OPA699: opa699

Hello,

  Are you probing directly at the output of the amplifier? We recommend placing an isolation resistor at the output of the amplifier, then placing the probe or to an SMA connector. 

  Is this device on a dedicated PCB or breadboard? 

Thank you,
Sima

it's on the output of the amplifier,with an SMA connect to the oscilloscope, and this device is on a dedicated PCB

Hello,

  That would help with avoiding oscillations. We usually do not recommend using amplifiers as comparators due to limits at differential voltage, but this device does seem to not have restrictions with abs max diff voltage of +/-Vs. But, for future designs, we do recommend using a dedicated comparator, here is a link to our portfolio of comparators: https://www.ti.com/amplifier-circuit/comparators/products.html

  For this device, it could be an issue with loading the output, is the scope set to 1Mohm or 50 ohms? If the later, try changing it to 1MOhm. Next try, if possible, to place a small isolation resistor near the output pin of the device. 

Thank you,

Sima

Hi Sima

  the scope is setted to 1Mohm, but it did't work. and I have tried to put an 100ohm resistor near the output pin ,the oscillations seem to have reduced, but the rising edge is still slow? it looks like the limite works ,but the amplifier's slew rate did't wok

Hello,

  Thank you for trying out the suggestions. Would you be able to use the scope to calculate the rise-time 20/80% and the max amplitude voltage? I want to double check expected slew rate and device saturation time. 

Best Regards,
Sima

the raise-time is about 1us. the Vh pin is open ; the Vl is to the gnd. the power is ±5V. the max amplitude voltage is 3.6V.  

Hello,

  Thank you for the information. That is slower than we would expect. I will talk to my the design team, and get back to you by tomorrow. I think the device might be slow to saturation, but would think the limiter function would work as intended even when in non-linear region of the amplifier. I will need to know more about this device internal structure, and will update you with the results of the discussion. 

Best Regards,

Sima 

Hello,

  I apologize for the delay. I was able to talk to my team, and they brought up a good point to check input voltage. Voltage Feedback amplifiers' slew rate is dependent on the voltage across its inputs, so a small differential input step could result in a fraction of the data sheet's slew rate value. 

  What is your input voltage to the voltage divider? Would you be able to test with a 2V input at the input pin of the device (i.e. 4V to the equal resistor voltage divider)?

Thank you,
Sima