3 Experimental Results

Figure 3-1 shows the experimental board used for obtaining the test data. The experimental board has three isolated amplifiers with different output type (single-ended fixed gain, single-ended ratiometric output and differential output). Additionally, the board contains a modulator device that is not a subject of this application note.

Figure 3-2 and Figure 3-3 show the performance of two different operational amplifiers in the identical setup using the TMS320F28P650 microcontroller with 16-bit 1-MSPS multiplexed ADC. The sample and hold time is t_SMPL=425ns.

Waveforms in Figure 3-2 represent the OPA365. A high-performance, high-bandwidth (50MHz), rail-to-rail operational amplifier. This is visible during the three scenarios described previously, the operational amplifier output settles in less than 150ns. Waveforms in Figure 3-3 represent TLV9001, a low-cost, 1MHz bandwidth rail-to-rail operational amplifier. This is visible that the operational amplifier output requires more than 800ns to settle for the same three test conditions. As a consequence, the sample and hold switch opens before the input of the ADC has settled. This is visible especially for the undershoot scenario when V_SH<V_OUT. In this case the voltage used for AD conversion is 30mV higher than this needs to be. This is resulting in 1% reading error for a 3V voltage reference used in the system.