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• TI Thinks Resolved

# AFE4300: Internal Low-Pass filter bandwidth, settling time & R/C values

Part Number: AFE4300

1. In datasheet Pg no : 18  - setup by the two 1-kΩ resistors and an external capacitor)

1. In datasheet Pg no : 8   - BWDEMOD  specified as 3.5 ±20%  Hz with Internal resistor = 5 kΩ,. external capacitor = 4.7 μF

1. In datasheet Pg no : 5 – pin description

OUTM_I_FILT 47 — I channel demodulator low pass filter, connect 10 μF between both pins

OUTP_I_FILT 48

OUTM_Q_FILT 49  — Q channel demodulator low pass filter, connect 10 μF between both pins

OUTP_Q_FILT

1. In reference design schematics :   OUTM_I_FILT 47 & OUTM_Q_FILT 49   has a 10uF capacitor.

Q1. I presume the BWdemod of 3.5Hz is a function of RC value , 1/2*pi*RC ?– pl confirm

Q2. Is the internal resistor is 5 k or 1k ? contradicting in above references of datasheet.

Q3. Should the external Cap be 10uF or 4.7 uF? contradicting in above references of datasheet & reference design.

Q4. What is the LP filter output settling time?  When Input mux is switched the LP filter at the output of FWR (or IQ channels) it will need a certain settling time to reach the final output. This would depend on this bandwidth and order of the LP filter. What should be the settling time I should assume given my external cap is 10uF?

Q5.  Since I want a very fast scan of impedance measurement I cannot wait a long time bfore reading my ADC output register.

Q6. Kindly let me know if  I can assume settling time of  ts = 0.35/3.5Hz = 0.1 sec

• Hi Sundar,

Yes, the filter bandwidth is calculated as 1/(2*pi*R*C). Where R is the internal 5KOhm and C = 2*External Cap.
So if you have 4.7uF as external cap the filter bandwidth will be 1/(2*pi*5e3*9.4e-6) = ~3.38Hz.

Choice of external cap is a trade off between the settling time as noise. Bigger cap will give better noise reduction by lowering the filter bandwidth but it will increase the settling time ( as time constant = R*C) . Whereas lower cap will give better response time in expense of noise.
Hope this helps you to choose the cap.

Regards,
Prabin