Designed as an anti-aliasing filter for a 25 kSPS ADC front-end.
The objective was to attenuate frequencies above the Nyquist limit while maintaining a maximally flat passband response and low distortion.
Two cascaded 2nd-order Sallen-Key stages:
Input → Stage 1 (Q = 0.541) → Stage 2 (Q = 1.306) → Output
| Parameter | Value |
|---|---|
| Filter Type | Butterworth |
| Order | 4 |
| Cutoff Frequency | 10 kHz |
| Supply Voltage | ±12 V |
| Op Amp | OPA2134 |
| ADC Sampling Rate | 25 kSPS |
| Component | Value |
|---|---|
| R1A | 17.4 kΩ |
| R2A | 41.2 kΩ |
| C1A | 1 nF |
| C2A | 820 pF |
| Component | Value |
|---|---|
| R1B | 17.4 kΩ |
| R2B | 41.2 kΩ |
| C1B | 1 nF |
| C2B | 150 pF |
For each Sallen-Key section:
[ f_0 = \frac{1} {2\pi\sqrt{R_1R_2C_1*C_2}} ]
The Butterworth response is achieved by cascading two second-order sections whose pole locations correspond to:
| Stage | Q |
|---|---|
| 1 | 0.5412 |
| 2 | 1.3066 |
Although closed-form component values were calculated initially, final resistor and capacitor values were refined in LTspice to obtain the target 10 kHz Butterworth response.
See:
Measured overshoot and settling behavior match Butterworth expectations.
Input:
- 1 Vrms
- 1 kHz sinewave
Result:
- THD = 0.0011 %
(See results/thd_log.txt)
100-run component tolerance analysis confirms cutoff variation remains within design limits.
| Folder | Contents |
|---|---|
| calculations | Design derivations |
| ltspice | Schematics and simulations |
| results | Plots and logs |
| bom | Bill of Materials |
| docs | Full report |
-
Open LTspice.
-
Load:
ltspice/sk_lpf_4th_order.asc
-
Run AC analysis:
Simulate → Run
-
Run transient analysis:
Open simulations/step_response.asc
-
Run THD analysis:
Open simulations/thd_analysis.asc
-
Run Monte Carlo:
Open simulations/monte_carlo.asc
| Metric | Result |
|---|---|
| Cutoff Frequency | ~10 kHz |
| Filter Order | 4 |
| Response Type | Butterworth |
| Supply Voltage | ±15 V |
| THD | (from LTspice log) |
| Monte Carlo Runs | 100 |
| Op-Amp | OPA2134 |
Ekagra Maheshwari
IIT Guwahati B.Tech in Electronics & Electrical Engineering
Interested in Analog IC Design, Signal Chain Design, and Digital Systems.