LT1787 current monitor when low voltage?

[eem2am]

Hello,

On page 8 of the followiwng, fig 6 shows how LT1787 can be used to monitor current flow from supplys that are lower than 2.5V.

http://cds.linear.com/docs/en/lt-journal/LT1787_0999_Mag.pdf

How is this and why does it monitor from lower voltage supplys than fig 8 on page 9?

 

[daGenie]

The buffer there helps to isolate the output of the lt1787 and the input of the ltc1404. Due to the high impedance of a buffer, the amount of current needed to drive the input is relatively small and therefore can monitor very small changes in its input. If you consider it using Ohm's law, a very high value of R with a very small changes in the value of I would produce a very large change in the value of V. (V = IR).

Without the buffer, the output of the lt1787 would depend on the input impedance of the next stage that it tries to drive.