AnsweredAssumed Answered

AD8418, use with second amp to minimize Rsh?

Question asked by apullin on Sep 2, 2015
Latest reply on Sep 4, 2015 by apullin

Hi there,

 

I was just looking at the AD8418 part for an application in bidirectional current sensing in a brush DC motor drive.

For this application, board size and part count are pretty sensitive, so this part looks pretty good.
The only thing that I am unsure of it: since the amplifier gain is only 20, is it common practice to use a current sense amp like this one with a second opamp with a high gain?

 

For this application, it's a (-4.2,+4.2)v drive (Hbridge, single Li cell), and the current range for the motor is +-2A.
The micro and AD8418 Vs would be at 3.3v. So, if I want to cover the full range and get (0,3.3)v output for +-2A, then I'd need a ~40 mOhm shunt, I believe? Then the shunt power dissipation is a little high, above 1/8 W, and wasteful of an already small battery.

 

So, what's the best practice here? Use a 10mOhm (or less) shunt and have another 4x (or more) opamp on the output, to bring it back to approx 0-3.3 full scale, before going into the ADC?


If there's a part that is better suited and/or an app note that is relevant for this, please let me know.

 

Thanks,

Andrew

Outcomes