Trying to work with LMH6628 op-amp

[itayd100]

Hello,

I have the LMH6628 op-amp and I'm trying to get it going.

I have a 10MHz sine wave from signal generator and I want to amplify that (start with that and move to crystal output).
I connect the op-amp to 5+ and GND and make the circuit like the picture.


I can't make it work.
Any idea?

Itay

 

[FvM]

The appended circuit is useless for LMH6628 or similar 100 MHz class OPs in nearly any regard. Consider a feedback network with kOhm or less resistance level, also bias the non-inverting input to mid-supply.

 

[andre_luis]

As far as I see, this part requires symetrical power suply voltages, not ground referenced as you did.

 

[itayd100]

Hey,
I can't understand.
What could be a circuit to work with this op-amp with a reasonable gain?

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I saw in the specs: "Wide Supply Voltage Range: ±2.5V to ±6V" --> 0 to 5V should be ok, not?

https://www.ti.com/lit/ds/symlink/lmh6628.pdf

 

[andre_luis]

The preceding '±' symbol denotes a symmetrical value varying from 2.5 to 6.0 volts. Moreover, the pinout designators +Vcc/-Vcc reinforces this chance.

 

[itayd100]

Hey @andre,

I tried the attached circuit, but nothing happened - the signal from the op-amp is 0V.

 

[andre_luis]

the signal from the op-amp is 0V

For what value of Vin ?

 

[itayd100]

for 2Vpp is 0V.
for 3Vpp is jumping.

 

[FvM]

Resistor values are still too large for this OP (should try 100 ohm/ 1kohm), but not the reason why you get no output.
Why don't you show a complete schematic, including source connection and pin numbers?

It's basically possible to supply the OP with +5V only, but the circuit must be modified respectively.

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for 2Vpp is 0V.
for 3Vpp is jumping.

Input AC coupled without bias resistor?

 

[andre_luis]

The datasheet isn't too clear regarding to maximum allowed input voltages, but I wouldn't recommend apply 2Vpp when the Vcc is just ±2.5v.

 

[betwixt]

The big clue is just above all the graphs in the data sheet. Note the values for the load and feedback resistors.

If you are using a split supply, the input pin should be ground referenced, try capacitve coupling with a resistor from the input pin to ground. If you are using a single supply, bias the non-inverting input at half supply voltage and break the DC link to ground in the feedback resistors by adding a capacitor in series with the ground side of the shunt resistor. At 10MHz you absolutely must add decoupling acoss the supply pins too.

Brian.

 

[itayd100]

Hey FvM,

I connected 150ohm and 1.5Kohm.
I attached the schematic and the picture from the oscilloscope.



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Hey Brian,

Thanks for the answer.
I was lost in the middle.... Could you help me with schematic and values?

thanks...

 

[KlausST]

Hi,

did´nt you say you use +/-2.5V. Now I see 0V, 5V. Feedback is 0V referenced.
--> as been said earlier: This won´t work.

Please try as explained in the posts.

Klaus

 

[itayd100]

Hey Klaus,

I changed it to +/-2.5V.
And added capacitor of 1nF between the input and pin 3(+ of the op-amp).

And again, I just want any setup that will make it work. If I'm doing something wrong, just tell me. Still new in analog circuits.

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[FvM]

Dual versus single supply configurations

 

[itayd100]

Hey FvM,

Thanks for your schematics.
I tried the one with the single power supply and that's what I get:

 

[FvM]

Some explanation would be fine.

If yellow trace is input and light blue output voltage and both have same scaling factor, we would expect different waveforms.