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Part Number: INA223
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!
In reply to Ben Huang1:
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In reply to kai klaas69:
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.
Patrick Simmons, TI Sensing Products Applications Support
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In reply to Patrick Simmons:
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
Could you please help update the noise waveform to the datasheet officially?
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