Other Parts Discussed in Thread: , LM358A
Hi all,
I have a strange behavior regarding some LVM358s on our PCBs. The circuit is nothing special. It is an impedance converter followed by a comparator, see below.
The supply voltage is 5.0V. The analog voltage at pin 3 comes from a light barrier, which has the same supply of 5V. It varies from 0 to 4.5V max.
The purpose of this circuit is to buffer the signal coming from this light barrier with IC27A. The signal shall then be compared to the voltage that is applied to pin 6 of IC27B. The voltage at pin 6 can vary from 0 to 5V.
No HF signals are applied to the circuit.
An example of measured analog values for a good board is shown here:
| U_REF (pin3) |
Buffer out (pin1) |
Comp Ref (pin 6) |
Comp In (pin5) |
Comp Out (pin 7) |
Comments |
| 3.82V | 3.67V | 2.88V | 3.84V | 4.92V | Comparator operates as as expected |
| 4.52V | 4.29V | 2.88V | 4.47V | 4.93V | Comparator operates as as expected |
| 4.54V | 4.30V | 4.90V | 4.43V | 0V | Comparator operates as as expected |
| 4.51V | 4.30V | 0V | 4.47V | 4.92V | Comparator operates as as expected |
An example of measured analog values for a bad board is shown here:
| U_REF (pin3) |
Buffer out (pin1) |
Comp Ref (pin 6) |
Comp In (pin5) |
Comp Out (pin 7) |
Comments |
| 3.87V | 3.72V | 2.88V | 3.88V | 4.92V | |
| 4.53V | 4.89V | 2.88V | 5.08V | 4.92V | Output of buffer and input of comparator implausibly high |
| 4.52V | 4.89V | 4.90V | 5.08V | 4.93V | Output of buffer and input of comparator implausibly high. No output change possible |
| 3.86V | 3.72V | 0V | 3.88V | 4.92V | Buffer out and Comp In very noisy. Can be smoothened with ice spray |
| 4.52V | 4.56V | 0V | 4.73V | 4.93V | Buffer out and Comp In very noisy. Can be smoothened with ice spray |
I cannot explain the red marked values here.
I have the following problems:
- The comparator does not work for input voltages of >4V and creates implausibly high values at pin 1.
- If I apply a voltage between 3.8V and 4.5V to pin3, then the output on pin 1 is very noisy. This can be eliminated with a little bit of cold spray and is gone as long as the part is cooled.
- 3 OpAmps out of a batch of 50 show this behavior. The rest does not.
- If I take a functioning and a "defective" board and I swap the parts on these boards then the " defective" board works as intended and the previously functioning board does not work anymore. So the problem can be transferred with this part.
The marking codes for these parts are:
Does anyone have an idea what the problem might be?
Best Regards
Sascha
