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MAX7034 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX7034 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 14 page Data Filter The data filter is implemented as a 2nd-order lowpass Sallen-Key filter. The pole locations are set by the com- bination of two on-chip resistors and two external capacitors. Adjusting the value of the external capaci- tors changes the corner frequency to optimize for differ- ent data rates. The corner frequency should be set to approximately 1.5 times the fastest expected data rate from the transmitter. Keeping the corner frequency near the data rate rejects any noise at higher frequencies, resulting in an increase in receiver sensitivity. The configuration shown in Figure 1 can create a Butterworth or Bessel response. The Butterworth filter offers a very flat amplitude response in the passband and a rolloff rate of 40dB/decade for the two-pole filter. The Bessel filter has a linear phase response, which works well for filtering digital data. To calculate the value of C5 and C6, use the following equations, along with the coefficients in Table 1: where fC is the desired 3dB corner frequency. For example, to choose a Butterworth filter response with a corner frequency of 5kHz: Choosing standard capacitor values changes C5 to 470pF and C6 to 220pF, as shown in the Typical Application Circuit. Data Slicer The data slicer takes the analog output of the data filter and converts it to a digital signal. This is achieved by using a comparator and comparing the analog input to a threshold voltage. One input is supplied by the data filter output. Both comparator inputs are accessible off- chip to allow for different methods of generating the slicing threshold, which is applied to the second com- parator input. The suggested data slicer configuration uses a resistor (R1) connected between DSN and DSP with a capaci- tor (C4) from DSN to DGND (Figure 2). This configura- tion averages the analog output of the filter and sets the threshold to approximately 50% of that amplitude. With this configuration, the threshold automatically adjusts as the analog signal varies, minimizing the possibility for errors in the digital data. The values of R1 and C4 affect how fast the threshold tracks to the analog ampli- tude. Be sure to keep the corner frequency of the RC circuit much lower than the lowest expected data rate. Note that a long string of zeros or ones can cause the threshold to drift. This configuration works best if a cod- ing scheme, such as Manchester coding, which has an equal number of zeros and ones, is used. To prevent continuous toggling of DATAOUT in the absence of an RF signal due to noise, add hysteresis to the data slicer as shown in Figure 3. C k kHz pF C k kHz pF 5 1 000 1 414 100 3 14 5 450 6 1 414 4 100 3 14 5 225 . .. . . = ()( )( )( ) ≈ = ()( )( )( ) ≈ Ω Ω C b ak f C a kf C C 5 100 6 4 100 = ()( )( ) = ()( )( ) π π 315MHz/434MHz ASK Superheterodyne Receiver 10 ______________________________________________________________________________________ RSSI RDF1 100k Ω RDF2 100k Ω C5 19 DFO 21 OPP 22 DFFB C6 MAX7034 FILTER TYPE a b Butterworth (Q = 0.707) 1.414 1.000 Bessel (Q = 0.577) 1.3617 0.618 Figure 1. Sallen-Key Lowpass Data Filter Table 1. Coefficents to Calculate C5 and C6 |
Similar Part No. - MAX7034_09 |
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Similar Description - MAX7034_09 |
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