AD8671/AD8672/AD8674

Rev. C | Page 13 of 16

AD8671

BAND-PASS FILTER

LOW NOISE OP AMP

MIXER

DEMODULATOR

LOW-PASS FILTER

VGA

ADC

AD10200

AD831

AD8671

AD630

AD8610

AD8369

CODE GENERATOR

Figure 39. Simplified Block Diagram of a GPS Receiver

GPS RECEIVER

GPS receivers require low noise to minimize RF effects. The

precision of the AD8671 makes it an excellent choice for such

applications. Its very low noise and wide bandwidth make it

suitable for band-pass and low-pass filters without the penalty

of high power consumption.

Figure 39 shows a simplified block diagram of a GPS receiver.

The next section details the design equations.

BAND-PASS FILTER

Filters are useful in many applications; for example, band-pass

filters are used in GPS systems, as discussed in the previous

section. Figure 40 shows a second-order band-pass KRC filter.

18k

Ω

10k

Ω

2.25k

Ω

R3

2.25k

Ω

R2

2.25k

Ω

R1

1nF

C2

1nF

C2

Figure 40. Band-Pass KRC Filter

The equal component topology yields a center frequency

RC

fo

π

=

2

2

and

K

Q

−

=

4

2

where:

A

B

R

R

K

+

=1

The band-pass response is shown in Figure 41.

Hz

100k

1k

100

10k

1M

10M

Figure 41. Band-Pass Response

PLL SYNTHESIZERS AND LOOP FILTERS

Phase-lock loop filters are used in AM/FM modulation.

Loop filters in PLL design require accuracy and care in their

implementation. The AD8671/AD8672/AD8674 are ideal

candidates for such filter design; the low offset voltage and low

input bias current minimize the output error. In addition to the

excellent dc specifications, the AD8671/AD8672/AD8674 have

a unique performance at high frequencies; the high open-loop

gain and wide bandwidth allow the user to design a filter with a

high closed-loop gain if desirable. To optimize the filter design,

it is recommended to use small value resistors to minimize the

thermal noise. A simple example is shown in Figure 42.

10k

Ω

R1

1nF

VCO

C1

CHARGE

PUMP

PHASE

DETECTOR

IN

D

Figure 42. PLL Filter Simplified Block Diagram