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THS3001: 3001HV version Oscillation at 125MHz and current increases to 70mA on negative pulse >6V (12mA load)

steve smith1
Intellectual 340 points
Part Number: THS3001

Hi This is my first time using the 3001HV version which I'm trying because it has the built in cooling pad. I'm using the part as a differential amplifier with a +/-15V supply, gain of 2.5, Vicm of 2.5V  and Rfb = 500 Ohms 20 Ohms R out then a few tens of pF capacitive loading. Layout is pretty tight with supply decoupling really close to power pins Rfb comprising two 0805 resistors close to part with minimal C loading on -input. What I'm seeing is in the positive direction it works fine with output up to 12.5V as expected. On the negative side though it only works correctly to -6V output (12mA load) beyond that it suddenly flips into an alternative state where I see 125MHz 0.5V pk-pk superimposed onto say a -7V pulse and the supply current increases to 70mA on both rails. If I then reduce the pulse back to even 1V the oscillation and high current remains. Switching power off and back on restores the part to it's correct function at lower pulse amplitudes. To begin with I thought I'd blown the first part probing the fine pin pitch or that the solder tab wasn't anchored and the part had overheated but not so problem repeats after 3 different parts. I know 3001 can reliably produce +/-10V with a +/-15V supply and 500 Ohm Rfb so what is the issue and workaround ? Is it the die problem that Michael has mentioned ? Either way what is the recommended fix?  I'm assuming it is a part issue because effect is on negative output only so less likely to be layout etc. This is a current production part so TI must have a solution for this.

  • 0
    TI__Mastermind 23049 points

    Hello Steve,

    I agree this behavior is troubling; I do have some action items I can take on my end to investigate.  In the interim, I did review various output characteristics curves on the THS3001 datasheet.  I am not sure if you have tried this, but have you tried keeping the same gain settings but using a larger Rf & Rg?  I am not sure that would explain the single-sidedness of the negative swing being the issue, but I would be remiss not to ask.

    Otherwise, I will take a look at your description.  If it came to it, would you be willing to share a portion of the schematic or layout, related to the supplies for the device?

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Intellectual 340 points

    Hi Alec, Rout = 20 ohms with 10pF shunt C, removing the C helps issue but on the edge of stability. I also noticed is the engineer who designed PCB had forgotten to add ground relief around Vin- which may not be the issue but doesn't help. Finally since I posted i tried increasing Rf to 750 Ohms which appears to fix the problem. Vales that caused the issue Rin- = 200R, Rf = 500R, Rs+ = 200R, R+ to ground = 500R. Rout =20R with, Cs = 10pF, output then taken from Rout/Cs node via 30R resistor to coaxial cable 210pF open circuit at load end. Supply is +/-15V. Noise gain = 3.5. V- signal gain is x2.5.

    Hopefully that's clear enough to picture schematic. My guess is for Noise gain=3.5 the optimum value for Rf is somewhere around 578R but let me know.

  • 0 in reply to steve smith1
    TI__Mastermind 23049 points

    Hello Steve, 

    Thank you for your response.  I am in the lab today & tomorrow, but I will take a look over your circuit details as I can do so.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Intellectual 340 points

    Hi Alec,

    Were you able to reproduce the issue and do you have any comments.

    Best,

    Steve

  • 0 in reply to steve smith1
    TI__Mastermind 28915 points

    Hello Steve,

      Since this is a current feedback amplifier architecture, feedback resistor values are critical for stability and even overall frequency response. This is the reason why the oscillations disappeared when you increased the resistor values. Here are some figures that match your observations in the lab:

      Lower feedback values even at the same gain increase peaking in the frequency response which means the overall configuration's phase margin is lower and, thus, lower stability. 

      Our current feedback amplifiers datasheet contains a table of recommended resistors at some popular gains:

      However, these types of amplifiers are pretty good at driving capacitive loads and as higher output power stages. Most likely removing the capacitive loading increased stability by lower the peaking from the low value resistors. 

    Thank you,
    Sima