Many sensors have high source impedances that may range up to 10 MΩ. The output signal of sensors must often be amplified or otherwise conditioned by means of an amplifier. The input bias current of this amplifier can load the sensor output and cause a voltage drop across the source resistance, as shown in Figure 8-4, where V_IN+ = V_S – I_BIAS * R_S.

Figure 8-4 Offset Error Due to I_BIAS

The last term, I_BIAS * R_S, shows the voltage drop across R_S. To prevent errors introduced to the system due to this voltage, an op amp with very low input bias current must be used with high impedance sensors. An amplifier with low input bias also has low input current noise, further improving the accuracy of systems with high source resistance.

Figure 8-3 shows one channel of the LMP7704-SP configured for a gain of 25. A high source impedance is placed between the input signal and the noninverting input of the amplifier to represent the output impedance of the sensor.

With the ultra-low input bias current of the LMP7704-SP, even with a signal source that has high output impedance, the system output maintains very good linearity to the ideal output voltage (that is, the output of an ideal amplifier in the same configuration). Figure 8-5 shows the output voltage vs input voltage of the LMP7704-SP with a 10-MΩ source impedance. Figure 8-6 shows the output voltage vs input voltage for an ideal amplifier with no input bias current. Comparing the two graphs shows that the LMP7704-SP maintains high accuracy even with a large source impedance connected to an input.