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:

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

100k

Ω

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