3.1 Smart Analog Combo Cascading

The analog circuit is usually complicated which requires different functions like amplification, filtering, buffering combined in one circuit. The multiple requirements cannot be achieved by a single amplifier so that amplifier cascading is widely used. One typical use case for two-stage cascaded amplifiers is that the first-stage amplifier set as high gain amplifier and the second-stage amplifier set as buffer. By this way, both high gain and high output drive strength can be achieved. MSP430FR2355 integrates four SAC modules (SAC0, SAC1, SAC2, and SAC3) with flexible interconnection. SAC0 is paired with SAC2, and SAC1 is paired with SAC3. The SAC0 output is connected to the SAC2 positive and negative inputs. The SAC2 output is connected to the SAC0 positive and negative inputs. SAC1 and SAC3 have interconnections similar to the interconnections of SAC0 and SAC2. With the internal connections of the MSP430 MCU, the cascaded SAC modules can be configured multiple operation modes, and board layout is also simplified.

Figure 9 shows one example of two cascaded noninverting amplifiers. The noninverting output of the first OA is internally connected to the noninverting positive input of the second OA by selecting the paired OA output as the positive input (PSEL = 10b).

Figure 10 shows the oscilloscope waveform of SAC cascading example 1. Channel 1 is the input signal of first stage SAC. Channel 2 is the output signal of first stage SAC. Channel 3 is the output of second stage SAC. As Figure 10 shows, the input signal is amplified twice without inverting. The first stage amplifier amplifies the signal by 3 times and the second amplifier amplifies the signal by 2 times. The total gain is 6.

Figure 11 shows another example of two cascaded inverting amplifiers. The output of the first inverting OA is internally connected to the resistor ladder of the second inverting OA as the negative input. This configuration can be achieved by setting SAC PGA mode into cascade OA inverting mode (MSEL = 11b).

Figure 12 shows the oscilloscope waveform of SAC cascading example 2. Channel 1 is the input signal of first stage SAC. Channel 2 is the output signal of first stage SAC. Channel 3 is the output of second stage SAC. As Figure 12 shows, the input signal is amplified twice with inverting. After inverting amplification twice, the output signal has the same phase as the input signal, and its amplitude is 4 times the amplitude of the input signal. The first stage amplifier amplifies the signal by 2 times, and the second amplifier amplifies the signal by 2 times.