Hi Everyone,


I am trying to generate a programmable negative Bias voltage from 0 to -2V with <0.5mV error.

I was able to generate a programmable positive bias voltage of similar range using DAC. based on this resource .

http://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-detail

My idea is to have a negative 5 Voltage supply on the board and connect 0V to 5V, -5V to the ground, So everything is inverted, it will just give me the negative bias voltage with the same accuracy.

However, My problems is that The DAC requires SPI communication from uC which is 3.3 V. I could not talk to the negative section of the board since They are at completely different voltage level.

Is there anyway for me to get around this? Do i have to get another uC on the board just to talk to that DAC?

Thanks a lot for the help.
Eddy

 

You could use a circuit like this. Depending on what speed the SPI is working at, you could increase the value of the resistors.

 

Hi,

Another option is to construct an inverting circuit that converts positive voltages to negative voltages. That would mean driving the circuit with a positive voltage which you can get from your positive DAC.
As the dac went from 0 to 0.1 to 0.2 to 0.3v etc., the output of the circuit would go from 0 to -0.1 to -0.2 to -0.3 etc. An op amp would do this.

 

An inverting op amp circuit would work but 0.5mV accuracy out of 2V is 0.025% so you want to use an op amp with less than 0.1mV offset and a pot in the inverting resistor network to calibrate the output since 0.025% accurate resistors are not readily available.