9.1.2.3 Input Bandwidth

The input bandwidth response is shown in Figure 63 and depends on the measurement network. The impedance of the driving source or any series resistance in the driving network greatly impacts the measured bandwidth. Three separate measurement methodologies are shown here. The first, shown in Figure 64, is the network used to measure the reported bandwidth in the performance tables. It uses a 50-Ω source, high bandwidth balun, and custom input network.

Another measurement using the simplified network of Figure 65 demonstrates the bandwidth of the ADC using a low impedance source near the ADC pins. The peaking in the frequency response is caused by the resonance between the package bond wires and input capacitance as well as a parasitic 0.5-nH series trace inductance leading to the device pins. This peaking is typically made insignificant by the anti-aliasing filter that precedes the ADC input.

The third measurement network of Figure 65 also assumes a low impedance voltage source but shows the bandwidth flattening impact of adding 10-Ω in series with the VIN+ and VIN– input pins.

Figure 63. Measured Input Voltage Frequency Response

Figure 64. 50-Ω Source, Bandwidth Measurement Network

Figure 65. Low-Ω Source Bandwidth Measurement (Simplified)

Figure 66. Low-Ω Source, Series 10Ω R, Bandwidth Measurement (Simplified)