ADS6444IRGCT View Datasheet(PDF) - STMicroelectronics
Part Name
Description
Manufacturer
ADS6444IRGCT Datasheet PDF : 75 Pages
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www.ti.com
APPLICATION INFORMATION (continued)
ADS6445, ADS6444
ADS6443, ADS6442
SLAS531 – MAY 2007
1
0
−1
−2
−3
−4
−5
−6
0
100 200 300 400 500 600 700
fIN − Input Frequency − MHz
G073
Figure 83. Analog Input Bandwidth (represented by magnitude of TF_ADC, see Figure 85 )
Drive Circuit Requirements
For optimum performance, the analog inputs must be driven differentially. This improves the common-mode
noise immunity and even order harmonic rejection.
A 5-Ω resistor in series with each input pin is recommended to damp out ringing caused by the package
parasitics. It is also necessary to present low impedance (< 50 Ω) for the common mode switching currents. For
example, this is achieved by using two resistors from each input terminated to the common mode voltage
(VCM).
In addition to the above, the drive circuit may have to be designed to provide a low insertion loss over the
desired frequency range and matched impedance to the source. While doing this, the ADC input impedance has
to be taken into account. Figure 84 shows that the impedance (Zin, looking into the ADC input pins) decreases
at high input frequencies. The smith chart shows that the input impedance is capacitive and can be
approximated by a series R-C upto 500 MHz.
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