AFE5818_15 Datasheet, PDF (64/156 Pages) Texas Instruments – AFE5818 16-Channel, Ultrasound, Analog Front-End with 140-mW/Channel Power, 0.75-nV/√Hz Noise, 14-Bit, 65-MSPS or 12-Bit, 80-MSPS ADC, and Passive CW Mixer

AFE5818
SBAS687B – FEBRUARY 2015 – REVISED AUGUST 2015
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When this feature is enabled, the overall transfer function of the CW summing amplifier across frequency is as
shown in Figure 95.
0
-3
Fc_HPF
Fc_LPF
-6
-9
-12
-15
-18
-21
-24
-27
100
1k
10k
100k
Frequency (Hz)
1M
D066
Figure 95. CW Summing Amplifier Transfer Function
The high-pass corner and low-pass corner shown in Figure 95 is given by Equation 7 and Equation 8,
respectively:
Fc_hpf = R_s / (2 × π × R_h × C_h × 2000 Ω)
(7)
Fc_lpf = 1 / (2 × π × R_s × C_s)
(8)
The following rules can be used to calculate the value of different components:
1. R_s is the resistor value determined by how much signal gain is needed.
2. C_s is selected depending upon the value of Fc_lpf.
3. R_h is determined by the amplitude of the undesired low-frequency signal required to be rejected. For
instance, suppose that without enabling the CW HPF feature, the peak-to-peak amplitude of the low-
frequency signal at the CW summing amplifier output is given by V_opp. Then the value of R_h has the
relationship shown in Equation 9. The previous constraint occurs because the CW HPF amplifier output can
only swing up to 4 V_pp.
R_h < 4 × R_s / V_opp
(9)
NOTE
Higher resistance values of R_h provide better noise performance.
4. C_h is the selected value of this capacitor depending upon the value of Fc_hpf.
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