The Log Detector block of the LOG300 supports both single-ended and differential input signals. Irrespective of the type of input signal, the impedances from the Log_Inp and Log_Inm pins to AGND have to be matched.

The requirement for impedance matching arises from the fact that any supply noise or externally coupled noise passes through these impedances and generates an input voltage. If the impedances are mismatched at the two inputs a differential voltage is seen by the Log Detector block thereby increasing the minimum output voltage at the Log_Out pin. This results in degradation of the dynamic range.

Impedance matching for differential input case can be easily achieved by replicating source impedance on both pins.

However for single-ended inputs, impedance matching can be quite complex. If board space is not limited TI recommends to copy the input structure at the Log_Inp pin as, is on to the Log_Inm input. Please take note of the default internal output impedance of the LNA of 1kΩ when copying the structure to the Log_Inm pin.

If the above recommendation is not possible, match the impedances between the two pins at the frequency with the highest probability of noise coupling. For example if the VCC supply is being derived by a DC/DC converter switching at a frequency of 500kHz, using option 1 gives the best impedance matching between Log_Inp and Log_Inm. If the VCC ripple is at 100kHz then both option 1 and option 2 result in the same rejection response.

AFE design with BPF of 1MHz

Figure 7-1 Impedance Structure at Log_Inp and Log_Inm

Figure 7-2 Impedance vs Frequency at Log_Inp and Log_Inm