8.2.1.2 Detailed Design Procedure

The LOG300 supports a 5V VCC. Add a 10Ω and 10µF || 10nF close to the VCC pin to provide sufficient

decoupling and immunity from external noise. This supply filter has a pole of 1.59kHz that is sufficiently less than

the frequency of interest, which is 1MHz.

The absolute maximum voltage rating of pin LNA_In is ±1V. Add a back-to-back diode along with a series

resistor (RS) at the input of the LNA (see also Figure 8-1). The back-to-back diode protects the LNA_In pin

from being exposed to any high voltages, especially during the transmit operation. Choose the series resistance

value in accordance to the maximum power rating (PMAX) of the back-to-back diode. The added series resistor

contributes to the input noise and deteriorates the minimum input sensitivity.

Rs= 0.7V× Vmax−0.7V

Pmax

(2)

The maximum expected output voltage of the LNA with a back-to-back diode placed at the input is:

11V/V×0.7VP=7.7VP

(3)

Since the LNA is only powered from a 5V supply; the maximum output is only 2.5VP.

The maximum input for the Log_Inp pin is 1.7VP for 5VCC (see also Section 5.1); therefore, add a band-pass

filter (BPF) of appropriate attenuation in the pass-band region so that the detector block absolute maximum

voltage rating is not violated. In this particular case, ensure that the BPF has an attenuation of at least −3.3dB. A

BPF of −4.3dB is shown in Figure 8-1.

Choose an Offset_Cap value based on Section 7.3.1.

Choose the value of CF based on the required rise time of the Log_Out pin voltage (VLog_Out). A lower-value CF

improves the rise time at the cost of higher ripple on the output envelope. For reference plots see also Section

5.8. Connect an oscilloscope at the Log_Out pin, triggered during the receive burst operation, to find the correct

balance between the required rise time and the acceptable ripple.

The RF resistor decides the input-to-output slope. The value of RF in kΩ equals the input-to-output slope in

mV/dB. In this example, calculate RF using the below set of equations:

Slope mV/dB =RFkΩ= Saturatedoutputvoltage−Minimumoutputvoltage

20×log MaximumLNA_In/MinimumLNA_In

(4)

Slope mV/dB =RFkΩ= 4.5V−0.5V

20×log 200mV–7µV

(5)

Slope mV/dB =44mV/dB,henceuseRFkΩ=44kΩ

(6)

Note

The maximum and minimum Log_Out values have been relaxed to design for the output to operate

well within the linear range. RF accuracy effects the slope accuracy.

The voltage measured at Log_Out can be traced back to calculate the input amplitude using the below equation:

Log_OutA=Slope

×20×Log Log_InA

Log_InB +Log_OutB

(7)

Log_InA= 10 Log_OutA–Log_OutB

Slope×20

×Log_InB

(8)

Where : A stands for values of Log_Out and Log_In at the required measurement point and B stands for

Log_Out and Log_In values at a known input value measured during factory calibration or production.

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LOG300

SLOSEB7B – SEPTEMBER 2024 – REVISED DECEMBER 2024

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