The sagging of the SMPS causes a voltage in the photodiode line like a camel hump.

[Muhammed Güler]

Hello,

I have a system of photodiode and led to detect the presence of a substance.

I am producing 2 different 5V with SMPS for led and transimpedance amplifer.

my problem is that the led voltage sags while switching. and this sagging causes changes in the photodiode line, such as a camel hump.

I have to work with a small battery i don't want to power the led with LDO.CH1 led switch signal

CH2 Pd ADC input

CH3 led 5V supply

how do i get rid of this camel hump-looking voltage?

thanks

[Javier1]

Could you share a drawing/schematic?

Where is your ADC connected to?

Why is the led 5v power supply dropping before the switching signal happens?

[Muhammed Güler]

this is my circuit

1st channel LED1_CNT

2nd channel ADC

3rd channel VCC2

[Piranha]

1. U9:A is a transimpedance amplifier and U7:A is an inverting amplifier. Both of those must have their positive inputs connected to the signal ground like in this example. In your schematic the inputs are reversed. The connection of U7:B is correct as it is a non-inverting amplifier.
2. If R3 pull-up is necessary at all, you should probably move it to the other side of R25 (U7:B output) so that it doesn't have an impact on signal level by forming a voltage divider with the same R25.
3. Q2 gate probably needs some 10k-100k pulldown to keep it off, if the LED1_CNT line can be floating, but that depends on the rest of the schematic and connections.
4. I cannot see the purpose of the R10.
5. For BAV99S schematic symbol the names of pins 3 and 6 are swapped.
6. D10 schematic symbol (arrows) are for LED, not photodiode.