THS3491: Gain setting

ttd

Mastermind 9395 points

Part Number: THS3491
Other Parts Discussed in Thread: DAC3482, THS3217

Dear Technical Support Team,

How do I extimate the gain such as G=8 for d Noninverting amp?

THS3491 is current feedback, so resistor value is important.

I use RF=576ohm and RG=82ohm for G=8. Is it correct?

How about G=6 and G=7, G=9 for example?

This is my TINA TI simulation.

THS3491_Non_invert_G=8_250M_Sine.TSC

Best Regards,

ttd

6 months ago

0 Ignacio Vazquez Lam1 6 months ago

TI__Mastermind 22860 points

Hi,

For current feedback devices, the value of Rf does play a part in the stability/behavior of the device however choosing between 576ohms and 499ohms will not make much of a difference so using 576ohms will be fine for your circuit. The concern is when the suggested value is 576ohms in a certain gain, but someone uses 100ohms. In gains of 6,7, and 8V/V I would use 576ohms for Rf as this should be fine. If you would like to extend bandwidth with a certain configuration you could reduce the Rf resistor slightly (ex. ~499ohms) and see if this response is better, or if you want to reduce peaking (at the expense of bandwidth) you could increase the Rf resistor. However as long as you're around the recommended, most current feedback devices will behave appropriately.

Best Regards,

Ignacio

0 ttd 6 months ago in reply to Ignacio Vazquez Lam1

Mastermind 9395 points

Hi Ignacio,

Thank you for you reply.

My configuration is below.

Max Power output： 20 dBm (6.325Vpp)

Frequency： 30MHz(min) and 250MHz(Max)

Input Voltage: 1Vpp form DAC3482(IOUTP/N → I/V →Transformer(1:1 , Differential to SingleEnd, 1Vpp max )→THS3491→ 20 dBm (6.325Vpp) 50Ω load)

I suppose that this configuration can be implemented with THS3491. If you have any concerns, please advise.

Do you have any application note or reference design?

Best Regards,

ttd

0 Ignacio Vazquez Lam1 6 months ago in reply to ttd

TI__Mastermind 22860 points

Hi,

I do not think the THS3491 would have an issue these frequency requirements. I would like to mention our THS3217 as well. This device was designed specifically to interface with current output DACs. 250MHz might be towards the limit of its bandwidth performance at 6.3Vpp but it's worth considering due to its integrated features which allows interfacing with a current DAC much easier. As far as collateral I am attaching some I have found on this subject.

Interfacing Op Amps to High-Speed DACs, Part 1: Current-Sinking DACs

Interfacing op amps to high-speed DACs, Part 2: Current-sourcing DACs

Passive Interface for Current Output DACs

Wideband Complementary Current Output DAC Single-Ended Interface (Rev. A)

Best Regards,

Ignacio