6 Gain configurations enabling beyond-the-rail voltage monitoring

DAs are typically in a unity gain configuration (meaning that the gain = 1) but can vary from 0.5 up to 2. Changing the values of the resistor networks in a DA makes it possible to achieve a wide range of gain ratios for different applications that may require greater attenuation to scale the voltage down to the input range of an ADC (3.3V or 5V). As shown in Figure 2, changing resistor network values achieves greater attenuation.

An overlooked benefit of DAs is their ability to allow your input to go beyond the rails. Because the resistor ladders divide down the input voltages of the DA, the input of the integrated amplifier will only see the attenuated voltage. With standard op amps, the supply voltage limits the common-mode voltage range. This flexibility of the DA makes them suitable for monitoring higher voltages when the amount of available power rails is limited. As shown in Figure 3, the input voltage range extends further than the recommended supply voltage of the DA.

In higher-power-density systems, the increase in switching frequencies and parasitic inductances caused by printed circuit board traces can lead to additional voltage disturbances that affect voltage monitoring accuracy, as common-mode noise cannot be completely eliminated. While using an integrated DA with high CMRR will remove any common-noise observed across the inputs, achieving high CMRR with the external resistors in a discrete DA will be difficult to achieve given the small mismatches of the resistors, especially at higher gain ratios.

Equation 2 expresses how parasitic inductances and switching frequencies influence the amount of voltage disturbance of your signal:

Figure 4 illustrates the CMRR performance over frequency for an integrated DA.