INA180: Output distortion and wrong ratio

Hello Yishan,

I am looking this over and will respond shortly.

Sincerely,

Peter

Hey Yishan,

There are a couple concerning issues with these waveforms.

1. Why is Vs (purple, channel 3) oscillating down below 2.7V? The device is not supposed to operate with Vs under 2.7V, nor is it able to operate accurately with VS oscillating with large magnitude?

2. How is blue, channel 4 representing Vout? Is this Vout of the INA180 or another part of circuit because device pin voltage cannot reach high voltage up to 40V?

Other than this, more information on the following would be helpful:

A. How is the shunt resistor connected to IN+ and IN- traces? Layout images are best.

B. How many systems have been tested and how many show normal behavior?

C. Can other waveforms be adjusted so that the waveforms are expanded vertically and with max and min values listed, specifically for load current and Vout?

Sincerely,

Peter

Hi Peter,

Thank you for the reply.

First, Sorry for the misunderstanding, Purple line is the output of the INA180.

Vout means the output of the converter, not the output of INA180. And the load is the resistive load, so Vout and sensing current (Iout) follow the ohom law.

For the follow A and C question, I will check with customer. 

For question B, they are just now testing with their prototype.

Is the schematic good right now? And could you please tell me if the resistor divider is acceptable or not?

Thank you,

Yishan

Hey Yishan,

The resistor divider is perfectly fine and seems to be correctly dividing down Vout.

However, there definitely is some major source of offset error. The device's gain is actually working when you calculate linear gain as (Vout2 - Vout1)/(Vsh1-Vsh2) = ~102 V/V. There is something other than simple distortion going on with the circuit.

The offset error could be coming from various things: differences in ground potentials or bad shunt resistor layout.

2. What is channel 4 representing?

3. Would they please make measurements with load = 0A?

4. Would they please measure for any high-impedance connections (or transient voltage differences with a differential probe) between INA180 ground, and ADC ground and load ground and power ground?

I will await answers for previous questions.

Sincerely,

Peter

Hi Peter,

For the previous question, here are layout images and the waveforms are expended vertically.

Resistor place:

Differential signal trace:

INA180:

Current and the output of INA180:

Channel 4 is the output voltage of the power stage.

I am asking customer to test the high impendence trace and  waveform with 0A output.

Thank you very much.

Yishan

Hi Peter,

I have reviewed their PCB layout.

Maybe the reason is they connected pin 2 and pin 3 directly?

Although they are the same net but not they should connect directly because they will bypass the parasitic inductor which means the common signal noise only effect the in+.

Is that right?

Thank you 

Yishan