AD6657A(Rev0) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD6657A Datasheet PDF : 36 Pages
| |||
AD6657A
Data Sheet
Differential Input Configurations
Optimum performance is achieved when driving the AD6657A
in a differential input configuration. For baseband applications,
the AD8138, ADA4937-2, and ADA4938-2 differential drivers
provide excellent performance and a flexible interface to the ADC.
The output common-mode voltage of the ADA4938-2 is easily
set with the VCMx pin of the AD6657A (see Figure 36), and the
driver can be configured in a Sallen-Key filter topology to provide
band limiting of the input signal.
15pF
VIN
76.8Ω
0.1µF
200Ω
90Ω
33Ω
15Ω
5pF
ADA4938-2
120Ω
200Ω
33Ω
15Ω
15pF
VIN– AVDD
ADC
VIN+
VCM
Figure 36. Differential Input Configuration Using the ADA4938-2
For baseband applications where SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in Figure 37. To bias the
analog input, the VCM voltage can be connected to the center
tap of the secondary winding of the transformer.
2V p-p
49.9Ω
C2
R1
C1
R2
VIN+
ADC
R1
R2
VIN– VCM
0.1µF
C2
Figure 37. Differential Transformer-Coupled Configuration
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few megahertz (MHz). 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
the true SNR performance of the AD6657A. For applications in
which SNR is a key parameter, differential double balun coupling
is the recommended input configuration (see Figure 38). In this
configuration, the input is ac-coupled and the CML is provided to
each input through a 33 Ω resistor. These resistors compensate
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 and may need to be reduced
or removed. Table 10 lists recommended values to set the RC
network. At higher input frequencies, good performance can be
achieved by using a ferrite bead in series with a resistor and
removing the capacitors. However, these values are dependent
on the input signal and should be used as a starting guide only.
Table 10. Example RC Network
Frequency
Range
R1 Series
(MHz)
(Each)
C1 Differential
0 to 100
33 Ω
5 pF
100 to 200 10 Ω
5 pF
100 to 300 10 Ω1
Remove
R2 Series
(Each)
15 Ω
10 Ω
66 Ω
C2 Shunt
(Each)
15 pF
10 pF
Remove
1 In this configuration, R1 is a ferrite bead with a value of 10 Ω @ 100 MHz.
An alternative to using a transformer-coupled input at frequen-
cies in the second Nyquist zone is to use the AD8352 differential
driver (see Figure 39). For more information, see the AD8352
data sheet.
2V p-p
0.1µF
PA
SS
0.1µF
33Ω
P
0.1µF 33Ω
C2
R1
R2
C1
0.1µF
R1
R2
C2
VIN+
ADC
VIN–
VCM
Figure 38. Differential Double Balun Input Configuration
VCC
ANALOG INPUT 0.1µF 0Ω 16
1
2
CD
RD RG 3
4
5
ANALOG INPUT
0.1µF 0Ω
8, 13
11
AD8352
10
14
0.1µF
0.1µF
0.1µF
0.1µF
R
200Ω
200Ω
C
R
0.1µF
VIN+
ADC
VIN– VCM
Figure 39. Differential Input Configuration Using the AD8352
Rev. 0 | Page 20 of 36
|
|
Share Link: 