9.2.3 Rx Block

The Rx block consists of Rx PGA1, the Rx Filter, and Rx PGA2. Both Rx PGA1 and Rx PGA2 are high-performance programmable gain amplifiers. Rx PGA1 can be configured through the SPI to operate as either an attenuator or in gain. The gain steps of the Rx PGA1 are 0.25 V/V, 0.5 V/V, 1 V/V, and 2 V/V. The gain steps of the Rx PGA2 are 1 V/V, 4 V/V, 16 V/V, and 64 V/V. Configuring the Rx PGA1 as an attenuator (at gains less than 1 V/V) is useful for applications where the presence of large interference signals are present within the signal band. Attenuating the large interference allows these signals to pass through the analog Rx signal chain without causing an overload; the interference signal can then be processed and removed within the microprocessor as necessary.

The Rx Filter is a very low noise, unity-gain, fourth-order low-pass filter. The Rx Filter cutoff frequency is selectable between CENELEC A or CENELEC B, C, and D modes. The Control1 Register bit location 3 setting (CA CBCD) determines the cutoff frequency. Setting Control1 Register bit location 3 to '0' selects the CENELEC A band; setting Control1 Register bit location 3 to '1' selects the CENELEC B, C, and D bands. Because the Rx Filter is a very low noise analog filter, two external capacitors are required to properly configure the Rx Filter. Table 3 shows the proper capacitance values for CENELEC A, B, C, and D bands. Capacitor Rx C1 is connected between pin 24 and ground, and Rx C2 is connected between pin 23 and ground. For the capacitors shown, it is recommended that these components be rated to withstand the full AV_DD power-supply voltage

Figure 31 illustrates the recommended connections for the Rx signal chain.

1. For capacitor value C₁, f is the desired lower cutoff frequency and R_IN,PGA1 is the input resistance of Rx PGA1.

2. For capacitor value C₂, f is the desired lower cutoff frequency and R_IN,PGA2 is the input resistance of Rx PGA2.

3. Refer to Table 3.

Figure 31. Recommended Connections for Rx Signal Chain

As Figure 32 shows, a fourth-order passive passband filter is optional but recommended for applications where high performance is required. The external passive passband filter removes any unwanted, out-of-band signals from the signal path, and prevents them from reaching the active internal filters within the AFE031.

1. For capacitor value C, f is the desired lower cutoff frequency and R_IN,PGA1 is the input resistance of Rx PGA1. Refer to Table 3.

Figure 32. Passive Bandpass Rx Filter

The following steps can be used to quickly design the passive passband filter. (Note that these steps produce an approximate result.)

1. Choose the filter characteristic impedance, Z_C:
   • For –6-db passband attenuation: R₁ = R₂ = Z_C
   • For 0-db passband attenuation: R₁ = Z_C, R₂ = 10 ● Z_C
2. Calculate values for C₁, C₂, L₁, and L₂ using the following equations:

Table 4 and Table 5 shows standard values for common applications.

The Rx PGA1, Rx Filter, and Rx PGA2 components have all inputs and outputs externally available to provide maximum system design flexibility. Care should be taken when laying out the PCB traces from the inputs or outputs to avoid excessive capacitive loading. Keeping the PCB capacitance from the inputs to ground, or outputs to ground, below 100 pF is recommended.

Figure 33 shows the complete Rx signal path, including the optional passive passband filter.

1. For capacitor value C1,f is the desired lower cutoff frequency and R_IN,PGA1 is the input resistance of Rx PGA1.

2. For capacitor value C2,f is the desired lower cutoff frequency and R_IN,PGA2 is the input resistance of Rx PGA2.

3. Refer to Table 3.

Figure 33. Complete Rx Signal Path (with Optional Bandpass Filter)