Datasheet

AD9634
Rev. 0 | Page 20 of 32
Differential Input Configurations
Optimum performance can be achieved when driving the
AD9634 in a differential input configuration. For baseband
applications, the AD8138, ADA4937-1, and ADA4930-1
differential drivers provide excellent performance and a flexible
interface to the ADC.
The output common-mode voltage of the ADA4930-1 is easily
set with the VCM pin of the AD9634 (see Figure 47), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
V
IN
76.8Ω
120Ω
0.1µF
200Ω
200Ω
90Ω
AVDD
33Ω
33Ω
15Ω
15Ω
5pF
15pF
15pF
ADC
VIN–
VIN+
VCM
ADA4930-1
0.1µF
09996-044
Figure 47. Differential Input Configuration Using the ADA4930-1
For baseband applications where SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in Figure 48. To bias the
analog input, connect the VCM voltage to the center tap of the
secondary winding of the transformer.
2V p-p
49.9Ω
0.1µF
R1
R1
C1
ADC
VIN+
VIN–
VCM
C2
R2
R3
R2
C2
R3
0.1µF
09996-045
Figure 48. Differential Transformer-Coupled Configuration
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
the true SNR performance of the AD9634. For applications where
SNR is a key parameter, differential double balun coupling is
the recommended input configuration (see Figure 49). In this
configuration, the input is ac-coupled and the VCM voltage 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. 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. Tabl e 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 the
R1, R2, R3, C1, and C2 components shown in Figure 49.
Table 9. Example RC Network
Frequency Range (MHz) R1 Series (Ω) C1 Differential (pF) R2 Series (Ω) C2 Shunt (pF) R3 Shunt (Ω)
0 to 100 33 8.2 0 15 49.9
100 to 300 15 3.9 0 8.2 49.9
ADC
R1
0.1µF
0.1µF
2V p-p
VIN+
VIN–
VCM
C1
C2
R1
R2
R2
0.1µF
S
0.1µF
C2
33Ω
33Ω
SP
A
P
R3
R3
0.1µF
09996-046
Figure 49. Differential Double Balun Input Configuration