improved Howland current source - differences in configurations

Discussion in 'The Projects Forum' started by knoximator, Apr 25, 2016.

  1. knoximator

    Thread Starter New Member

    Apr 25, 2016

    i'm building a precision current source for some sensors in my university lab.
    i've decided to go with the improved howland current source for it's simplicity and minimal number of components.
    my current configuration is based on using a difference amplifier, shorting the sense with the output, using a "current gain" resistor, with feedback going back to the 'ref' pin. (see attached fig1)

    now, i've managed to stumble upon the application note of the lt1990 where they use it as a current source.
    looking at the inside configurations of the chip, you notice that the 'ref' pin has two smaller valued resistors connected to each input of the op-amp. (see spec sheet, or fig2 )

    now, i've managed to simulate both circuit configurations in ltspice, with minimal differences (one circuit has a slightly higher output current vs input voltage gradient)

    my question is, (disregarding resistor values) what effect do these two resistor in the lt1990 have and is one configuration superior to the other?