9.2.2 High-Impedance Sensor Interface

Many sensors have high source impedances that may range up to 10 MΩ, or even higher. The output signal of sensors often must be amplified or otherwise conditioned by an amplifier. The input bias current of this amplifier loads the sensor output and causes a voltage drop across the source resistance, Figure 34 shows (V_(+INx) = V_S – I_(BIAS) × R_(S).)The last term (I_(BIAS) × R_(S)) shows the voltage drop across R_(S). To prevent errors introduced to the system as a result of this voltage, use an op amp with low input bias current with high-impedance sensors. This low current keeps the I_(BIAS) × R_(S) error contribution less than the input voltage noise of the amplifier so that input voltage noise is not the dominant noise factor. The OPAx355 op amps feature low input bias current (typically 200 fA), and as a result, a preferred choice for these applications.

Figure 34. Noise as a Result of I_(BIAS)