ADS6149 datasheet(49/70 Pages) TI

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Drive Circuit Requirements

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f-Frequency-MHz

ADS6149/ADS6129

ADS6148/ADS6128

www.ti.com ..................................................................................................................................................... SLWS211B – JULY 2008 – REVISED OCTOBER 2008

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 to 15 Ω resistor in series with each input pin is

recommended to damp out ringing caused by package parasitics. It is also necessary to present low impedance

(< 50

Ω) for the common mode switching currents. This can be achieved by using two resistors from each input

terminated to the common mode voltage (VCM).

Note that the device includes an internal R-C filter from each input to ground. The purpose of this filter is to

absorb the glitches caused by the opening and closing of the sampling capacitors. The cut-off frequency of the

R-C filter involves a trade-off. A lower cut-off frequency (larger C) absorbs glitches better, but also reduces the

input bandwidth and the maximum input frequency that can be supported. On the other hand, with no internal

R-C filter, high input frequency can be supported, but now the sampling glitches need to be supplied by the

external driving circuit. This has limitations due to the presence of the package bond-wire inductance.

In ADS61x9/x8, the R-C component values have been optimized while supporting high input bandwidth (up to

750 MHz). However, in applications where high input frequency support is not required, the filtering of the

glitches can be improved further using an external R-C-R filter (as shown in Figure 99 and Figure 100).

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

must be considered. Figure 97 and Figure 98 show the impedance (Zin = Rin || Cin) looking into the ADC input

pins.

Figure 97. ADC Analog Input Resistance (Rin) Across Frequency

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ADS6149 Datasheet(PDF) 49 Page - Texas Instruments

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