The analog output of a DAC may be either voltage or current. The output impedance is one of the more important parameters for any output type.
For voltage mode output, a binary-weighted resistor network is generally applied for digital-to-analog conversion.
Figure 1. Voltage mode binary weighted resistor DAC conversion structure
If there is no buffering, the output is usually high impedance, which will lead to output voltage error for any loading, the impedance can be reduced by adding a “buffer” (op-amp).
Figure 2. DACs output buffering
Current-output DACs have code-dependent output impedance, so its output must drive a virtual ground op-amp before the load in order to maintain output linearity. Let’s take Analog Devices’ AD5545 as en example.
The current output IOUT is connected to the OP_AMP inverting input, so the OP_AMP acts as an I-V converter.
It leads VOUT to only refer to your VREF value, and thus can increase the output stability of the DAC.
Figure 3. Current mode binary weighted resistor DAC conversion structure
Due to the necessity of “Buffer Amps”, manufacturers have buffer built-in products, this reduces the number of parts and costs. “Buffered” DAC means the output is buffered, it will indicate this on the datasheet and the output impedance will be low. An indicator in the datasheet that the DAC is unbuffered is that the parameters will be specified as “unloaded”.
Manufacturers also offer “unbuffered DACs” to the market. The benefit is that designers can choose the amplifiers suitable for their applications, such as higher current drive, larger power supply range or more bandwidth. Amplifiers, etc., so it has a higher degree of design freedom.