In this noise measurement experiment, filters with closely-matched time constants at the reference path and at the input path are used. The diagram in Figure 3-3 shows a typical four-wire configuration using matched RC filters.

Figure 3-3 Example of a Four-Wire Ratiometric Configuration with RC Filters (R _Lead Removed for Simplicity)

The resistance at the input is swept from 0 Ω to 250 Ω to produce a full-scale voltage of 250 mV. Resistors R1 and R2 are selected to be 6.04 kΩ in order to reduce the effect of the resistance change of the RTD on the input frequency. Using Equation 3, the differential-mode filter corner frequency at the input path can be calculated as:

Equation 9.

The differential-mode input corner frequency changes very little (258 Hz to 253 Hz) as the sensor resistance changes from 0 Ω to 250 Ω (full-scale). Using Equation 6, the differential mode filter corner frequency at the reference path can be calculated as:

Equation 10.

The differential mode filter corner frequency at the reference path is approximately 260 Hz.

The input-referred noise of the ADS1248 is typically 0.350 µVrms when the device is configured with a PGA gain of 8 V/V with a data rate of 20 SPS. The full-scale range in this case is 250 mV. The reference path and input path filter corner frequencies are closely matched; therefore, the input-referred noise remains constant as the differential voltage increases. Figure 3-4 shows the input-referred noise in the measurement versus input differential voltage.

Figure 3-4 Input-Referred Noise vs Input Voltage Using Matched RC Filters