kai
  • In reply to kai klaas69:

    Hi Kai


    Thanks for recommendation!
    Noise is everywhere; Other than Lab, I should move all the setup to another quite room (we have no dedicated chamber, meeting room may be suitable) and keep away from any electronic devices


    Due to difficulty of the correlation to the result of datasheet
    I am very curious that whether it is possible to have an estimation for the contribution of INA223 itself?

    Since the output show the overall noise distribution? INA223 and others

    RMS (Overall) = RMS (INA223) and RMS (Others)

    If the noise distribution of others is realized,

    It may be a good alternative


    Regards
    Ben
  • In reply to Ben Huang1:

    ello Ben,

    Sorry about not getting back to you sooner.  I tried duplicating a setup similar to yours and I also got measurements that were larger than what is shown in Figure 28 of the datasheet.   Upon investigating this, I think there may be a scaling issue with that figure as there is a discrepancy between this figure and the collected data.   Despite this, we do have a 0.1 to 10 Hz noise specification in electrical characteristics section of the datasheet called voltage noise density.  Based of this specification we can calculate the typical peak to peak noise we would expect to measure.  To calculate this we take the number provided in the specification and multiply by the square root of the upper bound of the bandwidth and we also multiple this by 6 to convert from RMS to peak-to-peak. This gives us (235nV/sqrt(Hz))*sqrt(10Hz)*6=4.46uV.  This actually correlates well with some measurements I was able to make.  

    For my setup, I added an additional gain stage in front to ensure, I would not have any issues with running into the noise floor of the SLAU522 board.  The reason I did this is that the SLAU522 board is designed for a high gain (1000V/V) dut.  The added gain stage utilized another opa827 with a 50V/V gain. From cascading my INA223 in series with the added gain stage and the original gain stages, I got an output with 450mV peak to peak.  If I divide out the gain I get 0.45/100/50/20= 4.5uV peak to peak referred to the input noise.

    For my setup I used a copper clad box to help shield out external noise sources. I also used a linear power supply to power the OPA827 gain stages as opposed to a switch mode power supply that couples more noise into the supplies.  From this linear supply I used the shortest connector leads I could to reduce how much noise they might pick up.  As I used solder with water soluble flux, I cleaned the board with a ultrasonic cleaner to reduce the impact of flux creating additional parasitic paths.   For my setup, I actually ended up using separate pcbs for the INA223 and the filter gain stages.  The INA223 board was our standard EVM that can be powered and programmed through a SM-USB-DIG connected to a laptop.  As the SM-USB-DIG that converts USB to I2C potentially could contribute noise, I dangled this part outside of the copper clad box.   On the scope I used a direct 1x probe DC coupled to ensure that the scope did not filter out any of the desired lower frequency content.  My measurements, board, and setup can be observed below.

    Best Regards,

    Patrick Simmons, TI Sensing Products Applications Support

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