Q: Output buffering and unbuffering of DAC
The analog output of a digital to analog converter (DAC) can be either voltage output or current output. Output impedance is one of the important parameters for any output type. For voltage mode output, binary weighted resistance networks are generally used for digital to analog conversion.
Figure 1. Voltage mode binary weighted resistor DAC conversion structure
If there is no buffer, the output is usually high impedance, which can lead to output voltage error for any load. In this case, the impedance can be reduced by adding a "buffer" (operational amplifier).
Figure 2. DAC output buffering
Current output DAC has code related output impedance, so its output must drive virtual ground operational amplifier before load to maintain output linearity. Taking Analog Devices‘ AD5545 as an example
The current output IOUT is connected to the inverted input of the operational amplifier, so the operational amplifier acts as an I-V converter.
In this way, VOUT only references the VREF value, thereby improving the output stability of DAC.
Figure 3. Current mode binary weighted resistor DAC conversion structure
Given the necessity of ‘buffer amplifiers‘, manufacturers have introduced built-in buffer products, thereby reducing the number of parts and costs. A "buffered" DAC indicates that the output has been buffered (as specified in the specifications), so the output impedance will be very low. If the DAC is not buffered, the relevant parameters in the specification will be specified as‘ not loaded ‘.
In addition, manufacturers also offer "unbuffered DACs" to the market. The advantage of doing so is that designers can choose amplifiers that are suitable for their application, such as higher current drive, larger power range or more bandwidth, amplifiers, etc., thereby increasing the freedom of design.
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