AD6672 Analog Devices, AD6672 Datasheet - Page 17

• Manufacturer Part Number: AD6672
• Description: IF Receiver
• Manufacturer: Analog Devices

Specifications of AD6672

• Resolution (bits): 11bit
• # Chan: 1
• Sample Rate: 250MSPS
• Interface: LVDS
• Analog Input Type: Diff-Uni
• Ain Range: 1.75 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

Differential Input Configurations
Optimum performance can be achieved when driving the
in a differential input configuration. For baseband applications, the
AD8138, ADA4937-1, and
excellent performance and a flexible interface to the ADC.
The output common-mode voltage of the
set with the VCM pin of the
driver can be configured in a Sallen-Key filter topology to
provide band-limiting of the input signal.
For baseband applications where SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in Figure 26. To bias the
analog input, connect the VCM voltage to the center tap of the
secondary winding of the transformer.
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few megahertz. Excessive signal power can also cause
core saturation, which leads to distortion.
At input frequencies in the second Nyquist zone and above, the
noise performance of most amplifiers is not adequate to achieve
VIN
2V p-p
0.1µF
Figure 25. Differential Input Configuration Using the
76.8Ω
Figure 26. Differential Transformer-Coupled Configuration
49.9Ω
120Ω
90Ω
0.1µF
ADA4930-1
200Ω
200Ω
ADA4930-1
2V p-p
AD6672
R1
R1
C1
33Ω
33Ω
C2
C2
R3
15pF
5pF
0.1µF
R3
(see Figure 25), and the
P
differential drivers provide
A
15pF
R2
R2
15Ω
15Ω
ADA4930-1
Figure 28. Differential Double Balun Input Configuration
VIN+
VIN–
S
VIN–
VIN+
ADA4930-1
0.1µF
ADC
0.1µF
S
ADC
VCM
AVDD
VCM
is easily
AD6672
P
0.1µF
0.1µF
Rev. 0 | Page 17 of 32
33Ω
33Ω
the true SNR performance of the AD6672. For applications where
SNR is a key parameter, differential double balun coupling is
the recommended input configuration (see Figure 28). In this
configuration, the input is ac-coupled and the VCM voltage is
provided to the input through a 33 Ω resistor. This resistor
compensates for losses in the input baluns to provide a 50 Ω
impedance to the driver.
In the double balun and transformer configurations, the value
of the input capacitors and resistors is dependent on the input
frequency and source impedance. Based on these parameters,
the value of the input resistors and capacitors may need to be
adjusted or some components may need to be removed. Table 9
displays recommended values to set the RC network for
different input frequency ranges. However, these values are
dependent on the input signal and bandwidth and should be
used only as a starting guide. Note that the values given in Table 9
are for each R1, R2, C2, and R3 component shown in Figure 26
and Figure 28.
Table 9. Example RC Network
Frequency
Range
(MHz)
0 to 100
100 to 300
An alternative to using a transformer-coupled input at
frequencies in the second Nyquist zone is to use an amplifier
with variable gain. The
(DVGA) provides good performance for driving the AD6672.
Figure 27 shows an example of the
through a band-pass antialiasing filter.
NOTES
1. ALL INDUCTORS ARE COILCRAFT
2. FILTER VALUES SHOWN ARE FOR A 20MHz BANDWIDTH FILTER
0.1µF
WITH THE EXCEPTION OF THE 1µH CHOKE INDUCTORS (0603LS).
CENTERED AT 140MHz.
AD8375
Figure 27. Differential Input Configuration Using the
R1
R1
C2
C1
C2
R3
1µH
1µH
R3
R1
Series
(Ω)
33
15
R2
R2
1000pF
1000pF
VPOS
1nF
VIN+
VIN–
180nH
301Ω
180nH
C1
Differential
(pF)
8.2
3.9
AD8375
ADC
0.1µF
5.1pF
220nH
220nH
®
VCM
0603CS COMPONENTS
3.9pF
digital variable gain amplifier
165Ω
165Ω
AD8375
R2
Series
(Ω)
0
0
VCM
15pF
1nF
driving the
68nH
C2
Shunt
(pF)
15
8.2
AD8375
2.5kΩ║2pF
AD6672
AD6672
AD6672
R3
Shunt
(Ω)
49.9
49.9