This thread has been locked.
If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.
I have years of analog designs, normally doing complicated, or fast or high precision designs, however this time I was requested to design a simple photocell data logger with only 8 bits of precision therefore I used a simple LM324 device as a transconductance amplifier with 32.4Kohm gain with the +Input grounded and the -Input to the diode and the feedback resistor (+a 1nF high frequency cut of capacitor also in parallel to the 32.4Kohm feedback resistor) Power supply is only positive 15.5V.
I was surprised it didn't work , until I touched the output of the amplifier with the 10Mohm scope probe.
I notice that by loading the OpAmp output by a 4.7Kohm to GND it worked OK.
I assumed the device was faulty and I replaced with an other from an OTHER batch, with the same results.
By the specs the input common mode includes GND , therefore I have NO explanation on the behavior.
I delivery the circuit with the 4.7Kohm load... but why?
Does someone has a clue?
It troubles me that I have no problems making conditioners with 24bits precision, acquisition systems with 64GBPS throughput... but this simplest project didn't work as expected, it shows that we can never be sure.
Eli Jacob
Eliezer,
Seeing that it didn't work until it was touch makes me suspect a bad connecting in the circuit.
Does the output connect to anything other then the feedback resistor and cap? I ask because LM324 can only have an output near ground when sourcing current or sink only a small current <30uA.
Can you provide a couple input current and output voltage points for not working and working (with 4.7k)
Does it not work for a certain input / output range or is the gain incorrect for the whole range?
This is a simulation
Hi
I did that, in fact I tested removing the cap completely and also changing it to 100nF (so see if I can worsen the behavior, also gives info)
I also connected 2.2nF and 100nF between the inputs.
I so than 100nF between the inputs without feedback capacitor helps, but not so much than the 4.7K load.
The point that 4.7K load fix the problem makes me think that is something related to DC biasing.
I don't know the detailed internal diagram of the LM324, but from the simplified diagram there is a 6uA current source to the input stage and also other current source of 100uA for the output transistors drive.
Can be that lack of load on those transistors affect the current source , and that the implementation of the current sources causes a reduction of the magnitude of the input stage current source, leading to the problem?
Eliezer,
Input common mode at ground is fine. Usage with or without with the feedback capacitor is also fine. I even tried to raise GND to +200mV thus making the noninverting inputs 200mV below GND and the LM324 worked well (for VOUT >200mV because output VOL is relative to GND pin).
The absolute maximum input voltage is -0.3V per data sheet page 4. If the input current precedes LM324 VCC then input voltage spec can be violated. A schottky clamp diode should be sufficient to keep input voltage below 300mV. This would require adding a component which requires a new circuit board layout. A quicker fix is to make LM324 VCC power up first or try the LM324A which still has a -0.3V maximum input however it is resists phase inversion at the inverting input better.