Active Filter Noise Study
Submitted by admin on Sun, 11/30/2008 - 09:33
An often used active filter is a second order Butterworth realized using the Sallen and Key circuit. There are several sources on the net for designs of Sallen and Key filters, including the ECircuit Center.
The ECircuit page provides a SPICE file for the filter design which is reproduced here:
The noise table details the sources of noise in the circuit at the requested frequencies. Note that the op-amp dominates at very low frequencies due to the 1/f noise, but the resistors eventually produce most of the noise in the circuit.
OPSALKEY1.CIR - OPAMP SALLEN-KEY LOW-PASS FILTER * 2ND-ORDER BUTTERWORTH * VS 1 0 AC 1 * R1 1 2 11.2K R2 2 3 11.2K C1 2 5 2000PF C2 3 0 1000PF * * UNITY GAIN AMPLIFIER, RA=OPEN, RB=SHORT RA 4 0 100MEG RB 4 5 1 XOP 3 4 5 OPAMP1 * * SINGLE RC FILTER FOR COMPARISON R10 1 10 15.9K C10 10 0 1000PF * * OPAMP MACRO MODEL, SINGLE-POLE * connections: non-inverting input * | inverting input * | | output * | | | .SUBCKT OPAMP1 1 2 6 * INPUT IMPEDANCE RIN 1 2 10MEG * DC GAIN (100K) AND POLE 1 (100HZ) * GBWP = 10MHz EGAIN 3 0 1 2 100K RP1 3 4 1K CP1 4 0 1.5915UF * OUTPUT BUFFER AND RESISTANCE EBUFFER 5 0 4 0 1 ROUT 5 6 10 .ENDS * * ANALYSIS .AC DEC 10 100 1MEG * VIEW RESULTS .PLOT AC V(5) .PROBE .ENDThe Sallen and Key SPICE input file is readily converted to the corresponding CCICAP input file. Here we also add a noise analysis and create a Noise Table from the results:
.title OPSALKEY1.CIR - OPAMP SALLEN-KEY LOW-PASS FILTER * 2ND-ORDER BUTTERWORTH * .opt txt .include op27.mdl .ac 20 10 10. 1.MEG .plot ac png eout eout > .plot ac png erc erc > .plot ac png eout erc NDM INM ' noise from all elements .noise .plot ac png NDM INM 'integrated (total) noise table .n_table inte 20 .ckt v ein 1 0 1. * r R1 1 2 11.2K r R2 2 3 11.2K c C1 2 5 2000.PF c C2 3 0 1000.PF * * UNITY GAIN AMPLIFIER, oa a1 3 5 5 op27 * SINGLE RC FILTER FOR COMPARISON r R10 1 10 15.9K c C10 10 0 1000.PF * vm eout 5 0 vm erc 10 0 .END .go .stopThe response plot from the filter (dB and phase) and a plot of both spot (NDM) and total noise (INM) from the CCICAP run are shown here:
![]() |
OPSALKEY1.CIR - OPAMP SALLEN-KEY LOW-PASS FILTER Component Integrated Noise Table Noise data at T = 2.982E+02 K Frequency, total spot noise, total integrated noise: 1.0E+01 4.0E-08 0.0E+00 Component integrated noise table: R1 0.000E+00 R2 0.000E+00 a1 0.000E+00 R10 0.000E+00 Frequency, total spot noise, total integrated noise: 1.0E+02 2.4E-08 2.6E-07 Component integrated noise table: a1 1.845E-07 R2 1.288E-07 R1 1.288E-07 R10 0.000E+00 Frequency, total spot noise, total integrated noise: 1.0E+03 2.2E-08 7.1E-07 Component integrated noise table: R2 4.286E-07 R1 4.272E-07 a1 3.752E-07 R10 0.000E+00 Frequency, total spot noise, total integrated noise: 1.0E+04 2.2E-08 2.3E-06 Component integrated noise table: R2 1.579E-06 R1 1.265E-06 a1 1.046E-06 R10 0.000E+00 Frequency, total spot noise, total integrated noise: 1.0E+05 3.7E-09 3.3E-06 Component integrated noise table: R2 2.409E-06 a1 1.684E-06 R1 1.435E-06 R10 0.000E+00 Frequency, total spot noise, total integrated noise: 1.0E+06 3.0E-09 4.4E-06 Component integrated noise table: a1 3.320E-06 R2 2.479E-06 R1 1.435E-06 R10 0.000E+00
»
- 18604 reads