Howland circuits have been widely used as powerful source for exciting tissue over a wide frequency range. When a Howland source is designed, the components are chosen so that the designed source has the desired characteristics. However, the operational amplifier limitations and resistor tolerances cause undesired behaviors. This work proposes to take into account the influence of the random distribution of the resistors in the modified Howland circuit over the frequency range of 10 Hz to 10 MHz. Both output current and impedance of the circuit are deduced either considering or the operational amplifiers parameters. The probability density function due to small changes in the resistors of the circuit was calculated by using the analytical modeling. Results showed that both output current and impedance are very sensitive to the resistors variations. In order to get higher output impedances, high operational amplifier gains are required. The operational amplifier open-loop gain increases as increasing the sensitivity of the output impedance. The analysis done in this work can be used as a powerful co-adjuvant tool when projecting this type of circuit in Spice simulators. This might improve the implementations of practical current sources used in electrical bioimpedance.
 P. Pouliquen, J. Vogelstein and R. Etienne-Cummings, “Practical Considerations for the Use of a Howland Current Source for Neuron-Stimulation,” Proceedings of the IEEE Biomedical Circuits and Systems Conference, Baltimore, 20-22 November 2008, pp. 33-36.
 E. Basham, Z. Yang and W. Liu, “Circuit and Coil DESIGN for in-Vitro Magnetic Neural Stimulation Systems,” IEEE Transactions on Biomedical Circuits and Systems, Vol. 3, No. 5, 2009, pp. 321-331.
 K. Sooksood, T. Stieglitz and M. Ortmanns, “An Active Approach for Charge Balancing in Functional Electrical Stimulation,” IEEE Transactions on Biomedical Circuits and Systems, Vol. 4, No. 3, 2010, pp. 162-170.
 D. X. Chen, X. Deng and W. Q. Yang, “Comparison of Three Current Sources for Single-Electrode Capacitance Measurement,” Review of Scientific Instruments, Vol. 81, No. 3, 2010, pp. 1-3. http://dx.doi.org/10.1063/1.3367879
 R. A. Pease, “A Comprehensive Study of the Howland Current Pump,” 2008.
 J. Frounchi, F. Dehkhoda and M. H. Zarifi, “A Low-Distortion Wideband Integrated Current Source for Tomography Applications,” European Journal of Scientific Research, Vol. 27, No. 1, 2009, pp. 56-65.
 A. Keshtkar, Z. Salehnia and B. Shokouhi, “Bladder Cancer Detection Using Electrical Impedance Technique (Tabriz Mark 1),” Pathology Research International, Vol. 2012, 2012, pp. 1-5.
 P. Aberg, I. Nicander and S. Ollmar, “Minimally Invasive Electrical Impedance Spectroscopy of Skin Exemplified by Skin Cancer Assessments,” Proceeding of the 25th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Cancun, 17-21 September 2003, pp. 3211-3214.
 P. Aberg, I. Nicander, J. Hansson, P. Geladi, U. Holmgren and S. Ollmar, “Skin Cancer Identification Using Multifrequency Electrical Impedance-A Potential Screening Tool,” IEEE Transactions on Biomedical Engineering, Vol. 51, No. 12, 2004, pp. 2097-2102.
 D. H. Sheingold, “Impedance & Admittance Transformations Using Operational Amplifiers,” Lightning Empiricist, Vol. 12, No. 1, 1964, pp. 1-8.
 P. Bertemes-Filho, R. G. Lima, M. B. P. Amato and H. Tanaka, “Performance of an Adaptative Multiplexed Current Source Used in Electrical Impedance Tomography,” Proceeding of the 20th Brazilian Congress on Biomedical Engineering, Sao Pedro, 22-26 October 2006, pp. 11671170.
 F. Seoane, R. Bragós and K. Lindecranz, “Current Source for Multifrequency Broadband Electrical Bioimpedance Spectroscopy Systems. A Novel Approach,” Proceedings of the 28th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, New York, 31 August-3 September 2006, pp. 5121-5125.
 F Seoane, R Macías, R Bragos and K Lindecrantz, “Simple Voltage-Controlled Current Source for Wideband Electrical Bioimpedance Spectroscopy: Circuit Dependences and Limitations,” Measurement Science and Technology, Vol. 22, No. 11, 2011, pp. 1-11.
 P. Bertemes-Filho, B. H. Brown and A. J. Wilson, “A Comparison of Modified Howland Circuits as Current Generators with Current Mirror Type Circuits,” Physiological Measurement, Vol. 21, No. 1, 2000, pp. 1-6.
 P. Bertemes-Filho, L. H. Negri, A. Felipe and V. C. Vincence, “Mirrored Modified Howland Circuit for Bioimpedance Applications: Analytical Analysis,” Journal of Physics: Conference Series, Vol. 407, No. 1, 2012, pp. 18. http://dx.doi.org/10.1088/1742-6596/407/1/012030