Hi,
I am performing Noise Analysis of this amazing low power AFE chip LMP91000. I find that the noise spectral density follow the expected curves with only exception.
Let me first explain my experimental setup . I am using a custom pcb designed for LMP91000. I am providing an external reference voltage of 2.5V from a 9V battery supply which is regulated to provide 2.5V . The LMP91000 is configured to set the Vref_divider voltage to 67% and Vbias percentage is set to 24%. The RIA is set to 2.75k ohm and Rload = 10 ohm.
The RE and CE Pins are shorted using a 1k ohm resistor. The WE pin is connected to grounded using an external source resistor. This external source resistor is increased for each measurement so that the gain of the TIA decreases.
Now, when I am increasing the source resistor from 10 ohm to 1M ohm, I find the curves follow the Op-amp characteristics keeping GBW product constant.
The exception which I encountered is when I connect the WE pin directly to ground without any external resistor i.e. direct short to ground using a wire. This curve for this Setup does not match the expected results. Infact the curve closely resembles to the Setup when I have a 1K ohm resistor connected to the WE and ground.
I have attached the plot which can help to better understand my measurements. Do you have any Explanation as to why I am observing this behavior and has anyone seen a similar behaviour.
Note: In the plot below, the red curve with the legend L1 is the measurement when the WE is directly shorted to ground using a wire and only the internal Rload of 10 ohm is conncted to the TIA.
Thank you.
Regards,
