The LMP7704-SP has a very-low input bias current, very-low input current noise, and very-low input voltage noise. As a result, this amplifier is an excellent choice for circuits with high-impedance sensor applications.

Figure 8-2 shows the typical input noise of the LMP7704-SP as a function of source resistance where:

• e_n denotes the input-referred voltage noise.
• e_i is the voltage drop across source resistance due to input-referred current noise or e_i = R_S * i_n.
• e_t shows the thermal noise of the source resistance.
• e_ni shows the total noise on the input, where:
  

Figure 8-2 Total Input Noise

The input current noise of the LMP7704-SP is so low that this noise does not become the dominant factor in the total noise unless the source resistance exceeds 300 MΩ, which is an unrealistically high value.

As is evident in Figure 8-2, at lower R_S values, total noise is dominated by the amplifier input voltage noise. If R_S is larger than a few kilo-ohms, then the dominant noise factor becomes the thermal noise of R_S. As mentioned previously, the current noise will not be the dominant noise factor for any practical application.