Timeline for Transimpedance amplifier with OPA838 - instability
Current License: CC BY-SA 4.0
16 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Apr 8 at 20:37 | comment | added | Kevin White | You put the resistor in series with the load after the feedback resistor/capacitor but before any significant capacitance load. I don't expect 18pF would don't be a problem though. Be careful about using x1 scope probes as they have a very high input capacitance, maybe 100pF. A x10 probe will be ~10pF and should not cause a problem. | |
| Apr 8 at 16:43 | comment | added | Andromeda | Hello @ Kevin White I noticed my calculator shows 171pF per meter length (0.171pF/mm), the length is 110mm, so it would be18 PF. Would you please let me know that one ne side of the 50 ohm resistor should be connected to the OPA838 output and the other side to GND? | |
| Apr 8 at 16:01 | comment | added | Kevin White | 170pF - that is extremely large and could cause instability by increasing the phase shift at the output in conjunction with the opacmpp output impedance. Isolate that trace/connector with a 50 ohm resistor (value not critical) close to the opamp. | |
| Apr 8 at 14:44 | comment | added | Andromeda | Hello @ Kevin White I also noticed the output of the OPA838 is connected to a connector for measurement (No load is connected to OPA838) and the trace on the PCB has a capacitance around 170 pF, can this cause the instability? if yes, how can I do a quick test? to verify this is the issue? | |
| Apr 5 at 22:16 | comment | added | Andromeda | @Kevin White would you please see my layout in here: electronics.stackexchange.com/questions/708779/… | |
| Apr 5 at 21:30 | comment | added | Kevin White | As others have indicated - +/- 1V is below the allowed operating supply of the OPA838. That opamp has a very high GBW so requires extremely good PCB layout, especially regarding supply decoupling. You need to ensure that there is very low inductance between the device supply pins and the capacitors. | |
| Apr 5 at 21:07 | comment | added | Andromeda | @Kevin White As an update I connected the transducer and it was still oscillating, I used also a 1nF Cap, instead of the transducer, and it was still oscillating. it is not oscillating if I use +/-1V for the supply or (V-=0&V+=3.5V)->not using any negative supply, so I wonder what is the reason, that it is very dependent to the supply voltage and the negative rail . | |
| Apr 2 at 12:23 | vote | accept | Andromeda | ||
| Apr 2 at 9:32 | comment | added | Andromeda | As it is clear in the schematic as well, you can see that, there is a DC blocking Cap (10 nF) between the OPA838 and the sensor, do you think I can directly connect that Cap (10 nF) to ground for a quick test?? | |
| Apr 2 at 9:16 | comment | added | Andromeda | @Kevin, thank you vey much for the guidance, so I will use a capacitor and let you the results, one pin of the 1nF cap with be tied to the inverting input of the OPA838, and the other pin will be tied to GND? right? | |
| Apr 2 at 9:08 | comment | added | Kevin White | If the sensor model is accurate and it does have 1nf of capacitance that should bring the loop gain to a stable point. For testing you could just replace the sensor with a 1nF capacitor. | |
| Apr 2 at 9:02 | comment | added | Andromeda | Hello Kevin, thank you for the nice comment. No for my initial tests the sensor was not attached to the OPA838, do you think if I attached the sensor it will be stable even with +/- 2.5 V? | |
| Apr 2 at 8:25 | answer | added | Andy aka | timeline score: 2 | |
| Apr 2 at 8:14 | comment | added | Kevin White | Are the sensors attached when performing these tests? The OPA838 is not unity gain stable, it needs the capacitance of the sensor to bring the loop gain to the point where it is stable. | |
| S Apr 2 at 8:00 | review | First questions | |||
| Apr 2 at 10:33 | |||||
| S Apr 2 at 8:00 | history | asked | Andromeda | CC BY-SA 4.0 |