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Uploaded the load of TIA amplication circuits, there are four same ciruits.

Some updates: I've made two layouts and one of them is much easier to have the osilation.

I've designed and tested with LT3045 transferring 5V to 4.5V.

There is a large oscillcation (around 20~30mVpp) sine wave oscillation on the input and smaller one (around 2mVpp) on the output. The frequency is around 810kHz.

At first I thought it was a ripple from front DC-DC converter so I used a battery (about 7.6V) but the oscillation still exist.

Then I try to reduce some load current (removed some TIA opamps) , then the osillation frequency changes and the mag get smaller. But still I can see there a peak on the spectrum.

The best way so far I find is to short the two beads on the input (but also can see a little peak on the spectrum around 800kHz). Adding capacitor on input or end doesn't seem to work.(I've tried even 220uF aluminum capacitor...)

I think I've almost followed the reference layout.

It would be very appreciated if anyone can give some hints.

Thanks in advance. enter image description here enter image description here enter image description here

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    \$\begingroup\$ Is the GND pad (11) soldered? \$\endgroup\$ Nov 19, 2022 at 3:56
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    \$\begingroup\$ Did you follow the recommended low ESR and low ESL for the output capacitor? (See datasheet section: "Stability and Output Capacitance"). Similar issues for the input cap. There is also a warning of high input inductance, so why did you add ferrites on the input and gnd lines? (see the datasheet section "Stability and Input Capacitance"). \$\endgroup\$
    – Nedd
    Nov 19, 2022 at 4:12
  • \$\begingroup\$ @SpehroPefhany Yes it should have been \$\endgroup\$
    – FNJU
    Nov 19, 2022 at 11:10
  • \$\begingroup\$ @Nedd I've tired different capcitor and also combination of different capacitors, but it seems not to work... \$\endgroup\$
    – FNJU
    Nov 19, 2022 at 11:12
  • \$\begingroup\$ @FNJU what is the load? \$\endgroup\$
    – Voltage Spike
    Nov 19, 2022 at 16:57

2 Answers 2

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Remove the ferrite from the ground line, it is creating common mode noise on ground. If you need to worry about conducted emission currents from entering the input of the LT3404, the best way would be to use some very low ESL/ESR capacitors on the input, not to block them with impedance on the ground

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  • \$\begingroup\$ Sorry i tried again but seems I miss tested or somthing weired. I short all the beads but there's still a sine wave. \$\endgroup\$
    – FNJU
    Nov 21, 2022 at 10:18
  • \$\begingroup\$ Take a diff probe and measure between grounds, then post the results. When you say shorted, how did you short? 0 ohm resistor? \$\endgroup\$
    – Voltage Spike
    Nov 21, 2022 at 14:32
  • \$\begingroup\$ I removed the bead and used a short metal pin cut from a aluminum capacitor \$\endgroup\$
    – FNJU
    Nov 22, 2022 at 6:50
  • \$\begingroup\$ And I used a coaxial cable to test the voltage \$\endgroup\$
    – FNJU
    Nov 22, 2022 at 7:03
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The parallel circuit of C4 and C5 and/or C3 probably has a strong resonance near 1 MHz ballpark. To damp it, you need a low (but resistive enough) impedance to ground.. Sometimes this is also called a snubber circuit, even if not in a switching context here.

This is typically achieved with an Al electrolytic capacitor. Do you have this at the input? Even if elsewhere on the board..

As others have said, I would get rid of the GND separating bead, too. That will increase the damping effect of an Al electrolytic cap on the input voltage rail and GND_P

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  • \$\begingroup\$ Hi Tobalt, I've removed the c3,c4 the bead but the sine wave still exsit. And I tried adding al capacitor on the input and output but seems not to work.. \$\endgroup\$
    – FNJU
    Nov 22, 2022 at 7:06
  • \$\begingroup\$ @FNJU Hmm hard to say more at this point. If I were in your position, I would go to SPICE and try to rebuild the circuit including some parasitics until I can sort of reproduce the behavior and work from there. 1 Mhz can be already related to trace inductances on the order of a few nH, if the layout is not tight and caps are low ESR and large C>~10 µF. I still suspect the input node because the oscillation is much larger there \$\endgroup\$
    – tobalt
    Nov 22, 2022 at 10:00

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