LM324LV: About the change in the output of the opamp when using offset voltage

Youngjun Kim

Prodigy 30 points

Part Number: LM324LV
Other Parts Discussed in Thread: ADS1115

Tool/software:

Hi,

I am designing a photodiode amplifier circuit by referencing to this document.

This is my schematic.

(I set the V_OFFSET to 0.1V using resistor.)

I prevented light from entering the photodiode to measure the dark current of the photodiode in this circuit.

When I set feedback resistor(R1) to 3.2Mohm, VOUT is about 200mV.

When I set feedback resistor(R1) to 1Mohm, VOUT is about 700mV.

I expected the VOUT to be output as (Ipd x R1) + V_OFFSET in my circuit.

But, when i changed feedback resister(R1) from 3.2M ohm to 1M ohm, VOUT inceased about 500mV.

I was wondering why the VOUT increased even though I lowered the feedback resistor.

Is there something wrong with my circuit?

Regards,

Youngjun.

23 days ago

0 Clemens Ladisch 22 days ago

Guru 252020 points

This schematic shows a comparator circuit (with positive feedback).

Please show the schematic that corresponds to your actual circuit.

0 Youngjun Kim 22 days ago in reply to Clemens Ladisch

Prodigy 30 points

Oh, thanks.

Above schematic was wrong.

This is my actual circuit. (negative feedback)

Regards,

Youngjun.

+1 Clemens Ladisch 21 days ago in reply to Youngjun Kim

Guru 252020 points

Are you sure that you have a LM324LV, and not some other device with higher input bias currents?

How are the other three amplifiers connected?

You are trying to measure nanoamperes, so you have to ensure that there are no leakage currents. Can you show a picture of your board?

0 Youngjun Kim 21 days ago in reply to Clemens Ladisch

Prodigy 30 points

Other channel of my LM324LV is not connected(floating state).

I made a mistake because I didn't have enough knowledge about the circuit.

I fixed my schematic like this.

I'll try to fix my circuit and test it..

Regards,

Youngjun.

0 Clemens Ladisch 21 days ago in reply to Youngjun Kim

Guru 252020 points

You do not need separate voltage dividers. And you can use any valid voltage, such as V_OFFSET.

0 Ashley Kang 19 days ago in reply to Clemens Ladisch

TI__Genius 10320 points

Hi Youngjun,

In addition to Clemens' comments, is there anything that is connected to the output of the circuit? Is the op amp driving a load?

What is the intended gain and output range you are trying to achieve with the circuit?

Regards,
Ashley

0 Youngjun Kim 19 days ago in reply to Ashley Kang

Prodigy 30 points

Hi Ashley Kang,

In actual circuit, output(VOUT) connected to input of ADS1115.

I'm trying to measure the amount of light change input into the photodiodes.

I set the output range of this circuit to 0.1V to 3.2V and set the gain.

Regards,

Youngjun

0 Ashley Kang 18 days ago in reply to Youngjun Kim

TI__Genius 10320 points

Hi Youngjun,

How much current are you expecting from the photodiode when you are testing this configuration between the 3.2M and 1M feedback resistor? Also, is the ADS1115 connected during this testing?

Regards,
Ashley

0 Youngjun Kim 18 days ago in reply to Ashley Kang

Prodigy 30 points

Hi Ashley,

I expect the current as below.

- 3.2M ohm : 3mA ~ 4mA

- 1M ohm : about 1mA

When i test above circuit, ADS1115 connected to VOUT.

Regards,

Youngjun.

+1 Ashley Kang 17 days ago in reply to Youngjun Kim

TI__Genius 10320 points

Hi Youngjun,

Have you tried to test the circuit without connecting to the ADC? What do the waveforms look like?

Also, another recommendation is to add a feedback capacitor (10pF or larger) to stabilize the circuit.

Please let me know if any of these changes impact the output of the op amp and what the respective oscilloscope images are.

Thank you!
Best Regards,
Ashley

0 Youngjun Kim 17 days ago in reply to Ashley Kang

Prodigy 30 points

Hi Ashley.

I added 15pF feedback capacitor. (It's the smallest capacitor I have.)

And the result is like below.

- no feedback capacitor

- feedback capacitor added

The waveform was clean and the noise of the ADC result measured by ADS1115 was reduced.

Thank you Ashley.

Regards,

Youngjun.

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LM324LV: About the change in the output of the opamp when using offset voltage