AD9874BST AD [Analog Devices], AD9874BST Datasheet - Page 23

• Manufacturer Part Number: AD9874BST
• Description: IF Digitizing Subsystem
• Manufacturer: AD [Analog Devices]

Based on these characterization curves, a LNA/mixer bias setting
of 3_3 is suitable for most applications since it will provide the
greatest dynamic range in the presence of multiple unfiltered
interferers. However, portable radio applications demanding the
lowest possible power may benefit by changing the LNA/mixer
bias setting based on the received signal strength power (i.e., RSSI)
available from the SSI output data. For instance, selecting an
LNA_Mixer bias setting of 1_2 for nominal input strength condi-
tions (i.e., <–45 dBm) would result in a 4 mA current savings
(i.e., 18% reduction). If the signal exceeds this level, a bias setting
of 3_3 could be selected. Lastly, refer to the Typical Performance
Characteristics (TPCs) for more performance graphs character-
izing the LNA and mixer’s effect upon the AD9874’s noise and
linearity performance under different operating conditions.
A 16 dB step attenuator is also included within the LNA/mixer
circuitry to prevent large signals (i.e., > –18 dBm) from overdriving
the - modulator. In such instances, the - modulator will
become unstable, thus severely desensitizing the receiver. The
16 dB step attenuator can be invoked by setting the ATTEN bit
(Register 0x03, Bit 7), causing the mixer gain to be reduced by
16 dB. The 16 dB step attenuator could be used in applications
in which a potential target or blocker signal could exceed the IF
input clip point. Although the LNA will be driven into compression,
it may still be possible to recover the desired signal if it is FM.
Refer to TPC 7c to see the gain compression characteristics of the
LNA and mixer with the 16 dB attenuator enabled.
Address
(Hex)
0x00
0x01
0x01
0x03
REV. 0
Figure 11c. LNA/Mixer Frequency Response vs. Bias Setting
Table IX. SPI Registers Associated with LNA/Mixer
–1
–2
–3
–4
–5
–6
–7
–8
0
0
Bit
Breakdown
(7:0)
(7:6)
(5:4)
(7)
100
1_2 SETTING
LNA_MIXER
FREQUENCY – MHz
200
Width
8
2
2
1
LNA_MIXER
3_3 SETTING
300
Default
Value
0xFF
0
0
0
400
Name
STBY
LNAB
MIXB
ATTEN
500
–23–
Band-Pass Sigma-Delta (
The ADC of the AD9874 is shown in Figure 12. The ADC contains
a sixth order multibit band-pass - modulator that achieves very
high instantaneous dynamic range over a narrow frequency band.
The loop filter of the band-pass - modulator consists of two
continuous-time resonators followed by a discrete-time resonator,
with each resonator stage contributing a pair of complex poles.
The first resonator is an external LC tank, while the second is an
on-chip active RC filter. The output of the LC resonator is
ac-coupled to the second resonator input via 100 pF capacitors.
The center frequencies of these two continuous-time resonators
must be tuned to f
center frequency of the discrete-time resonator automatically scales
with f
MXOP
Figure 13a shows the measured power spectral density measured
at the output of the undecimated band-pass - modulator. Note,
the wide dynamic range achieved at the center frequency, f
is achieved once the LC and RC resonators of the - modulator
have been successfully tuned. The out-of-band noise is removed
by the decimation filters following quadrature demodulation.
Figure 13a. Measured Undecimated Spectral Output of
Bandwidth of 3.3 kHz
-
OUTPUT
CLK
MIXER
Figure 12. Equivalent Circuit of Sixth Order
Band-Pass - Modulator
Modulator ADC with f
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
, thus no tuning is required.
0
0
MXON
IF2N
IF2P
1
EXTERNAL
LC
CONTROL
CLK
DAC1
GAIN
2
/8 for the ADC to function properly. The
NATOR
RESO-
3
FREQUENCY – MHz
–2 dBFS OUTPUT
RC
-
CLK
) ADC
4
= 18 MSPS and Noise
f
CLK
NATOR
RESO-
5
SC
= 13 MSPS–26 MSPS
6
NBW = 3.3kHz
LEVEL
FLASH
f
CLK
NINE-
ESL
7
AD9874
= 18MHz
8
TO DIGITAL
FILTER
CLK
9
/8,