THS4503: OPA627+THS4503

Amy Luo

Part Number: THS4503
Other Parts Discussed in Thread: OPA627, OPA656, LMH6551, THS4541

Hi team,

The voltage follower is constructed by OPA627, and then connected to THS4503 amplifier. It is correct to test the output of OPA627 separately. When the input signal is directly connected to the THS4503 amplifier circuit, the output is also correct. But when OPA627 voltage follower is connected with THS4503 amplifier, the output is not correct. I'd like to know why and how to fix it output correctly. The attachment is Tina simulation file.

I consider the output resistance (55 Ω) of OPA627. The amplifier circuit of THS4503 adopts the parameters in Table 4 of the datasheet.

diff0822.TSC

Best Regards,
Amy Luo

over 3 years ago

0 Michael Steffes over 3 years ago

Guru 47435 points

You don't need that extra resistor to ground on the OPA627 output. Also, that is a pretty slow device for this, you might look at the OPA656 if using +/-5V there. You will be losing the DC precsion of the OPA627 in the FDA stage

Also, the R's a bit high around the FDA, here is a better solution I think,

0 Michael Steffes over 3 years ago in reply to Michael Steffes

Guru 47435 points

I see now what you mean by not working, the FDA output is way attenuated, I can't see a set up problem, might be the model? What is your maximum output differential Vpp that you need and max frequency out of the FDA,

0 Hasan Babiker31 over 3 years ago

TI__Genius 15555 points

Hey Amy,

Not entirely sure what the issue was, but I rebuilt the circuit on a new file and the attenuation issue looks to be fixed.

diff0822_updated.TSC

Best,

Hasan Babiker

0 Michael Steffes over 3 years ago in reply to Hasan Babiker31

Guru 47435 points

l ran your file Hasan, looks to be still attenuating to me?

0 Michael Steffes over 3 years ago in reply to Michael Steffes

Guru 47435 points

I had simplified this circuit also and made some necessary changes - even dropped in the LMH6551 model, still attenuating - so I went the THS4541 model which I know works, and it pops in fine at gain of 20dB. I also swapped out the OPA627 for the OPA656, comparative response shapes here,

And then the TINA files,

OPA656 with THS4541.TSC

OPA627 with THS4541.TSC

0 Hasan Babiker31 over 3 years ago in reply to Michael Steffes

TI__Genius 15555 points

Hey Michael,

That's strange, this doesn't appear for me anymore with the most recent file I posted.

Best,

Hasan

0 Hasan Babiker31 over 3 years ago in reply to Hasan Babiker31

TI__Genius 15555 points

Looks like this model doesn't function properly under default TINA parameters. For the THS4503 model to work, you must go into analysis parameters & set "Always Starts with Source Step" to yes.

Best,

Hasan Babiker

0 Michael Steffes over 3 years ago in reply to Hasan Babiker31

Guru 47435 points

Good to know, but I can't find that option?

0 Hasan Babiker31 over 3 years ago in reply to Michael Steffes

TI__Genius 15555 points

Hey Michael,

You can see the full list of options by clicking on the hand on the bottom right and clicking on "View All"

Best,

Hasan Babiker

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THS4503: OPA627+THS4503