DESIGN NOTES Number 16 in a series from Linear Technology Corporation October, 1988 Switched-Capacitor Low Pass Filters for Anti-Aliasing Applications Richard Markell Nello Sevastopoulos INTRODUCTION Many signal processing applications require a front end low pass fil- ter to bandwidth limit the signal of interest. This filter is often crucial to the system designer since it determines the number of bits which the system can resolve by its noise and dynamic range. Until now, the designer rejected the use of switched-capacitor filters as being too noisy, having too much distortion, or because they were not usable at a high enough frequency. The LTC1064-1 8th order Cauer filter can compete directly with the discrete operational ampiifier design. Not only that, but the cost and performance advantages are tremendous. The LTC1064-1 is a complete 8th order, clock tunable Cauer (also known as elliptic) low pass switched-capacitor filter with internal thin film resistors. The passband ripple is +0.1dB and the stopband attenuation at 1.5 times the cutoff frequency is 72dB. The device is available in a 14-pin DIP or 16-pin surface mount package, The LTC1064-1 boasts internal thin film resistors factory adjusted to optimize the Cauer 8th order response. The LTC1064-1 attains wide- band noise (2kHz-102kHz) of 150uVpus and a total harmonic distor: tion of 0.03% for Vin=3Vpys. No external components are required for cutoff frequencies up to 20kHz. For cutoff frequencies over 20kHz two small value capacitors are required to maintain passband flatness. By way of comparison, older switched-capacitor filters had noise in the millivolts, THD in the percents, and maximum corner frequencies limited to <20KHz. This note compares the performance of the LTC 1064-1 8th order Cauer filter with internal thin film resistors to that of the equivalent filter built with operational amplifiers. The LTC1064-1 quad switched- capacitor filter competes favorably with op amp RC designs in most parameters of interest to the designer and wins easily when printed circuit board space is considered. Since it is tunable, the 1701064-1 can replace not just one, but many op amp RC designs, if multi-fre- quency filtering is required. The specification comparisons become even more favorable to the LTC1064-1 as the frequencies become higher. COMPARING THE LTC1064-1 WITH RC ACTIVE FILTERS UTILIZING OPERATIONAL AMPLIFIERS Performance The Cauer filter has target design specifications as follows: a cutoff frequency of 40kHz, + 0.05dB passband ripple and a - 72dB attenua- tion at 1.5 times the cutoff frequency. This filter is realized with stop- band notches and it is considered a quite complex and selective filter realization. Figure 1 details the frequency response of this design. An 8th order active RC was designed using a fully inverting state vari- able topology. This topology is considered state-of-the-art for active filters since all non-inverting inputs of the op amps are grounded. The discrete active RC version of the Cauer filter is quite complex requir- ing 16 op amps, 31 resistors and 8 capacitors. The op amps used for this comparison were TLO84 quad FET input amplifiers. The circuit topology was optimized to yield the maximum useful input voltage swing. Test Results Figure 1 shows the frequency response of the LTC1064-1 connected as shown in Figure 3. The shape of the frequency response of the ac- live RC state variable filter was very similar and its differences cannot Vout/ Vin Vin=1V AMPLITUDE (dB) 10 2 40 100 = 200 INPUT FREQUENCY (kHz) 1000 Figure 1. LTC1064-1 Frequency Response DN16-1 be easily shown here. Figure 2, curve (a), details the TLOB4 state vari- able filter response near the 40kHz cutoff frequency. Laboratory tweaking of resistor values could not produce any better response than shown here. This is a passband ripple of approximately 2d8(V) a (a); TLOS4 FILTER 10B/01V ~ #04008 (b); LTC1064-1 FILTER RIPPLE ~ +0.1508 -6 15kKHz 40kHz 65kHz Figure 2. Passband Ripple +0.45dB. For comparison, the LTC1064-1 passband ripple is +0.15dB as shown in Figure 2, curve (b}. This is for a clock to center frequency ratio of 100:1, or a 4MHz clock. The measured filter amplitude re- sponse al 1.5 times the cutoff frequency for the TLO84 active RC filter was about ~ 65dB while that of the LTC1064-1 was - 68dB. The noise for the TLO84 state variable implementation was 111Vams while that for the LTC1064-1 was 145pVays. Second harmonic distortion mea- surements were also made on both filters and they are included on the summary chart, Table 1. Table 1 compares the LTC 1064-1, the switched capacitor implementa- tion of the 8th order Cauer low pass filter, to the active RC. Both cir- cuits operate with dual + 7.5V supplies or a single 15V supply, System Considerations Not only does the LTC1064-1 compare favorably on individual speci- fications, but it wins easily when system considerations are evaluated. Suppose four sharp cutoff frequencies are needed. The Table 1. 8th Order Cauer (Elliptic) LPF with a 40kHz Ripple Bandwidth closest active RC solution is a 7th order single cutoff frequency Cauer filter. Four of these non-tunable devices (each a 2 x3" hybrid) would be required for the four cutoff frequencies. This would be 24 square inches of PC board space. The discrete approach using operational amplifiers requires even more space. Since the LTC1064-1 is tunable, four frequencies can be selected merely by tuning the clock to the LTC1064-1. A complete LTC-1064-1 system with tunable clock is esti- mated to occupy oniy 4 square inches of board space. This is a whop- ping savings of 6 times in board area. The LTC1064-1 wins easily in this category. SUMMARY In summary it can be seen from Table 1 that the LTC1064-1 is the equal of the active RC filter. In the pure specification battle there is no clear winner, but when the amazing difference in hardware complexity, the full clock tunability and the simple method of application of the LTC1064-1 device are all considered it is the sure winner. Figure 3. The LTC1064-1, Monolithic 8th Order Cauer Low Pass Filter Operating with a 4MHz Clock and Providing a 40kHz Cutoff Frequency WIDEBAND] DISTORTION | Vos MEASURED EXT | #EXT | #EXT NOISE, | Vin=1Vans,3ams | OUT| Isuppy |ATTENUATION PASSBAND OP AMPS|Rs, 1% CAPS, 5% (TUNABLE! RMS (dB) (m}s|_ (mA) | AT60kHz | RIPPLE | TRIMMING? RC Active TLO84 16 | 31 |B Na -87, -87 55 | 33 66dB | + 0.45dB Yes LTC1064-1| None! | None | 1 Yes | 145,V -70, -70 30 | 18 | 68dB | +0.18dB None Note 1: An output inverting buffer (LT118} was used for driving cables during measurements. Note 2: To obtain the + 0.45dB ripple for the TLO84, 3 resistors were trimmed. Note 3: The output offset voltage numbers are as measured by DVM with the input of the filter grounded. Note 4: Measurement BW (2kHz-102kHz). For literature on our filter products, call 800-637-5545. For help with an application, call (408) 432-1900, Ext. 361. DN16-2 LT WEAR