Noise analysis of a Howland current source

We present a detailed noise analysis of a Howland current source that can supply up to 25 mA output current with a current gain of approximately 20. A modified topology is also considered which includes an additional compensation resistor at the non-inverting terminal of the amplifier to balance the input impedance when the amplifier is used with current gain. A lowest measured current noise of 13 pA/root Hz (for f > 40 Hz) is obtained for the balanced configuration. We find that the addition of a compensation resistor improves the thermal stability of the amplifier and this agrees well with theoretical predictions. Some care must be taken when matching the resistance ratios of the Howland current source as matching at the level of 1% results in a theoretical output impedance of approximately 22 k Omega for our design. The analytical expressions developed in this work should allow noise analysis to be performed for general resistance combinations.