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AD9225 数据表(PDF) 9 Page - Analog Devices

部件名 AD9225
功能描述  Complete 12-Bit, 25 MSPS Monolithic A/D Converter
Download  24 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

AD9225 数据表(HTML) 9 Page - Analog Devices

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AD9225
–9–
REV. A
Due to the high degree of symmetry within the SHA topology, a
significant improvement in distortion performance for differen-
tial input signals with frequencies up to and beyond Nyquist can
be realized. This inherent symmetry provides excellent cancella-
tion of both common-mode distortion and noise. Also, the
required input signal voltage span is reduced by a half which
further reduces the degree of RON 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., 4 V input span) and matched
input impedance for VINA and VINB. Only a slight degrada-
tion in dc linearity performance exists between the 2 V and
4 V input spans.
Referring to Figure 14, the differential SHA is implemented
using a switched-capacitor topology. Its input impedance and
its switching effects on the input drive source should be consid-
ered in order to maximize the converter’s performance. The
combination of the pin capacitance, CPIN, parasitic capacitance
CPAR, and the sampling capacitance, CS, is typically less than
5 pF. When the SHA goes into track mode, the input source
must charge or discharge the voltage stored on CS to the new
input voltage. This action of charging and discharging CS, aver-
aged over a period of time and for a given sampling frequency,
FS, makes the input impedance appear to have a benign resistive
component. However, if this action is analyzed within a sam-
pling period (i.e., T = 1/FS), the input impedance is dynamic
and hence certain precautions on the input drive source should be
observed.
The resistive component to the input impedance can be com-
puted by calculating the average charge that gets drawn by CH
from the input drive source. It can be shown that if CS is
allowed to fully charge up to the input voltage before switches
QS1 are opened, then the average current into the input is the
same as if there were a resistor of 1/(CS FS) ohms connected
between the inputs. This means that the input impedance is
inversely proportional to the converter’s sample rate. Since CS is
only 5 pF, this resistive component is typically much larger than
that of the drive source (i.e., 8 k
Ω at F
S = 25 MSPS).
The SHA’s input impedance over a sampling period appears as
a dynamic input impedance to the input drive source. When the
SHA goes into the track mode, the input source should ideally
provide the charging current through RON of switch QS1 in an
exponential manner. The requirement of exponential charging
means that the most common input source, an op amp, must
exhibit a source impedance that is both low and resistive up to
and beyond the sampling frequency.
The output impedance of an op amp can be modeled with a
series inductor and resistor. When a capacitive load is switched
onto the output of the op amp, the output will momentarily
drop due to its effective output impedance. As the output recov-
ers, ringing may occur. To remedy the situation, a series resistor
can be inserted between the op amp and the SHA input as
shown in Figure 15. The series resistance helps isolate the op
amp from the switched-capacitor load.
10 F
VINA
VINB
SENSE
AD9225
0.1 F
RS
VCC
VEE
RS
VREF
REFCOM
Figure 15. 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 ADC sampling rate, the selected op amp, and
the particular application. In most applications, a 30
Ω to 100 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 appli-
cations may require a larger resistor value as part of an antialias-
ing filter. In any case, since the THD performance is dependent
on the series resistance and the above mentioned factors, opti-
mizing this resistor value for a given application is encouraged.
The source impedance driving VINA and VINB should be
matched. Failure to provide that matching will result in degra-
dation of the AD9225’s superb SNR, THD and SFDR.
For noise sensitive applications, the very high bandwidth of the
AD9225 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 AD9225 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 10 pF of input capaci-
tance to set the filter’s 3 dB cutoff frequency.
A better method of reducing the noise bandwidth, while possi-
bly 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 capaci-
tance of the AD9225, 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 like a charge reservoir, sinking or sourcing the addi-
tional charge required by the hold capacitor, CH, further reduc-
ing current transients seen at the op amp’s output.
The effect of this increased capacitive load on the op amp driv-
ing the AD9225 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 dis-
tortion performance.


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