| Electronics > Beginners |
| Nanoamp current source troubleshooting |
| << < (5/6) > >> |
| ZeroResistance:
--- Quote from: exe on May 09, 2019, 01:58:45 pm ---Did you put compensation? Without compensation network it may not work stable as each element in feedback loop creates a delay. Delay == phase shift :). May be at 50KHz phase shift is 180 degrees. --- End quote --- 1. Would decoupling caps do the trick? I didn't bother to add decoupling since it was battery powered. 2. I thought that heavy capacitive loads on the op-amp output is one of the factors that cause unstability. However I don't have any load here except a scope probe. |
| Kalvin:
Try adding a 1 uF ... 10 uF capacitor in parallel with the BAT54. The capacitor will keep the ground reference stable and reduce noise. While you are at it, add 100 nF ... 1 uF capacitors across the op amps' U1, U2 and U3 power supply pins (although the circuit is battery powered) which will reduce noise in the power supply and reduce noise coupling in the circuit. You can try to tame the oscillation further by adding a small capacitors (1 nF, for example) across U1's and U3's In- and Out. Just check with simulator that the circuit is still stable and performs as you expect. Tweak the capacitors as necessary in the real circuit. |
| ZeroResistance:
--- Quote from: Kalvin on May 09, 2019, 05:50:12 pm ---Try adding a 1 uF ... 10 uF capacitor in parallel with the BAT54. The capacitor will keep the ground reference stable and reduce noise. While you are at it, add 100 nF ... 1 uF capacitors across the op amps' U1, U2 and U3 power supply pins (although the circuit is battery powered) which will reduce noise in the power supply and reduce noise coupling in the circuit. You can try to tame the oscillation further by adding a small capacitors (1 nF, for example) across U1's and U3's In- and Out. Just check with simulator that the circuit is still stable and performs as you expect. Tweak the capacitors as necessary in the real circuit. --- End quote --- I add a 22uF cap across the schottky but didn't make any difference. The 0.1uF decoupling made lot of difference and the 50Khz noise seems to have diminished now I get spikes at the output at 10ms intervals. Please see attached. |
| Kalvin:
10 ms equals to 100 Hz, so your circuit is picking up some noise from the rectified 50 Hz mains. |
| ZeroResistance:
--- Quote from: Kalvin on May 10, 2019, 07:20:21 am ---10 ms equals to 100 Hz, so your circuit is picking up some noise from the rectified 50 Hz mains. --- End quote --- I have a laptop next to the circuit but that's again not connected to mains but running on battery. I can see the noise even on the input. Is there a way to reject the power supply noise? |
| Navigation |
| Message Index |
| Next page |
| Previous page |