LMP7721: Capacitance Problem when is power on first time has problenm second wotrks correct

It's impossible to read your schematic but I had made some assumption regarding the circuit in question and have picked the values to assure stable operation - see below.

AC stability analysis shows a very stable system with 76 degrees phase margin whereas the minimum recommended is 45 degrees - see below.

Running transient analysis by exciting the output with +/-1mA current pulses confirms stable operation with no overshoot - see below.

Below I have attached Tina-TI schematics so you may modify the circuit as needed and perform your own simulations. If you need help, please send back a legible schematic of the circuit in question.

8308.LMP7721 AC Stability.TSC

7750.LMP7721 Transient Stability.TSC

I am atatching the file a bit bigger 

The photcell has a capacitance of 200 pF 

I think C146 is too big and create inconsitance in the reading of ADS1255. Could you verify the follwing :

• Rf = 20 kΩ
• Photodiode capacitance Cd ≈ 200 pF
• LMP7721 input capacitance is small vs 200 pF, but add stray (no guard) → total at IN− is typically ~220–260 pF

For this regime, a realistic compensation Cf is ~10–22 pF (NP0/C0G), not 1200 pF.

1. Recommendation

• Populate Cf = 15-22 pF NP0/C0Gas your default.
• If any overshoot/peaking: go up to 22 pF.
• If it is very slow (and stable): try 10 pF.

What bandwidth that implies

The “intentional” feedback pole is:

• Cf = 15 pF → f≈1/(2π⋅20k⋅15p)≈530 kHzf   
• Cf = 22 pF → f≈360 kHzf ≈ 360\text{ kHz}f≈360 kHz

That’s a normal compensation region; we can do any wanted low-pass filtering later (and more cleanly) at the ADC input.

Why 1200 pF is wrong here
It creates a dominant pole with Rf at:

Fc  ≈   / 2π⋅20k⋅1200p ​≈6.6 kHz

That turns your TIA itself into a low-kHz integrator and makes power-up settling/history effects much worse.

(Also: LMP7721 is explicitly intended for femtoamp work and expects guarding/clean layout at IN−; without guard, we already have extra capacitance/leakage sensitivity. )

AT