LOG200EVM: Current sources within EVM

Wouter Kragt

Intellectual 1930 points

Part Number: LOG200EVM
Other Parts Discussed in Thread: LOG200

Tool/software:

Hi PSC team,

Company x needs to measure very low currents into the pA, for that they are evaluating the LOG200EVM and have some questions, could you please have a look at them?

Now I have some questions about the internal (1uA) current-source of the EVM.

They need to measure very low currents down to 1pA (there they can have a bigger tolerance). For that they should go in to the board with a lower current on I2 (for example 100pA).

Should they put the 100pA with an external source, or can they use the 1uA current-source and make the 100pA with that?

If they have on the VCM node, the REF25, is it also possible to have a load resistor on the I2 to GND and then a part of the source current flows into that path?

Thank you.

Kind regards,

Wouter Kragt

over 1 year ago

0 Robert Clifton56 over 1 year ago

TI__Mastermind 28810 points

Hi Wouter,

Wouter Kragt said:
Should they put the 100pA with an external source, or can they use the 1uA current-source and make the 100pA with that?

Both options are valid. You'll need a resistor for either approach. But that also means the accuracy of the current source will be heavily influenced on the accuracy of the resistor.

Wouter Kragt said:
If they have on the VCM node, the REF25, is it also possible to have a load resistor on the I2 to GND and then a part of the source current flows into that path?

That's how I would subtract the current flowing into the I2 pin.

Best Regards,

Robert Clifton

0 Robert Clifton56 over 1 year ago in reply to Robert Clifton56

TI__Mastermind 28810 points

Hi Wouter,

I forgot to mention that it can be difficult to achieve a highly accurate reference current that's as low as 100pF. Another option would be for the customer to select an IREF value that is closer to the center of the expected current signal range.

Best Regards,

Robert Clifton

0 Wouter Kragt 11 months ago in reply to Robert Clifton56

TI__Intellectual 1930 points

Hi Robert,

They came with this solution, see figure and asked the following:

"Is it ok to use the internel 1uA IRef and two Resitors for example 1kR and 1MR to split the current ?

In that way you it should also be possible to have a reference of around 100pA ore 1nA at I2."

Do you think this will work?

Thank you.

Kind regards,

Wouter Kragt

0 Robert Clifton56 11 months ago in reply to Wouter Kragt

TI__Mastermind 28810 points

Hi Wouter,

I have not tried doing this on the LOG200 but the math seems to suggest that it should work as you aren't violating any of the datasheet specs.

Best Regards,

Robert Clifton

0 Wouter Kragt 11 months ago in reply to Robert Clifton56

TI__Intellectual 1930 points

Hi Robert,

Thank you for the feedback. I am taking it offline for my further questions.

Kind regards,

Wouter Kragt

0 Wouter Kragt 11 months ago in reply to Wouter Kragt

TI__Intellectual 1930 points

Hi Robert,

Would the approach below work? I think it could.

Kind regards,

Wouter Kragt

0 Robert Clifton56 11 months ago in reply to Wouter Kragt

TI__Mastermind 28810 points

Hi Wouter,

Was there a problem with the resistors?

I've been running some simulations on this. First, I noticed something weird with the I2 pin in the simulation.

Now I've recreated the circuit and found that it works but the input current into I2 is negative. I think that's from a simulation error. I'll talk with the model designer about this. 2772.LOG200EVM Current sources within EVM.TSC

Also since you are using the EVM, it might be easier to wire it up the LOG200 auxiliary amplifier. Unless they need it for something else in their evaluation process.

Best Regards,

Robert Clifton

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LOG200EVM: Current sources within EVM