LMP91000: Lower limit of current detection? Oscillations with measurements in 10 nA range with certain cells

WJ Michael Davis

Part Number: LMP91000

Working with a two electrode cell (with CE and RE shorted) I'm getting a consistent 400 Hz oscillation, only when the cell current is low (<10nA). Is the feedback circuit unstable at this low current or could it have to do with the geometry of the cell itself?

Is there anything I can do to improve the stability at very low currents?

over 6 years ago

0 Audrey Dearien7 over 6 years ago

TI__Expert 4875 points

Hello,

Where are you seeing the oscillation and where are you measuring current?

Thank you,
Audrey

0 WJ Michael Davis over 6 years ago in reply to Audrey Dearien7

Prodigy 30 points

Hi Audrey,

The oscillation is on the LMP91000 Vout, which is also where I'm measuring the current (proportionally).

Thank you,

Michael

0 Audrey Dearien7 over 6 years ago in reply to WJ Michael Davis

TI__Expert 4875 points

Hi Michael,

Thanks for the additional info. Have you measured your reference voltage to see if the oscillation also appears there? Also, what configurations are you using in the LMP91000?

Regards,
Audrey

0 WJ Michael Davis over 6 years ago in reply to Audrey Dearien7

Prodigy 30 points

Hi Audrey,

I have measured the reference voltage and don't see any oscillation there. The LMP91000 is configured for 3-lead amperometric measurement (with the counter and reference electrodes shorted in this case) with TIA gain of 350 kOhm and 10 Ohm load. The reaction I'm measuring is on test strips and the oscillations only occur with certain types of strips. This is what lead me to believe it may be an instability due to the impedance of those strips.

Thank you,

Michael

0 Audrey Dearien7 over 6 years ago in reply to WJ Michael Davis

TI__Expert 4875 points

Hi Michael,

Are these strips changed at the input of the LMP91000 or the output?

Additionally, you may consider adding a capacitor between C1 and C2 to filter the noise. The R_LOAD resistor and capacitor between C1 and C2 creates a low-pass filter.

Regards,
Audrey

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LMP91000: Lower limit of current detection? Oscillations with measurements in 10 nA range with certain cells