AD9240
REV. A
–10–
Figure 27 compares the AD9240’s THD vs. frequency performance for a 2 V input span with a common-mode voltage of 
1 V and 2.5 V. Note the difference in the amount of degradation in THD performance as the input frequency increases. 
Similarly, note how the THD performance at lower frequencies 
becomes less sensitive to the common-mode voltage. As the 
input frequency approaches dc, the distortion will be dominated by static nonlinearities such as INL and DNL. It is 
important to note that these dc static nonlinearities are independent of any R
ON
 modulation.
–50
–80
0.1 1 20
–70
–60
–90
10
THD – dB
FREQUENCY – MHz
VCM = 1.0V
VCM = 2.5V
Figure 27. THD vs. Frequency for VCM = 2.5 V and 1.0 V 
(A
IN
 = –0.5 dB, Input Span = 2.0 V p-p)
Due to the high degree of symmetry within the SHA topology, a 
significant improvement in distortion performance for differential input signals with frequencies up to and beyond Nyquist can 
be realized. This inherent symmetry provides excellent cancellation of both common-mode distortion and noise. Also, the 
required input signal voltage span is reduced a factor of two 
which further reduces the degree of R
ON
 modulation and its
effects on distortion.
The optimum noise and dc linearity performance for either 
differential or single-ended inputs is achieved with the largest 
input signal voltage span (i.e., 5 V input span) and matched 
input impedance for VINA and VINB. Note that only a slight 
degradation in dc linearity performance exists between the 
2 V and 5 V input span as specified in the AD9240 DC 
SPECIFICATIONS.
Referring to Figure 24, the differential SHA is implemented 
using a switched-capacitor topology. Hence, its input impedance and its subsequent effects on the input drive source should 
be understood to maximize the converter’s performance. The 
combination of the pin capacitance, C
PIN
, parasitic capacitance
C
PAR, 
and the sampling capacitance, CS, is typically less than 
16 pF. When the SHA goes into track mode, the input source 
must charge or discharge the voltage stored on C
S
 to the new
input voltage. This action of charging and discharging C
S 
which 
is approximately 4 pF, averaged over a period of time and for a 
given sampling frequency, F
S
, makes the input impedance ap-
pear to have a benign resistive component (i.e., 83 kΩ at F
S
 = 
10 MSPS). However, if this action is analyzed within a sampling period (i.e., T = <1/F
S
), the input impedance is dynamic 
due to the instantaneous requirement of charging and discharging C
S
. A series resistor inserted between the input drive source 
and the SHA input as shown in Figure 28 provides effective 
isolation.
10mF
VINA
VINB
SENSE
AD9240
0.1mF
RS*
V
CC
V
EE
RS*
VREF
REFCOM
*OPTIONAL SERIES RESISTOR
Figure 28. Series Resistor Isolates Switched-Capacitor 
SHA Input from Op Amp. Matching Resistors Improve 
SNR Performance
The optimum size of this resistor is dependent on several factors, which include the AD9240 sampling rate, the selected op 
amp and the particular application. In most applications, a
30 Ω to 50 Ω resistor is sufficient; however, some applications
may require a larger resistor value to reduce the noise bandwidth or possibly limit the fault current in an overvoltage 
condition. Other applications may require a larger resistor value 
as part of an antialiasing filter. In any case, since the THD 
performance is dependent on the series resistance and the above 
mentioned factors, optimizing this resistor value for a given 
application is encouraged.
A slight improvement in SNR performance and dc offset 
performance is achieved by matching the input resistance connected to VINA and VINB. The degree of improvement is dependent on the resistor value and the sampling rate. For series
resistor values greater than 100 Ω, the use of a matching resis-
tor is encouraged.
The noise or small-signal bandwidth of the AD9240 is the same 
as its full-power bandwidth. For noise sensitive applications, the 
excessive bandwidth may be detrimental and the addition of a 
series resistor and/or shunt capacitor can help limit the wideband noise at the A/D’s input by forming a low-pass filter. Note, 
however, that the combination of this series resistance with the 
equivalent input capacitance of the AD9240 should be evaluated for those time-domain applications that are sensitive to the 
input signal’s absolute settling time. In applications where harmonic distortion is not a primary concern, the series resistance 
may be selected in combination with the SHA’s nominal 16 pF 
of input capacitance to set the filter’s 3 dB cutoff frequency.
A better method of reducing the noise bandwidth, while possibly establishing a real pole for an antialiasing filter, is to add 
some additional shunt capacitance between the input (i.e., 
VINA and/or VINB) and analog ground. Since this additional 
shunt capacitance combines with the equivalent input capacitance of the AD9240, a lower series resistance can be selected to 
establish the filter’s cutoff frequency while not degrading the 
distortion performance of the device. The shunt capacitance 
also acts as a charge reservoir, sinking or sourcing the additional 
charge required by the hold capacitor, C
H
, further reducing
current transients seen at the op amp’s output.
The effect of this increased capacitive load on the op amp driving the AD9240 should be evaluated. To optimize performance 
when noise is the primary consideration, increase the shunt 
capacitance as much as the transient response of the input signal 
will allow. Increasing the capacitance too much may adversely 
affect the op amp’s settling time, frequency response and distortion performance.