# Opamp and photodiode. Try get to know where should be offset voltage?

#### Dever

Joined May 2, 2022
35
Hi,
I created simple circuit with photodiode for pulse oximeter. As you can see I use opamp in single supply mode. After removing DC offset I put 1V offset on non-inverting input on opamp. Then RC filter is connected to grond. Is everyting correct?
I wonder if I should connect REF on ADC to ground or 1V offset voltage?

#### ericgibbs

Joined Jan 29, 2010
18,983
hi Dever,
Are you aware your Vout will be at +3v with no signal input.?
E

#### ericgibbs

Joined Jan 29, 2010
18,983
Hi Dever,
Could you please describe the nature and shape, etc that you expect from the sensing diode.?
I could then use that data for a signal source.
E

#### Dever

Joined May 2, 2022
35
Hi,
Are you aware your Vout will be at +3v with no signal input.?
No.

My circuit is part of pulse oximeter. I would expect from the sense diode to get PPG (photoplethysmogram) signal. Then I will calculate SpO2 in blood. Photodiode is illuminated by RED and IR LED.

#### ericgibbs

Joined Jan 29, 2010
18,983
hi D,
The Gain of your output OPA is 1+490k/150k =4.26, so with a1V signal on the NI input the output is saturated at 3.3V, ie: the supply voltage.
E

#### crutschow

Joined Mar 14, 2008
34,673
The non-inverting gain of the second stage is actually 1+(490K) / (150k+56k)) = 3.38

I see no reason for R4.
If you remove that, then the non-inverting DC gain becomes 1, giving a DC output offset of 1V, with an AC inverting gain of 3.26 (below).
I wonder if I should connect REF on ADC to ground or 1V offset voltage?
Neither.
You want the ADC REF input to go to the desired ADC full-scale reference voltage.

#### ericgibbs

Joined Jan 29, 2010
18,983
hi Dever,
As my colleague post #6 points out the Gain is actually 3.38, careless oversight on my part, however your final OPA would still be in saturation with level gain and a +1Vinput on the NI pin.
Carry out the modifications as he has suggested.

Do you have the details of the signals expected from the Oximeter sensor, that you could post, so that we can check the circuits correctly?

E

#### Dever

Joined May 2, 2022
35
Thanks. At this point I don't have that photodiode in my hands to see how much offset voltage do I need. I can do research about PPG. In days I will think about my circuit and mayby ask for help. I used once "Photodiode Circuit Design Wizard".

I'd like to admit that I always got saturated opamp in simualtion without coupling capacitor on input, then I used current source with 1uA current and it worked.

#### crutschow

Joined Mar 14, 2008
34,673
then I used current source with 1uA current and it worked.
To what input?