DocID026799 Rev 1

9/19

AN4566

Example

18

2

Example

The example of the high speed use of the DAC is based on STM32F407, it shows how to

generate a 200 kHz sine wave by the DAC operating at 5 Msps.

2.1

External OpAmp choice

As indicated before, the external OpAmp will define the DAC total performance.

To choose the OpAmp, the following parameters must be considered.

•

slew Rate;

•

gain bandwidth (GBW);

•

open loop gain;

•

supply voltage range;

•

output voltage swing performance;

•

input common mode voltage range;

•

minimum stable gain.

If STM32 DAC is working at 3.3 V, it’s possible to use the OpAmp 3.3 V supply (it’s also

possible to consider another analog supply rail, actually this is the option used in the

example).

It is recommended to have minimum two times of sampling speed of the gain bandwidth, so,

for 5 Msps, GBW needs to be wider than 10 MHz.

To keep good DAC linearity, open loop gain must be higher than 60 dB.

If it’s needed to have the output voltage near the supply voltage, then the output voltage

swing of the OpAmp should preferably be rail to rail, otherwise, should voltage swing be

near to the supply or ground rail, signal will be saturated and this will result in distortion.

Even the OpAmp negative input is fixed at the reference voltage level, it is necessary to

verify that the input common voltage range covers the reference voltage level with a margin.

The used OpAmp gain is about -1, so the OpAmp must be stable at this gain.

By considering above criteria, LMH6645/6646/6647 from Texas Instruments fit the

requirements:

•

slew rate: 22 Volts/µs;

•

gain band width: 55 MHz;

•

open loop gain: 87 dB;

•

supply voltage range: 2.5 to 12 V;

•

input Common Mode Voltage 0.3 V beyond rails;

•

output Voltage Swing 20 mV from Rails;

•

stable from gain +1.