Bipolare adjust. precision current-source with 12-bit DAC and PICO RP2040
Here I test the current-source with 12-bit DACs from Microchip. At first I tested the I2C DAC MCP4725, but the result was not so good: 3 to 6 mV offset (expected 0...1 mV), also the linearity was bad. Therefore I tested it with a SPI-DAC MCP4822 (dual). He is better from the offset (1... 2mV) , also the linearity (1-2 %).
The isolated interface for SPI (3-4 Pins) is more complex compared with I2C ( 2 Pins).
Also the price for SPI-isolater chip (ADUM140x about 10 €) is higher compared with I2C (ADUM125x about 3 €) , therefore I tested it with four optocoupler . I found a small cheap breadboard ( Optoisolator HW-399) with optocoupler TLP281-4 and 4 transistors toturn threw the coupler inverted signal.
I used 3 for the SPI-interface (only CLOCK, MOSI, CS used for the DAC) and one for changing the
output polararity of the currentsource .
It functions, but only very low SPI frequency (15 kHz max).
The slewrate from normal LED-couplers is to small.
But it's ok for applications like transistor-curvetracer changing I-base every 20 s or so.
For quicker I have tested the currentsource with an ADUM1400, making a small PCB with the smae dimensions and pins like the HW-399 board, using the NC-Pin for +5V power supply.
(the ADUMxxxx isolators use magnetic-transformer and need on both side +3-5V power )
The max. speed I tested is 12 MHz .
Also the price for SPI-isolater chip (ADUM140x about 10 €) is higher compared with I2C (ADUM125x about 3 €) , therefore I tested it with four optocoupler . I found a small cheap breadboard ( Optoisolator HW-399) with optocoupler TLP281-4 and 4 transistors toturn threw the coupler inverted signal.
I used 3 for the SPI-interface (only CLOCK, MOSI, CS used for the DAC) and one for changing the
output polararity of the currentsource .
It functions, but only very low SPI frequency (15 kHz max).
The slewrate from normal LED-couplers is to small.
But it's ok for applications like transistor-curvetracer changing I-base every 20 s or so.
For quicker I have tested the currentsource with an ADUM1400, making a small PCB with the smae dimensions and pins like the HW-399 board, using the NC-Pin for +5V power supply.
(the ADUMxxxx isolators use magnetic-transformer and need on both side +3-5V power )
The max. speed I tested is 12 MHz .
Diskussion (0 Kommentare)