Hello,I use my new 3 LME49720 in a PCM1794 DAC - two of them are I/V,and one for LPF.They sound great,but my concern is the heat they produce- they're quite warm(all 3),well not hot,but certainly hotter than any other opamp I have.And I wonder if it's normal,or they just oscillate.What's your experience with this chip?Are the just hot or tend to oscillate and get hot?
Hot LME49720s
As they are very low distortion op amps the bias currents are likely to be on the high side which will make them warm i.e. they are operating closer to class A. As they are unity gain stable they are not likely to oscillate very easily. Make sure you have no rf getting into the supply pins by bypassing them properly with electrolytic and disc ceramic capacitors if this is not already done. Also check the load being driven if it is highly capacitive (over 100pF) then you might benefit by putting a resistor in series on each output to isolate the load. Usually values around 50 ohms are used for this purpose. Use an oscilloscope to check for excessive ringing and overshoot on the outputs which are caused by an unstable op amp. Highly likely they are operating as normal and it is nothing to be concerned about.
As they are very low distortion op amps the bias currents are likely to be on the high side which will make them warm i.e. they are operating closer to class A. As they are unity gain stable they are not likely to oscillate very easily. Make sure you have no rf getting into the supply pins by bypassing them properly with electrolytic and disc ceramic capacitors if this is not already done. Also check the load being driven if it is highly capacitive (over 100pF) then you might benefit by putting a resistor in series on each output to isolate the load. Usually values around 50 ohms are used for this purpose. Use an oscilloscope to check for excessive ringing and overshoot on the outputs which are caused by an unstable op amp. Highly likely they are operating as normal and it is nothing to be concerned about.
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Opamp rolling again? Oh my...
The LME49xxx are susceptible to oscillation just like any other high speed opamp. There NEEDS to be adequate local power supply decoupling to avoid it, specifically a small value ceramic of at least 100nF or more as physically close to the opamp supply pins as possible to a LOW IMPEDANCE ground, and a small electrolytic of at least 10uF or more. The PCB layout is important and might not be ideal for a given opamp in a given application. As mentioned before, using an O-scope to verify proper operation is a must.
Mike
The LME49xxx are susceptible to oscillation just like any other high speed opamp. There NEEDS to be adequate local power supply decoupling to avoid it, specifically a small value ceramic of at least 100nF or more as physically close to the opamp supply pins as possible to a LOW IMPEDANCE ground, and a small electrolytic of at least 10uF or more. The PCB layout is important and might not be ideal for a given opamp in a given application. As mentioned before, using an O-scope to verify proper operation is a must.
Mike
The LME49720 does have a higher quiescent current draw of 10ma, as stated, due to the higher biasing. With 30 volts across the rail, the IC will be dissipating 0.3 watt. That will certainly make it feel warm, but it depends on what "quite hot" means. Measure the temperature. If you cant, I'd say it would feel warm to the back of the finger, but not so hot that you can't hold it there.
BTW, this IC is identical to the LM4562. It was renamed to bring it under the LME high performance series. The LM part # remains due to its popularity (it won some awards).
BTW, this IC is identical to the LM4562. It was renamed to bring it under the LME high performance series. The LM part # remains due to its popularity (it won some awards).
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