Circuit functions and advantages

In a multi-channel DAC system, all outputs can be monitored at a single point, which is very beneficial for troubleshooting and diagnostic analysis. The circuit described in this paper uses a single channel SAR ADC to realize the output channel monitoring of multi-channel DAC.

  Figure 1: typical channel monitoring circuit

Figure 1: typical channel monitoring circuit

Circuit description

The circuit shown in Figure 1 uses the internal multiplexer of 40 channel and 14 bit dacad5380 to route all 40 output channels to a single output (mon_out) pin respectively, and then monitor through an external ADC (ad7476). This method utilizes far fewer circuits than the circuits required to monitor each channel individually.

The ad5380 is a complete 40 channel, 14 bit DAC powered by a single power supply. All 40 channels have a rail to rail on-chip output amplifier. The ad5380 has a built-in channel monitoring function, which is realized by a multiplexer addressed through the serial interface. Any channel output can be routed to the monitoring output (mon_out) pin to facilitate monitoring with an external ADC. Any channel to be routed to mon_ Out, the channel monitoring function must first be enabled in the control register. The microcontroller or processor can select the output channel to be monitored through its serial output port, and then read the ADC converted data through its serial input port.

Ad5380-3 is powered by 3 V power supply, and ad5380-5 is powered by 5 V power supply. Ad7476 ADC provides 12 bit resolution, uses 2.35 V to 5.25 V single power supply, integrates reference voltage source, has low power consumption, small size and serial interface, with throughput rate up to 1 MSPs, and adopts 6-pin SOT-23 package. The conversion rate is determined by SCLK, and the throughput rate can be up to 1 MSPs.

Ad5380 and ad7476 must have a large enough power bypass capacitance of 10 µ F, which is connected in parallel with the 0.1 µ f capacitor on each power pin, and be as close to the package as possible, preferably facing these devices (this state is not shown on the schematic diagram). ten μ F capacitor is tantalum bead capacitor.

0.1 µ f capacitor must have low effective series resistance (ESR) and low effective series inductance (ESL), such as ordinary ceramic capacitor that provides low impedance grounding path at high frequency, so as to deal with transient current caused by internal logic switch.

The power wiring shall be as wide as possible to provide a low impedance path and reduce the surge effect on the power line. The clock and other fast switching signals must be shielded by the ground line to avoid radiating noise to other devices on the circuit board, and should never be close to the analog signal. Laying a grounding line between sData line and SCLK line helps to reduce crosstalk between them (not required on multilayer circuit board because it has a separate grounding layer; however, grounding line helps to separate different lines). Overlapping of digital and analog signals shall be avoided. The routing on the opposite sides of the circuit board should be perpendicular to each other, which helps to reduce the feedthrough effect on the circuit board. Microstrip technology is recommended, but it may not always be feasible for double-sided circuit boards. When this technology is adopted, the component side of the circuit board is dedicated to the grounding layer, and the signal routing is arranged on the welding side. The circuit board needs at least four layers to achieve the best layout and performance: one ground layer, one power layer and two signal layers.

Common changes

In monitoring applications that can accept lower resolution conversion, ad7476 pin compatible products can be used. The ad7477 provides 10 bit resolution and the ad7478 provides 8-bit resolution.

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