Universal dual filter assembly-EEWORLD

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Filters are needed in data acquisition, telecommunications, and noise reduction systems. Designing filters using filter components is convenient and saves components, space and time. Linear Technology's LTC1067 universal filter assembly is packaged in a 16-pin narrow SSOP and consists of two identical rail-to-rail second-order filter units. Each unit can be used with 3 to 5 resistors to design different filter functions, such as band-pass, high-pass, low-pass, notch and all-pass filters. Key features of this universal filter assembly are: biquad filter, rail-to-rail input and output, single 3V to ±5V supply, greater than 80dB dynamic range (single 3.3V supply), 100:1 clock-to-center Frequency ratio, 200:1 internal sampling-center frequency ratio, less than ±0.2% center frequency error, low noise (<40μVRMS, Q≤5), etc.

The block diagram of the LTC1067 universal dual filter is shown in Figure 1. Among them, V + and V - are the power terminals; INV A and INV B are inverting inputs; LPA, BPA, HPA/NA, HPB/NB, BPB, and LPB are outputs. Each second-order filter unit has 3 outputs; SA, SB are addition inputs; CLK is clock input; AGND is analog ground. Users can use LTC's FIL TERCAD software (Version 1.956 or later, this software can be obtained from Linear Technology's website www.linear-tech.com) according to the desired filter (cutoff frequency, ripple and stopband attenuation). It is easy to implement a second order filter.

Figure 2 is a single 3.3V power rail-to-rail fourth-order 5kHz bandpass filter designed using the LTC1067. The total output noise of this bandpass filter is 90μVRMS, and the signal-to-noise is 80dB. Figure 2(b) is the frequency response curve.

Figure 3 is a 1.02kHz notch filter designed for telecommunications systems using LTC1067. R51, R61, and R62 in the circuit are all 0.1% tolerance resistors. C21 and C22 improve the notch degree, and their value is determined by the following formula:

(30) (I NOTCH ) <1/2π (R2X) (C2X) < (75) (I NOTCH )

When C21 and C22 are not used, the notch degree is limited to -35dB. The input VIN of this notch filter is ≤1.25Vp-p.

See Table 1 for other optional filter components.

Table 1 Filter components

model	illustrate	Note
LTC1067-50	Low Power Low Noise Ratio Component Block Filter	50:1 clock-to-fo ratio
LTC1064	Low Noise Low Power Four Component Block Filter	100:1 clock-to-fo ratio
LTC1068	Four Universal Component Block Filters	100:1 clock-to-fo ratio
LTC1164	Low Power Quad Universal Component Block Filter	50:1 and 100:1 clock-to-fo ratio
LTC1264	High Speed Quad Universal Component Block Filter	20:1 clock-to-fo ratio