INA202: about high Vout and CMPin voltage when load supply ramps up

Hello Joyce,

Thanks for reaching out on the forum.  I think your customer's issue might be related to the issues found in the posts here and here.  As such, adding an RC low pass filter to your RESET pin might help.   As for the shunt value, that seems fine.  After your customer attempts using a filter on the reset pin, if the part still does not work we can look into trying to reproduce your customer's test conditions.  However, we will need more details on how your customer is powering on the device and how they are cutting power to the device.

Hi Patrick,

I am afraid the two cases are not same as my case.

My case is not related to REset pin and customer does design RC filter for RESET pin.

My case is related to high output voltage of the amplifier when the common mode voltage of Vin+ and Vin- is around 0.5V. Please check the scope snapshot, blue line is the load supply power, green line is CMPin, when the load supply ramps to 0.5V, the CMPin will reach to 0.6V, which means the output of amplifier reaches to 2.4V when load supply is 0.5V. When load supply continues to ramp up, the cmpin voltage will become normal.

They also tried to short Vin+ and Vin-, making differential input=0V, while the load supply ramps to 0.5V, customer still observes high output voltage of the amplifier. it means this issue is related to common mode voltage is 0.5V, differential voltage=0V, the output is very high.

Please try to reproduce this on demo board to help us identify the problem. thanks a lot!

Hello Joyce,

Sorry for my misunderstanding.  I do not have INA202 samples right now, but I should have them tomorrow and will be able to test in the lab then.  When the customer's common mode is ramping, is their INA202 supply line also ramping or is it already stable at 5V?

Hi Patrick,

Yes, INA202 supply is stable at 5V. 

Joyce

Hello Joyce,

I ran some tests in the lab and I do see the same behavior your customer is seeing.  At the moment I am trying come up with a workaround to suppress that glitch.  I will try to get you some proposed fix by tomorrow.

Hi Patrick,

Good to know the demo board can see the similar phenomenon. will wait for your update. :) 

Thanks a lot for your support!

BR,

Joyce

Hi Patrick,

Thanks a lot for your effort on the work around solution. The workaround basically focused on the CMPin circuit.

Customer wants to know the root cause of Vout reaches to 2.4V while Vin and Vout Common mode voltage is around 0.5V? Can you please explain this? They need to be fully clear about the root cause before implementing any workaround solution. 

"My case is related to high output voltage of the amplifier when the common mode voltage of Vin+ and Vin- is around 0.5V. Please check the scope snapshot, blue line is the load supply power, green line is CMPin, when the load supply ramps to 0.5V, the CMPin will reach to 0.6V, which means the output of amplifier reaches to 2.4V when load supply is 0.5V. When load supply continues to ramp up, the cmpin voltage will become normal."

Thanks very much for your support.

Hello Joyce,

This actually can be found in the datasheet. The explanation is provided on page 20 section 7.4.2.4.

Hi Patrick,

I checked the datasheet 7.4.2.4 low Vsense case2. Customer test result is similar to datasheet description. However, there is some difference.

1. In datasheet, it describes Vout test limit for INA202 is 2V. My customer tested result is 2.4V. It exceeds the test limit. They also tested some normal parts, the CMPin is 0.3V, i.e. The Vout is 1.8V. This is normal. Why some parts Vout exceeds to 2.4V? Is this bad part that failed to be screened out?

In customer's application, Rshunt=10 mohm, R1=18.7kohm, R2=6.5Kohm.

         2.  you mentioned you also reproduce this phenomenon, I wonder how much is the Vout voltage?

Thanks a lot for your reply in advance.

 Joyce

Hey Joyce,

Although we covered some of this over email, I will also post here to the benefit of any other forum user encountering the same issue.   Also I do have some updates to the prior responses I gave,

1. Parts that exceed specifications in the electrical characteristics table should be screened out.  Turns out there is a section on page 6 that does specify a max limit for Vout when vsense is less than 20mV.  It is 2V like you said.  However, after consulting with a colleague this spec appears to be a DC specification that was likely taken after some settling time.  Therefore if overshoot is considered, its possible that Vout could be greater than 2V.

The difference in behavior among your devices could be attributed to manufacturing tolerances.

2.  For my test, the output went to roughly ~1V with CMPIN voltage at ~250mV.  Since, performing that test I have done a few additonal tests.  My key observations were that below 20mV Vsense on my device, the glitch was not always large.  Typically the largest glitch was found below 10mV.  I also noticed that changing the VCC potential had an impact on the size of the glitch.  Lower supply voltages typically had bigger glitches.  From this behavior I suspect that if your supply droops at all during the VCM ramp up, the affect may be even more pronounced.  The other key item I noticed was that a faster VCM ramp up yield a significantly smaller glitch.  This probably due to the device leaving the worst operating region more quickly.

Hi Patrick,

Agreed. Thanks a lot for your support!

BR,

Joyce.