source resistance

In DC coupled applications, input impedance is not as important as input current and its voltage drop across the source resistance.

Johnson noise may be reduced by lowering the temperature of the source resistance and by decreasing the bandwidth of the measurement.

After the bandwidth and temperature, the remaining factor in determining the system noise is the effective source resistance.

As shown in Figure 2-2, the input bias current (IBIAS) develops an error voltage across the source resistance (RS).

The effective resistance is the parallel combination of the source resistance and the feedback (or shunt) sensing resistance.

Table 2-3 summarizes minimum recommended source resistance values for various measurement ranges for a typical feedback ammeter.

Note that the recommended source resistance varies by measurement range because the RF value also depends on the measurement range.

In current measurements, the source resistance and the sensing resistance both contribute noise.

hence, the source resistance value usually determines the Johnson noise voltage.

Figure 2-52 shows Johnson noise voltage versus source resistance for various bandwidths (or rise times) at room temperature.

For example, if the desired accuracy is 1%, then the meter resistance must be more than 100 times the source resistance.

The input resistance is normally much larger than the source resistance;