AD9874 Analog Devices, AD9874 Datasheet - Page 7

• Manufacturer Part Number: AD9874
• Description: Low Power IF Digitizing Subsystem
• Manufacturer: Analog Devices

Specifications of AD9874

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 26MSPS
• Interface: Ser
• Analog Input Type: Diff-Uni,SE-Uni
• Adc Architecture: Sigma-Delta
• Pkg Type: QFP

REV. A
DEFINITION OF SPECIFICATIONS/TEST METHODS
Single-Sideband Noise Figure (SSB NF)
Noise figure (NF) is defined as the degradation in SNR perfor-
mance (in dB) of an IF input signal after it passes through a
component or system. It can be expressed with the equation
The term SSB is applicable for heterodyne systems containing a
mixer. It indicates that the desired signal spectrum resides on
only one side of the LO frequency (i.e., single sideband); thus a
“noiseless” mixer has a noise figure of 3 dB.
The AD9874’s SSB noise figure is determined by the equation
where P
the noise measurement bandwidth, –174 dBm/Hz is the thermal
noise floor at 293 K, and SNR is the measured signal-to-noise
ratio in dB of the AD9874.
Note that P
measured SNR due to phase noise from the RF and LO signal
generators. The IF frequency, CLK frequency, and decimation
factors are selected to minimize any spurious components
falling within the measurement bandwidth. Note also that a
bandwidth of 10 kHz is used for the data sheet specification.
Refer to Figures 22a and 22b for an indication of how NF varies
with BW. Also, refer to the TPCs to see how NF is affected by
different operating conditions. All references to noise figures
within this data sheet imply single-sideband noise figure.
Input Third Order Intercept (IIP3)
IIP3 is a figure of merit used to determine a component’s or
system’s susceptibility to intermodulation distortion (IMD)
from its third order nonlinearities. Two unmodulated carriers at
a specified frequency relationship (f
nonlinear system exhibiting third order nonlinearities producing
IMD components at 2f
sents the extrapolated intersection of the carrier’s input power
with the third order IMD component when plotted in dB. The
difference in power (D in dBc) between the two carriers and the
resulting third order IMD components can be determined from
the equation
SSB NF
IN
is the input power of an unmodulated carrier, BW is
Noise Figure
IN
=
is set to –85 dBm to minimize any degradation in
P
IN
−
{
D
10
1
= ×
=
– f
×
10 log(
2
log
2
and 2f
×
(
(
IIP
BW
3
2
SNR
1
)
– f
}
–
and f
−
P
1
IN
174
. IIP3 graphically repre-
IN
)
2
) are injected into a
SNR
dBm Hz SNR
OUT
)
−
–7–
Dynamic Range (DR)
Dynamic range is the measure of a small target input signal
(P
(P
characteristic of the component or system to degrade, thus
making it unable to detect the smaller target signal correctly. In
the case of the AD9874, it is often a degradation in noise figure
at increased VGA attenuation settings that limits its dynamic
range (refer to TPCs 15a, 15b, and 15c).
The test method for the AD9874 is as follows. The small target
signal (an unmodulated carrier) is input at the center of the IF
frequency, and its power level (P
SNR
3 dB prior to injecting the interferer signal. The offset frequency
of the interferer signal is selected so that aliases produced by
the decimation filter’s response as well as phase noise from the LO
(due to reciprocal mixing) do not fall back within the measurement
bandwidth. For this reason, an offset of 110 kHz was selected.
The interferer signal (also an unmodulated carrier) is then
injected into the input and its power level is increased to the
point (P
The dynamic range is determined with the equation:
Note that the AD9874’s AGC is enabled for this test.
IF Input Clip Point
The IF input clip point is defined as 2 dB below the input power
level (P
Unlike other linear components that typically exhibit a soft
compression (characterized by its 1 dB compression point), an
ADC exhibits a hard compression once its input signal exceeds
its rated maximum input signal range. In the case of the AD9874,
which contains a - ADC, hard compression should be avoided
because it causes severe SNR degradation.
TARGET
INTER
TARGET
). Typically, the large signal will cause some unwanted
IN
INTER
) in the presence of a large unwanted interferer signal
), resulting in the clipping of the AD9874’s ADC.
of 6 dB. The power of the signal is then increased by
) where the target signal SNR is reduced to 6 dB.
DR
=
P
INTER
–
P
TARGET
TARGET
+
) is adjusted to achieve an
SNR
TARGET
AD9874