SBOS778D April 2016 – April 2021 THS4551
The circuit of Figure 8-5 is a typical setup for common-mode measurements.
In Figure 8-5, the differential path is simply terminated back to ground on the two 1-kΩ input resistors and the VOCM control input is driven from a 50-Ω matched source for the frequency response and step response curves of Figure 7-43 and Figure 7-44. The outputs are summed to a center point (to obtain the average, or common-mode, output) through two 100-Ω resistors. These 100-Ω resistors form an equivalent 50-Ω source to the common-mode output for measurements. This common-mode test circuit is available as a TINA-TI™ simulation file. Figure 7-45 illustrates the common-mode output noise measurements with either a ground on the VOCM input pin or with the VOCM input pin floating. The higher noise in Figure 7-45 for a floated input can be reduced by including a capacitor to ground at the VOCM control input pin.