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LPV821: LPV821 Voltage buffer

Part Number: LPV821


I'm using the LPV821 op-amp for a buffer circuit (voltage follower). Below is my circuit, where the Vin is a non-deal source with very high output resistance (~700KOhm) and the desired output has the same voltage as the input but with low impedance. 


The circuit works only with ideal source (little or no impedance between Vin and +IN). But it does not output anything for my non-ideal voltage source (When there's a large resistor between Vin and +IN). I was wondering if I could have some suggestions on why I'm not getting the desired output.


  • Hi Vahid,

    Thanks for reaching out. Could you provide a full schematic and also any waveforms of the input and output signals?

  • Vahid
    I cannot think of any fundamental reason why you are experiencing that problem. One way that may help us track down the problem would be if you have a model for your sensing element. you described it as having high Z output, but that alone should not be a problem for the LPV821. on the trouble shooting side, can you go back to your ideal source and make sure the amplifier is still functioning properly? I know that sometimes I breadboard my preliminary circuits and all of a sudden my circuit stops working because a connection came loose or I damaged a device somehow. Lastly, when you say that the output doesn't output anything, does that mean the output is at ground? If you can use a dual oscilloscope and probe the non-inverting pin and the output at the same time, that will be interesting to see.
  • Hello,

    Here's the schematic of the circuit. Please note that the circuit outputs the input sine wave with the same amplitude when I remove the 700K resistor (R10), which is the desired output. But it does not output anything when I have the 700K resistor in the circuit. V2 is a sine wave (0.4V peak)

  • Hello, 

    So I tried the circuit again and below are my observations:

    1- When I connect the positive (1.4V) and negative (-1.4V) supplies to the op-amp and disconnect the other pins, all the pins are at 0V. 

    2- When I connect an ideal source (no R10), ALL the pins have the same voltage as the input. 

    3- When I add the R10 (700KOhm), the output goes to zero. I also measured the voltage at the non-inverting pin and it's at zero, even though the input was not zero. 

    4- When I paralleled a 100K with R10 the output was almost like input: a sine wave with minor drop in the amplitude. 

    5- When I replaced R10 with a 200KOhm resistor, the output was substantially attenuated but not zero.

    From 3,4, and 5, I can conclude that the current in the non-inverting pin is pretty high and this current causes a voltage drop across the R10. I don't know why this happens.

    Maybe the op-amp is damaged?  


  • I do not see anything fundamentally wrong with your circuit. Sorry for repeating my question but I want to make sure I understand what you are doing. You say that you remove R10. Does that mean that you short R10 out with a wire and see the sine wave at VOUT and then remove the shorting wire and the sine wave at VOUT goes away? What exactly do you see at VOUT when you remove the shorting wire? I am trying to understand what you mean by the amplifier does not output anything? if you could probe the input sine wave, the non-inverting input pin, and vout at the same time, that may give us some hint about what is happening. also, what is connected to VOUT? I presume you are using an oscilloscope probe. Is anything else connected. Sometimes it is good to monitor VOUT through a 10k resistor to make sure the output of the amplifier is not going unstable. Driving a scope probe directly should not be a problem. I am just trying to troubleshoot to see what is happening because I do not see any problem with what you have built.
  • sorry Vahid
    I was responding to your previous posting, not your latest.
    It does sound like the input is damaged. their are back to back diodes between the input pins. It is possible that they were damaged somehow. do you have another LPV821 to experiment with? the bias current is in the pA range, so that doesn't make sense for a good part.
  • 1- By removing R10 I mean I short it out. (change R10 to zero).
    2- When I set R10 to say 200KOhm, the output is extremely attenuated (but it's not zero). BUT, when I increase R10 to 700KOhm, the output will further reduce to zero. Same happens when I probe non-inverting pin and output at the same time. 
    3- The output is not connected to anything. I measure the output with a 1MOhm probe.

  • Unfortunately I don't have another one and this one was a sample that I got from TI! But I have the feeling that I damaged the chip during the soldering (I soldered it to a SMT Breakout )
  • Unfortunately I don't have another one of this. This was a sample that I got from TI. Maybe I damaged the chip during the soldering.
  • Unfortunately it sounds like the device is damaged. Do you have another sample to try?

    If you don’t, we will run an experiment on Monday to make sure that we are not missing something obvious.  Simulation doesn’t show any problem either but I didn’t expect it to since I cannot see any problem with how you are using the device. Sorry for your troubles.


  • Thanks for your time! I really appreciate it. I posted the same question on stack exchange and they're suggesting that the way I supply the op-amp is questionable and could have caused a damage to the chip. Do you have any comment on this?
  • I would really appreciate it if you could run an experiment to test this circuit out. Unfortunately this one was the only op-amp that I had (It was a sample that I got from TI). I probably need to order a new one.
    Also, I posted the same question on stack exchange and they're suggesting that the way I supply the op-amp is questionable and could have caused a damage to the chip. Maybe adding bypass caps around R13/R14 or adding Zeners could further protect the device.
  • Hi Vahid,

    V2, the 60Hz sine, has it something to do with mains voltage? Mains voltage is a very dirty voltage being contaminated with lots of spikes and surges. Such surges could have destroyed the OPAmp.

    Then this 17V supply powering the OPAmp. Where does it come from?

    Vahid, it's extremely difficult to help, when we must guess...

  • Sorry Vahid
    our testing is showing much lower bias current than you are experimenting. In regards to what may have happened to the device, any circuit with voltages that exceed the operating voltage range of a device are always prone to cause problems if a pin gets inadvertently connected to one of those voltages. Limiting the voltage across the device with a zener diode is a common example. You already limit the current with series resistance which is appropriate.
  • Thanks! I added a zener diode (3.6V) across the resistors (from V+ to V-) so this way the total supply to the op-amp will not exceed 3.6V. I also added 0.1uf bypass caps to the supply pins of the op-amp (this was mentioned in the datasheet). Another thing that was brought up by kai klaas69 is the spikes in the input. I forgot to mention that the input may have spikes and that might have damaged the IC. Would adding bypass caps to the input (non-inverting pins) help in this case?
  • Yes V2 might have spikes. Thanks for pointing to this. Would adding bypass caps to the op-amp non-inverting pin avoid the spikes?
  • Hi Vahid,

    a good cure is to have a series resistor at the input which limits the input current to less 10mA. See section 8.3.2 of datasheet.

  • Vahid
    At this point I am going to close this post with the understanding that the input structure somehow got damaged. If any additional support is required, feel free to reopen or create a new post.