cc2540 op amp input bias current current

Jay Hung

Prodigy 230 points

Other Parts Discussed in Thread: CC2540

Hello,

Do you know the spec of input bias current of internal op amp.? I can't get the data in datasheet.

Please reply, thank so much!

over 10 years ago

0 svendbt over 10 years ago

TI__Mastermind 29650 points

Hi Jay,

The opamp has a high impedance input as long as you don't have an external negative feedback from the output driving back into the input.

Regards,
Svend

0 Jay Hung over 10 years ago in reply to svendbt

Prodigy 230 points

Hi,

The internal op amp in our application is unit gain op amplifier. It is with an external negative feedback from the output driving back into the input. Is it available used as unit gain op amp?

0 BugKing over 10 years ago in reply to Jay Hung

TI__Intellectual 1600 points

Yes that should be fine, the opamp is unity gain stable.

0 Jay Hung over 10 years ago in reply to BugKing

Prodigy 230 points

As you mention before, could you let me know the around number of input impedance and input bias current?

In my application, the op positive input is from 3V divided voltage with a several mega ohm resistor. The large divided resistor is for power saving, but I have concern about op input driving. Please see attached figure and feedback the answer.

Thanks!

0 BugKing over 10 years ago in reply to Jay Hung

TI__Intellectual 1600 points

Hi Jay,

The input bias current of the opamp is simulated to be around 100pA so I think the 1M resistor should be fine.

I'm wondering do you need the battery sense voltage for external purposes or could you just use the battery monitor included in the CC2540 ADC? Or is it that this 3V supply you are monitoring is not connected to the CC2540?

Regards,

0 Jay Hung over 9 years ago in reply to BugKing

Prodigy 230 points

Hi BK,

Our production feature should have battery monitor. I need divided resistor due to cc2540 operating on 2.1V. For power saving, the divided resistor prefer to much bigger. I can't feed the divided voltage to ADC directly becuase the input bias current of ADC looks more than op amp. So I use unit gain op amp as a buffer, then feed to ADC.

0 BugKing over 9 years ago in reply to Jay Hung

TI__Intellectual 1600 points

Hi Jay,

That makes sense. Another option which some people use is that when they want to read the voltage they use a GPIO to activate a switch. The switch then allows current to flow through a low resistor divider. In this scenario you won't be burning current all of the time. You can see a schematic illustration in this post, but you have to be careful to select low enough resistors since the ADC impedance is 197k.

http://e2e.ti.com/support/low_power_rf/f/538/p/335327/1170097.aspx#1170097

Regards,

0 Jay Hung over 9 years ago in reply to BugKing

Prodigy 230 points

Hi BK,

Thanks for your sharing so much! The idea with switching path for battery sense is good. I will think about that.

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cc2540 op amp input bias current current