AN4566 datasheet(5/19 Pages) STMICROELECTRONICS

5 / 19 page

DocID026799 Rev 1

5/19

AN4566

The STM32 DAC system

1.1

DAC equivalent circuit

Digital-to-Analog converters (DACs) are routinely used to transform digital data into analog

signals. The structure of the DAC can be modeled as a digitally controlled voltage source

and the output impedance, as shown in Figure 1.

The output impedance of the DAC is constant, independently from the digital input signal.

When the output buffer is OFF, the DACINT and DACOUT are connected through the

resistor Rb, hence the output impedance of the DAC is Ra+Rb (Rb is equal to Ra), and

RDAC = 2*Ra (S1 and S2 switches are open).

When the buffer is enabled, the OpAmp is configured as an inverting amplifier with Av = -1,

and the output impedance is almost zero thanks to the feedback loop.

Figure 1. DAC equivalent circuit

1.2

DAC speed on the specification

When the output buffer is enabled on output of the DAC, the DAC speed is specified by the

output buffer performance. This number is indicated in the Tsetting or Update rate in the

product datasheet.

When output buffer is disabled, the output signal speed is simply following the RC constant

which is determined by the DAC output impedance RDAC (= 2*Ra) and the capacitive load

on the DACOUT pad.

As an example, the STM32F407 defines the impedance output with buffer off at a maximum

value of 15 kΩ. If a 10 pF capacitive load (including the parasitic capacitance of the

STM32F407 device on DACOUT pad) is considered, to get +/-1LSB of the final value (from

lowest code to highest code) we have

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TCR

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AN4566 Datasheet(PDF) 5 Page - STMicroelectronics