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# INA223: 0.1Hz to 10Hz Voltage Noise Measurement

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

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

• 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

Getting Started with Current Sensing Video Training Series

TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.

• In reply to Patrick Simmons:

Hi Patrick

Thanks for much effort on reproducing the waveform!

It is very detailed and clear description what and why you are doing

Voltage noise density of INA223, I consider it is a typo and neglect at the first sight, since the representation is different between INA223 and OPA827, one show density the other is peak to peak

I am referring the result of waveform recently collected to the input

8.24mV / 100 / 20 = 4.12uV

According to your calculation, the input voltage noise is 4.46uV typically

It is highly correlated

Regards

Ben

• In reply to Ben Huang1:

Hello Ben,

I cannot simply swap the plot out in our datasheet due to the internal approval process we have in place. To push the change forward, will require further investigation. This investigation would start with discovering the source of discrepancy between that figure and some measurement data I was able to find. As this data was collected 7 years ago, finding the source of error will be difficult.

In the meantime I encourage you to work with the specifications provided in our electrical characteristics section. The values for noise density from the table resulted in 0.1 to 10 Hz calculated noise which was in agreement with what I was able to measure in the lab. If you measure noise significantly lower than this there is very likely an error in the setup or calculations.

As we have provided the methods for improving your noise measurements and we now have figured out how to correlate my measurements and your measurements with the datasheet electrical characteristics table, I believe have serviced your initial issue and will therefore close this thread. However, once we have any developments for changing the plot in the INA223, I will be sure to let you know.

Best Regards,

Patrick Simmons, TI Sensing Products Applications Support

Getting Started with Current Sensing Video Training Series

TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.