AD8671 AD8672 AD8674 Analog Devices, AD8671 AD8672 AD8674 Datasheet - Page 13

• Manufacturer Part Number: AD8671 AD8672 AD8674
• Description: Precision, Very Low Noise, Low Input Bias Current Operational Amplifiers
• Manufacturer: Analog Devices

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.
The equal component topology yields a center frequency
where:
and
K
fo
= 1
=
Q
V
2
+
IN
π
=
BAND-PASS FILTER
RC
R
R
2
4
B
A
−
2.25kΩ
AD8671
2
R1
K
Figure 40. Band-Pass KRC Filter
1nF
C2
C2
1nF
2.25kΩ
R2
LOW NOISE OP AMP
AD8671
2.25kΩ
R3
V
V
CC
EE
R
R
03726-0-019
Figure 39. Simplified Block Diagram of a GPS Receiver
A
B
10kΩ
18kΩ
AD831
MIXER
Rev. A | Page 13 of 16
CODE GENERATOR
DEMODULATOR
AD630
The band-pass response is shown in Figure 41.
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; this minimizes
the thermal noise. A simple example is shown in Figure 42.
100
DETECTOR
LOW-PASS FILTER
V
PHASE
S
AD8610
= ±15V
Figure 42. PLL Filter Simplified Block Diagram
1k
Figure 41. Band-Pass Response
CHARGE
PUMP
IN
10k
AD8671/AD8672/AD8674
AD8369
D
VGA
Hz
100k
V
V
EE
10kΩ
CC
R1
AD10200
1nF
C1
1M
03726-0-018
ADC
03726-0-021
VCO
03726-0-025
10M