Answer: For a system that requires multiple channels, one AD7124-8 can support up to 8 differential/15 single ended input channels and one AD7124-4 can support up to 4 differential/7 single ended input channels. If the system requires a greater number of channels, either a single ADC with an external MUX can be used to increase the channel count as shown in Figure 1 or, multiple ADCs can be used (Figure 2).
Figure 1. 32 Channels using MUX and single AD7124-8
Figure 1 shows an example for 32 analog input channels using a single AD7124-8. Here, 4 8x1 muxes (ADG707) are connected to 4 of the ADC’s analog input channels, thus supporting 32 differential inputs. The multiplexer logic pins can be controlled via the AD7124-8’s general-purpose output pins. This option simplifies the analog front-end circuitry and digital software interface. However, converting multiple ADC channels will take time. At Full power mode 50/60Hz rejection (FIR 25sps) the throughput time for 32 channels is 41.72ms x 32 = 1.34s.
If the system requires faster measurement speed, it is recommended to use multiple AD7124-4/AD7124-8 devices. Figure 2 shows the connections for 4 AD7124-8s which support 32 channels in total. In this mode, the measurement speed will increase by the number of ADCs used. So for the same scenario as above, the throughput time for 32 channels will be reduced to 1.34s/4 = 0.335s. The ADCs can be synchronized so that a common SPI bus can be used by all the ADCs. Synchronisation ensures that the conversions are available from each ADC at the same time, ensuring that all data is captured correctly.
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Figure2. 32 Channels using multiple AD7124-8