2 Output Configuration

There are two buffers following the output of the DAC in Figure 2-1, each buffer provides negative feedback. One negative input of the buffer is connected to the output of the DAC, while the positive input is connected to the common-mode node. The output of each buffer is connected to one of the output pins. Depending on the application, one or both of these buffers can be used. The TAD5212-Q1 supports up to two channels of differential output, up to two channels of pseudo-differential output, and up to four channels of single-ended output. Each of the output channels can be independently configured for differential- or single-ended output.

Register 100 and 107 are used to configure the output connections, such as differential output, or single-ended, and so on for OUTxP and OUTxM, where x is the channel number corresponding to channel one or two. Each configuration and the allowable swing are discussed further in this section.

Figure 2-1 General Structure of DAC and Output Buffer Amplifiers

In fully-differential configuration, the DAC data is available differentially at both output pins. In this configuration, the load is attached between the two output pins. In differential mode, the load can be AC coupled, with a capacitor at the output before the load. Or the load can be DC coupled, connecting the outputs directly to the load. Figure 2-2 and Figure 2-3 show the AC and DC coupling in fully-differential mode. The maximum swing for the fully-differential configuration is 2Vrms. This maximum swing is 2Vrms because one output is 180 degrees out of phase with respect to the other output, effectively doubling the resulting swing, shown in Figure 2-4.

Figure 2-2 Fully-differential AC Coupling

Figure 2-3 Fully-differential DC Coupling

Figure 2-4 Signal Amplitude and Phase for Each Output and the Resulting Differential Signal

In single-ended configuration, the output can be on one output pin OUTP or OUTM) but needs to be AC-coupled because without the capacitor, a current draw can result. The current drawn depends on the load connected. Figure 2-5 shows an example of the single-ended configuration with AC coupling. The maximum swing for single-ended configuration is half of that the fully-differentiated configuration at 1Vrms.

Figure 2-5 Single-ended Configuration With Needed AC Coupling

Figure 2-6 Inversion of Signal in Single-ended Configuration

The pseudo-differential configuration is similar to fully-differential configuration but in this case the DAC output is on one pin while the other pin is connected to the common-mode voltage. The primary use-case of the pseudo-differential configuration is to avoid the AC-coupling capacitor. Similar to the fully-differential configuration, pseudo-differential configuration allows use of the load with or without the AC coupling capacitor. The maximum swing for the pseudo-differential configuration is 1Vrms. Figure 2-7 shows the pseudo-differential configuration with DC load coupling.

Figure 2-7 Pseudo-differential Configuration With DC Coupling

The TAD5212 can have a combination of drivers used in each mode. Typically, single-ended outputs use four channels, whereas fully-differential and pseudo-differential configurations use two channels. However, one distinct feature of this DAC is the capability to use all four channels, even in single-ended configuration, because the DAC acts as two half-DACs.

The DAC works off either a nominally 3.3V, 3V, or 1.8V supply. For a 3.3V supply, the internally-generated reference is 2.75V, which permits a 2Vrms swing differentially or 1Vrms swing in single-ended configuration.

When a 1.8V supply is used, then the reference drops to 1.65V with a common-mode voltage of 0.9V so that the output can swing 5V above and below the common-mode voltage without saturating the drivers.

In general, the headroom reduces as the supply voltage is reduced to be able to drive the load with the existing supply. Table 2-1 shows the supply voltages with the resulting reference voltage and output swing.