THS4031_16 Datasheet, PDF (27/47 Pages) Texas Instruments – THS403x 100-MHz Low-Noise High-Speed Amplifiers

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THS4031, THS4032
SLOS224H – JULY 1999 – REVISED JUNE 2016
10.2.2.4 Driving Amplifier Bandwidth Restriction
The restriction of bandwidth by an RC filter (after OPA3 in Figure 60) may result in better SNR and THD, but it
makes the operational amplifier difficult to settle within the required accuracy. If the output does not settle
properly, some residual charge of the previous channel remains in the next sampling. It appears as a crosstalk. If
the throughput of the ADC is reduced, allowing the output of the operational amplifier to settle properly, the
problem becomes smaller. Therefore, using a larger capacitor makes the operational-amplifier output settling
slower. So, within the ADC sampling frame, the operational-amplifier output does not settle to its final level. The
diagrams of Figure 62 and Figure 63 show SNR and crosstalk as a function of the filter capacitor.
The input settling behavior is shown in Figure 64 with three different bandwidths. The value of the capacitor is
changed to change the bandwidth. As the bandwidth increases, the settling time improves (see Equation 3).
Bandwidth @ ¹
2pR₁C₁
(3)
10.2.3 Application Curves
Figure 62. SNR vs Input Bandwidth
Figure 63. Crosstalk vs Input Bandwidth
Figure 64. Input Settings With Different Values of Capacitors
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