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Help with LMH6703

Adam Kopp
Prodigy 20 points

Other Parts Discussed in Thread: LMH6703, LMH730216

Hi,

I'm working with an LMH6703 op-amp (SOIC-8 configuration) and am having some issues.  I'm new to high speed/current feedback amplifiers, so any suggestions anyone can give me would be much appreciated.

I have a 10 MHz signal (coming from a crystal, at about 3V amplitude).  This signal is actually coming from another circuit, so I need to apply a buffer, or even 2V/V amplification so that I can rectify the signal for AM demodulation.  I have tried just about every configuration of the op-amps that I can think of, including the suggested circuits in the application section of the LMH6703 datasheet (both inverting and non-inverting).

I have tried different configurations with the shutdown pin (pin 8), applying ground, +2.5 V, and letting it float.  Applying ground seems to work as it should, as far as causing the op-amp to go into "shutdown mode", and making every input high impedance.  However, letting it float and applying +2.5 V (which should "activate" the op-amp) causes some weird effects.  For one thing, when I measure the input impedance (with a simple multimeter) between the inverting and non-inverting inputs, the non-inverting impedance is 8 kOhm - 20 kOhm (varying between several different LMH6703's), where it should be 1 MOhm.  The inverting impedance is really strange.  When I hook up my multimeter, it flashes between OL (inf. impedance) and another value (it happens too quickly to actually give me a number though).

At this point I'm really lost as to what to do.  The only hints I've gotten so far is that sometimes I'll see ringing at ~3-100 MHz at the output.  Right now, however, I'm seeing absolutely nothing at the output.  I've got everything grounded really well.  The only thing I'm doing "wrong", is that in the application section of the datasheet, it specifies a 6.8 uf one-way capacitor between ground and the power-rails,  I'm currently using a 1 uf capacitor because we don't have any 6.8 uf caps laying around, but I can't see that making a huge difference.

Thanks again for any help.  Here's a link to the datasheet:

http://www.ti.com/lit/ds/symlink/lmh6703.pdf

-Adam

  • 0
    TI__Mastermind 22825 points

    Hi Adam,

    If you are operating the LMH6703 with +/-2.5V supplies (you don't say it, I'm only guessing), it won't work since it needs +/-4V minimum (+/-5V nominal). The shutdown pin can be left floating for normal operation. So, you probably have other issues that is causing the shutdown misbehavior. You should attach a schematic of your circuit so we can discuss further.

    I recommend you obtain the LMH6703 EVAL board for your 1st time trouble shooting and debugging because the LMH6703's 1.2GHz bandwidth requires careful attention to layout, decoupling, signal routing or else you'll get oscillations, instability, or poor performance. Here is the LMH6703 EVAL board web site:

    http://www.ti.com/tool/lmh730216

    Hope this helps.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 20 points

    Hooman,

    Thanks for your reply.  I am supplying a +/- 5V supply to the op-amp.  The +2.5V is simply what I applied to the shutdown pin to try to activate the op-amp.  The datasheet said that to activate the op-amp, you must either leave the shutdown pin floating or apply > + 2.0 V, so when it didn't work floating, I tried applying the + 2.5 V signal, which yielded identical results.  At this point I think it is a decoupling (or some other transient effect) issue.  However, I'm pretty inexperienced with the practicalities of high speed circuitry, so if you have any tips for decoupling, I would be forever grateful.

    Other than than, I've been very careful to ground everything pretty well, and I've setup the circuit just like those specified in the applications section of the data sheet (you can refer to that for the schematic).  The only other idea I have is that I'm using a breadboard.  Could this be causing the problem?

    Thanks again, I really appreciate your time and efforts.

    -Adam

  • 0 in reply to Adam Kopp
    TI__Mastermind 22825 points

    Hi Adam,

    From your description, the 2.5V on the shutdown pin is correct and should put the device in normal operation. So, I'm not sure what is causing your issues!

    Please tell me which figure number schematic from the datasheet you are using? The device needs a feedback resistor, RF, of about 560ohm in any configuration (even Av=+1) because of its current feedback topology. The input Common mode voltage range is limited (~+/-1V). So, your 3V (peak, peak-to-peak?) crystal input may be violating that input range. Try limiting the input applied to +/-1V maximum and retest. Use a fast (< 2ns rise time) lab step generator for your initial trying to observe the output step response and watch for excessive overshoot or ringing. Perf boards (with holes you push hook-up wire into) won't work with a 1.2GHz bandwidth device. You need a continuous ground plane as well for any reasonable pulse / frequency response because ground inductance will get you in trouble. Try the EVAL board that can be ordered for this device. Also, power supply decoupling caps to ground should be less than 0.1" from the supply pins (use both aluminum or electrolythic and cermaic / low value types in parallel).

    Regards,

    Hooman