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REV. 0
AD9203
–13–
DIFFERENTIAL MODE OF OPERATION
Since not all applications have a signal preconditioned for differ-
ential operation, there is often a need to perform a single-ended-
to-differential conversion. In systems that do not need a dc
input, an RF transformer with a center tap is a method to gener-
ate differential inputs beyond 20 MHz for the AD9203. This
provides all the benefits of operating the A/D in the differential
mode without contributing additional noise or distortion. An RF
transformer also has the added benefit of providing electrical isola-
tion between the signal source and the A/D.
An improvement in THD and SFDR performance can be real-
ized by operating the AD9203 in differential mode. The perfor-
mance enhancement between the differential and single-ended
mode is most considerable as the input frequency approaches
and goes beyond the Nyquist frequency (i.e., f
IN
> F
S
/2).
49.9
V
0.1
m
F
10k
V
523
V
499
V
10k
V
20pF
49.9
V
20pF
499
V
499
V
0.1
m
F
AINP
+3V
DRVDD
AVDD
AVSS DRVSS
AINN
0.1
m
F
0.1
m
F
DIGITAL
OUTPUTS
+3V
AD9203
AD8138
49.9
V
10
m
F
10
m
F
Figure 26. AD8138 Driving an AD9203, a 10-Bit, 40 MSPS
A/D Converter
The AD8138 provides a convenient method of converting a
single-ended signal to a differential signal. This is an ideal
method for generating a direct coupled signal to the AD9203.
The AD8138 will accept a signal and shift it to an externally
provided common-mode level. The AD8138 configuration is
shown in Figure 26.
Figure 27 shows the schematic of a suggested transformer
circuit. The circuit uses a Minicircuits RF transformer, model
number T4–1T, which has an impedance ratio of four (turns
ratio of 2).
The center tap of the transformer provides a convenient means
of level-shifting the input signal to a desired common-mode
voltage. Figure 28 illustrates the performance of the AD9203
over a wide range of common-mode levels.
AINP
AINN
VREF
REFSENSE
2V
1V
AD9203
0.1
m
F
10
m
F
Figure 27. Transformer Coupled Input
Transformers with other turns ratios may also be selected to
optimize the performance of a given application. For example,
selecting a transformer with a higher impedance ratio (e.g.,
Minicircuits T16–6T with a impedance ratio of 16) effectively
“steps up” the signal amplitude, thus further reducing the driv-
ing requirements of the signal source.
The AD9203 can be easily configured for either a 1 V p-p input
span or 2 V p-p input span by setting the internal reference.
Other input spans can be realized with two external gain setting
resistors as shown in Figure 19 of this data sheet. Figures 32
and 33 demonstrate the SNR and SFDR performance over a
wide range of amplitudes required by most communication
applications.
–80
0
T
–70
–60
–50
–40
–30
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1.0V REF
0.5V REF
COMMON-MODE VOLTAGE – Volts
Figure 28. THD vs. Common-Mode Voltage vs. THD (AIN =
2 V Differential) (f
IN
= 5 MHz, f
S
= 40 MSPS)
–90
40.0
T
–80
–70
–60
–50
–40
42.5
45.0
47.5
DUTY CYCLE – %
50.0
52.5
55.0
57.5
60.0
THD
SNR
Figure 29. THD and SNR vs. Clock Duty Cycle (f
IN
= 5 MHz
Differential, Clock = 40 MSPS)