LMH6503: Circuit Input Impedance

Dennis Brown

Part Number: LMH6503
Other Parts Discussed in Thread: LMH730033

A question about using the LMH6503 VGA in a high impedance application.

In the app note AN-1808 LMH730033 Eval board, this is a circuit listed page two. It is meant as a 50ohm load to the preceding stage.

If one wants to use this circuit at at higher load impedance (with the same noninverting single ended input configuration) and R1 becomes 1kohm, does R2 need to follow suit? Or is it ok for R2 to stay at 50ohms?

over 3 years ago

+1 kai klaas69 over 3 years ago

Guru 140200 points

Hi Dennis,

have you noticed figure 65 of datasheet?

To remain the symmetry of input stage I would make both inputs to see exactly the same impedances.

Kai

+1 Sima Jalaleddine over 3 years ago

TI__Mastermind 28865 points

Hello Dennis,

I agree with Kai, it is best to also change R2 to 1kohm as well. If you keep R2 at 50 ohms, you might see a some offset voltage errors due to the input bias current. Also, if your application requires minimal offset, I would look at nulling method in Figure 61 as well.

Thank you,

Sima

0 Dennis Brown over 3 years ago in reply to kai klaas69

Prodigy 230 points

Hi Kai,

Yes I did see that figure before but since it was configured for a +/-Vin, so I thought maybe it would be a bit different if running single ended. In any case I will make both resistors the same. Thanks for the help.

0 Dennis Brown over 3 years ago in reply to Sima Jalaleddine

Prodigy 230 points

Hi Sima,

Thank you for the comments. I will proceed as both you and Kai have stated with same resistor values.

dennis brown

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LMH6503: Circuit Input Impedance