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THS6002: THS6002 Output Issues

Part Number: THS6002

My customer is seeing the following issues with the THS6002:

I'm using this part for its high-speed/bandwidth current FB capabilities, but not for an ADSL application. I targeted this part for its "quad" packaging as I need 64-channels of analog output in a minimum footprint. I also ran several p-spice sims to evaluate various parts for this challenging application.

After implementing the design, I'm seeing strange "recovery" type behaviors which are triggered by full-scale negative going steps (only) that are followed by an approximate 2 usec period of random reduced amplitude output.  The part exhibits no problem with similar full-scale positive edge step responses.

I am using the parts (trying) to create a uni-polar 0-to-12V output with a small (hi-Z) load. I'm running them with a 1.2K FB resistor in a  6 gain configuration to achieve a 12V output from a 2V p-p input. The worst case signal input is effectively a 25 MHz square wave from a DAC running at 25 MHz clock rate.

Thinking the "negative only" behavior was perhaps a supply headroom issue, I increased the negative supply rail as far as -15V (initially designed for  15/-5V). The increased supply headroom has no affect on the strange behavior.

The 0-2V input waveform has some ringing (overshoot/undershoot) and a fast rise/fall time of approximately 2 nsec.

Is this asymmetrical behavior possibly due to some kind of internal saturation issue specific to the THS6002 device?  What can I do to correct it?

Unfortunately, the circuitry is at my clients facility and I did not think to capture any scope trace pictures earlier that I can send you. It will be next week before I can do that.

I have attached schematics of both the source signal buffers and the amplifier circuits annotated with notes.  I realize the whole configuration is far from ideal, but its what I was given (as a design consultant) to make system “speed improvements” on.  I have provided a re-design of the video buffer cards as well as new amplifier cards that are arranged in two-card stacks with ribbon cable interconnects.  Each card contains 32-channels of signal amplification.

Both the buffer and amplifier pcbs are 8-layer with all input/output signal traces on controlled impedance internal layers (stripline) between ground and power plane layers. In a rush to get the pcbs finished, I did not take the time to add plane layer cutouts around the amp input pins to minimize capacitance.  Since the components were so close and traces so short, I gambled that wouldn’t be necessary. 

Also, prior debug steps have included:

Temporary increase of decoupling (handheld on amp pins) – no observable changes noted.

Reduced FB resistor values (and gain) slightly – no improvement, actually made erratic response worse.

Maximized power supply headroom – no improvements noted.

Added stray capacitance on input pins (via fingers etc) – no observable changes noted.

The amp behavior doesn’t really look like an instability since its only associated with negative transitions? I would think a phase induced oscillation would be present all the time, instead of just on negative step responses?  I have since noticed in the datasheet the 70ns “settling time” for 01%, and some rather low BW numbers for “full power” operation. Even these do not explain a 2 usec recovery time after a full-scale negative transition.

I have attached his schematics.

THS6002 Output Issues_THS6002 Amp Sch.pdf

THS6002 Output Issues_Video Buffer Input Sch.pdf

My question is whether he can set up a gain of 6 and still be able to meet the specifications in the data sheet. The only references to gain that are given are either 1x or 2x. Is he having a bandwidth issue when the fast negative signal appears on the input? The recovery time also seems odd, but there is nothing specified in the data sheet regarding the recovery from an over-voltage situation. He is driving the outputs near to the rail.

I am sorry I cannot provide a scope shot of this behavior right now, but I hoped that you might be able to diagnose the problem by just seeing the schematics and an explanation of the circuit.

Please let me know if you have additional questions for the customer.

Thanks for your help with this!

Richard Elmquist

13 Replies

  • I have further information from the  customer:

    I did manage to capture some scope traces this afternoon.  As you can imagine, the behavior was extremely difficult to trigger the scope on and capture a stable trace. Since the amp outputs aren’t perfectly repeatable, most traces just look like random garbage.  But I’ve put together some slides with annotations pointing out and answering some of your questions hopefully.

    THS6002 scope.pdf

     The “negative” edges I was referring to aren’t negative voltage. I just meant (digitally speaking) falling edges or “negative going”.  The op-amp inputs are 0-2Vp while the needed outputs are 0-12Vp.  I’m hoping the designed power rails of +14V and –5V provide enough headroom to prevent saturation from that perspective.  I have temporarily increased the negative supply to –14V (as a check) and that made no change or improvement in the amplifier’s outputs.

     I have also discovered a different test image that improved the DC offset a bit (between true/inverse image periods) such that both falling and rising edge steps were a bit closer to “full-scale”. That image managed to break the amps output all the time.  In that case (sorry I didn’t capture a pic), the amps were recognizably trying to produce the “ramp” signal waveforms, but with reduced effective gain.  In other words, the signal output was only swinging between 3-4V on the low side and 8-9V on the high side.  The slew rate was still plenty fast, just lost amplitudes. Not sure I would recognize that as a BW issue, but there’s obviously some amp limitation.

     Bottom line question for your designers – is the THS6002 capable of doing this?  I chose it based on seemingly adequate BW specs in the datasheet, before noticing the relatively pathetic “full power BW” numbers in the details.  If this is not the correct part, what would your recommendation be to achieve this pipe-dream? 

    This looks to me like the op amp is saturating and then takes a while to recover. I have asked him to increase Vcc to +/- 15V and see if this has any effect. I believe that this might also be a byproduct of not enough bandwidth as the customer asks.

    Please let me know what you think. Let me know if you have any further questions for the customer.

    Thanks for your help with this!

    Richard Elmquist

  • In reply to Richard Elmquist:

    Hi Richard,

    To answer your questions in the first post, it is possible to set up the THS6002 for a gain of +6 and still be able to meet the specifications in the data-sheet. If you look at Figure 30 and Figure 51 in the datasheet, they both give the pulse response for G=+5 with Vo = 20Vpp at tr/f = 5nsec.

    Regarding the recovery scenario, the min output voltage swing tested on +/-15V supply is from +11.8V to -11.5V (see page 4 in the datasheet). If the THS6002 output swing is from 0V to 12V in the customer's application, I don't think this is a hard enough saturation. In such a scenario, the recovery from this saturation should be quick for both the positive and negative going input transitions. One way of quickly verifying that it is indeed the saturation that is causing the issue is by increasing the positive supply voltage. Would it be possible for the customer to increase the positive supply voltage to say +17V or +20V keeping the negative supply voltage at -5V and see if the issue goes away?

    To answer your questions in the second post, it is possible that the THS6002 does not have sufficient bandwidth as the customer mentions. However, the reduced BW would show up for the positive transition as well. So, I am not quite sure whether the reduced BW could really relate to the negative transition issue that the customer sees. One possibility of increasing the BW is by reducing the Rf value to 680 ohms and accordingly scale the Rg to 136 ohms for G=+6 to see if the Trise/fall of the output transition decreases.

    It also seems that the LMH6739 outputs directly drive a ribbon cable interface via 73-ohm pcb stripline and then connect to the THS6002 input. Usually, ribbon cables are notorious for doubling the source voltage due to signal reflections if there is not adequate source termination. As a result, would it be possible to include some series resistance (on the order of 30ohms to 73ohms) at the LMH6379 output to make sure there are no signal reflections at its output? You could try scaling the THS6002 gain accordingly to compensate the loss in those series resistors at the LMH6379 output.

    Best Regards,

    Rohit

  • In reply to Rohit Bhat:

    Rohit,

    Thanks for your response!

    I will send your comments to the customer and will let you know if this fixes the issue.

    Thanks again.

    Richard Elmquist

  • In reply to Richard Elmquist:

    Rohit,

    Here are some further scope shots once the customer has expanded the supply voltage to +/- 15V.

    THS6002 scope2.pdf

     Here are the customer’s comments:

    Here’s some scope pics of today’s testing results.  I re-wired the THS6002 amps directly to the system’s +/-15V power supplies. The amplifiers seemed to behave a small bit better in that the problem responses were more stable/periodic and it was easier to trigger the scope and capture repetitive traces.  Otherwise, the increased supply headroom did not have a dramatic effect.

     Some of the zoomed in scope pics reveal what looks more like a BW issue when the amp is not responding to the small incremental waveform steps during the “recovery” periods either. The small stair-steps look to be filtered into a smooth ramp response.  But again this is only after a large falling edge step!

     Hopefully your factory engineer can make some sense out of this.  I left a tech today attempting to haywire a “filter” on a few of the LMH6739 inputs to try and get rid of some of the high frequency ringing out of them. I also received some components today for setting up some individual part breadboards to more conveniently play with gains and such. It will be next week before I can report much in results from these efforts.

     Question – is it possible that parasitic pcb capacitance on the THS6002 input pins could be causing this type of behavior?

     Could it be that the supply voltage was still not high enough?

     Could the trace capacitance possibly be causing the issue?

     Or does this appear to be a BW issue?

     Thanks for your help with this!

    Richard Elmquist

  • In reply to Richard Elmquist:

    Rohit,

    The customer has done some further testing and the results are shown in the attached file;

    THS6002_scope_3.pdf

    Here are his comments on this:

    I have re-created the THS6002 circuit on a breadboard setup that allows somewhat easier tweaking.  The chip is on an SOIC-to-DIP adapter pcb with pins plugged into an old (classic) breadboard and components plugged in as close as possible. I’m not sure the parasitics are much better, but they are at least different!

    I managed to accidentally capture the amp doing something similar to my production pcb, but surprisingly it was triggered by varying the input signal amplitude!  The attached charts capture an entire sequence of scope pics showing the circuit response to inputs from 0-1Vp up to 0-2.1Vp.  The amp behaved reasonably up to around 1.5Vp when it started saturating/oscillating similar to before.  Increasing the input to 1.8V stabilized the response, but the output amplitude was greatly reduced and with a positive offset.  Bumping the input up to around 2.1Vp seemed to snap the amp out of its reduced amplitude mode and almost back to expected levels but still with a small positive offset.

    What is going on??   Insights from the factory experts will be appreciated.

    I can do another pcb layout to improve parasitics or decoupling, etc., if that’s the issue. But if this behavior is due to a part limitation I need to know soon so I can pursue a different amplifier component.  Perhaps the THS3092 can do a better job for this application?

    He has increased the supply voltage to +/-15V. I am going to suggest that he increase it even higher.

    Do you have any idea of what is causing this? I can understand the loss of amplitude if the op amp cannot gain up the signal properly, but it seems from his test that it varies and that an offset is also seen. Can you reproduce this? Would the THS3092 possibly be a better option?

    Please let me know if you have any further questions for the customer.

    Thanks for your help with this!

    Richard Elmquist

  • In reply to Richard Elmquist:

    Hi Richard,

    Let me take a look and get back with a reply or alternative soon.

    Best Regards,
    Rohit
  • In reply to Rohit Bhat:

    Hi Richard,

    Looking at the scope capture waveforms in THS6002_scope2 pdf file, it is possible that the fast falling edge issue seen at the THS6002 output could be related to the internal thermal shutdown of the part. In the Applications Section of the THS6002 datasheet, it is mentioned that the part has a built-in thermal shutdown protection feature if the internal junction temperature rises above 180'C. Does the customer have any heat-sink on the part that dissipates heat out of the package, or is it only the bottom thermal pad that conducts the heat? If there is no heat-sink on the part, I would try the below experiment with a heat-sink to see if the issue goes away.

    I think the issue is more related to thermal shut-down rather than BW because if I look at the below scope capture, the two input and output waveform cycles track each other somewhat. However, the waveform in-between them and subsequently after the second good waveform, it seems the THS6002 goes into an unknown state indicating the part could be turned-off until the internal junction temperature cools down. Also if it was BW limited, I would expect both the two good waveforms to be distorted as well. In addition to that, it seems one channel worked good based upon the comments in THS6002 scope pdf file.

    For increasing the supplies to +/-15V, I don't think this scenario is any different than the +15V & -5V supply because the output really does not go down to the negative rail. A better experiment would be is to make the THS6002 supply as +20V & -5V which will eliminate any possibility of the output saturation that couples with the customer's issue. However, since the thermal shutdown still exists with +20V & -5V supply case, the customer should still see the distorted waveform indicating thermal shutdown to be the main culprit.

    I think going forward in the next design, it would be better to switch to THS3092 because there are lesser devices per package which will help in keeping the internal junction temperature lower. Also, it has higher large signal BW compared to the THS6002 which will make the rise/fall edges sharper. However, before switching to the THS3092, I would still perform the heat sink experiment on the existing board to make sure it is indeed the thermal shutdown that's causing the problem. The customer could also try powering on only one out of the eight devices to see if the board heating is causing any trouble.

    Best Regards,

    Rohit

  • In reply to Rohit Bhat:

    Rohit,

    Thanks for your response!

    I has not thought of the thermal effect. This could be the issue.

    I will have the customer try the tests that you recommend and I will let you know if he has any further questions.

    Thanks for your help!

    Richard Elmquist

  • In reply to Richard Elmquist:

    Rohit,

    Please see the customer’s comments below:

    I’m having difficulty believing the behavior is due to thermal shutdown due to actual thermal issues.  On my production pcb’s the parts are well heats inked through the “power pad” on the bottom and soldered to two internal ground planes.  As a result the parts get a bit warm to the touch and the pcb warms up globally as well indicating that the thermal pads are working. There are no additional heat sinks on the top of the packages as it doesn’t seem necessary.  The parts only get a bit warm to the touch (I guess around 40C) so I can’t imagine the junction temperatures are anything close to 180C!  Perhaps the thermal shutdown circuitry is sensitive to power supply noise or something else?

    On my breadboard, I have increased the supply rails to both  +17/- 17V as well as +20/-10V with no effect on the output response.  No headroom issues evident.

    I’ve also varies the waveform frequency several magnitudes.  The output looks good at really slow (10KHz range) but then it has plenty of time to “recover” from any edge.  When the frequency gets close to 1MHz it starts exhibiting problems all the way to 20MHz (which is the max range on my signal generator).  The response varies a bit over the range, but so does the input waveform due to reflections.  I think the main effect of frequency is slight variation of amplitudes/reflections on the input.

    Doesn't sound like the factory engineer has seen or responded specifically to the last set of pics I sent you showing the input amplitude sensitivity??  I would be very interested in their take on that.

    I have suggested that he try the THS3092 since he is bread boarding this, It seems to me that the bread board could be causing some of the issue that he is seeing. Can you look at the comments and let me know if you have any additional questions or comments?

    Thanks for your help with this!

    Richard Elmquist

  • In reply to Richard Elmquist:

    Rohit,

    Here are some further comments from the customer about this issue.

    On my pcb’s I have no practical way to selectively power off some of the amps.  It’s all or none unfortunately. We had played with some “cold spray” on some of them and “freezing” them did have some effect on the output waveforms, but did not “fix” the shutdown looking issue.

    My breadboard chip does get much warmer since it is not connected to any heatsink. As a result, I have been limiting its operation to a few minutes at a time.

    I did procure a TI eval board for the THS3092 in preparation for having to proceed with a potential re-design. I can power that up and check its operation for this application.

    Meanwhile, I will continue to play with the breadboard THS6002 chip to see if I can discover any other correlations to the behavior other than signal amplitude shown in the “THS6002_scope_3” file.

    Please let me know if you have any further replies that I can send to the customer.

    Thanks for your help with this!

    Richard Elmquist