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I have an RF signal conditioning circuit that uses two AD8065 opamps for coarse gain. The board is powered via +/-12V, and is stepped down to +/-6V using MC7906 and MC7806 PMIC. The gain is controlled via the DIP switch. With both switches off, the regulators are warm. With both switches engaged, the regulators quickly heat up and begin to give off that smell... I also notice the AD8065s get hot. I understand that PMICs get warm, but I've never experienced them getting hot enough to burn skin.

Is this possibly due to the values of the feedback resistors, or using +/-6 instead of +/-12V? Where can I begin troubleshooting this?

schematic

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  • \$\begingroup\$ Is it still stable after increasing the gain? Or do you have an oscillator? \$\endgroup\$
    – user287001
    Aug 1 at 17:04
  • \$\begingroup\$ It is still stable after both switches are engaged. No oscillation \$\endgroup\$
    – earl
    Aug 1 at 17:07
  • \$\begingroup\$ Do you have a DMM monitoring the supply when you change the switch. Maybe there is a short somewhere. \$\endgroup\$
    – Rodo
    Aug 1 at 17:08
  • \$\begingroup\$ I would suspect oscillation. Or something that's not in the schematic. What are the DC voltages at the op-amp outputs? They should be fairly low even with the switches closed (DC gain of ~10). You can also easily measure the four supply currents by measuring the voltages across the 4.7 ohm resistors. \$\endgroup\$ Aug 1 at 17:14

1 Answer 1

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The opamp has a rail to rail output, so assuming it outputs 6V peak, into 242 ohms that's 25mA, so the opamp's maximum output current spec is not exceeded.

Average current from each power supply should be around 8mA per opamp due to output current, plus the idle current.

The opamps' dissipation doesn't seem too high either.

Even if the opamps are clipped and both output +6V, that's still only 50mA on the supply.

Look for oscillations with a x10 probe and a scope with more bandwidth than the opamps.

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  • \$\begingroup\$ Thanks, will check this out and report back \$\endgroup\$
    – earl
    Aug 1 at 17:07

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