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Posts: 9
Registered: ‎08-01-2017

How to achieve constant voltage output

Can a constant output voltage be achieved on the thunderboard react without using an external voltage regulator?

Posts: 18
Registered: ‎02-24-2017

Re: How to achieve constant voltage output

a R/C DAC Filter:

R= Resistor, C=Capacitor.

 

if you PWM a output, and it's 50% duty, your voltage will be half of Vcc after the r/c filter.

There will be some small ripple and current draw should be kept under 5mA

 

So you can change the Duty from 0 to 100% and you get a proportional voltage.

 

Posts: 18
Registered: ‎02-24-2017

Re: How to achieve constant voltage output

P.S many SiLabs MCU's now have IDAC, you for example use a 10K pull-up resistor and the idac can adjust it's current sink and that result is a adjustable analog voltage out below the pull-up resistor.

 

Posts: 471
Registered: ‎01-18-2004

Re: How to achieve constant voltage output


abaz21 wrote:

Can a constant output voltage be achieved on the thunderboard react without using an external voltage regulator?


Constant voltage output of what voltage value, exactly, and for what use?

Posts: 9
Registered: ‎08-01-2017

Re: How to achieve constant voltage output

I would like a constant voltage of 3.3V, but 3.0V or 2.7V could work as well.

 

I am developing an iPhone application that receives readings (via BLE) from a sensor wired to the thunderboard react. I retrieve a reading from the sensor through the ADC. The value returned is an integer between 0 and 65535 (16 bit). I then convert this integer to voltage: V_across_sensor = V_reference * (sensor_reading/64435). I was hoping to get a constant voltage input to my sensor so that I can simply stream the sensor_reading value to the iPhone and calculate the V_across sensor with a constant value for V_reference.

 

 

 

 

 

 

 

Posts: 2,326
Registered: ‎10-14-2014

Re: How to achieve constant voltage output

I have below question:

1. which device you are trying to use?

2. now you need the device to out a constant voltage from the MCU that apply to your sensor, let us assume it was Vc.

3. Is the Vc work as power supply to your sensor, how much current the sensor need?

4. The sensor would output a voltage to be measure by the MCU ADC peripheral. does the sensor output level shift with Vc, does the sensor output range scale with Vc?

5. Since you could use the internal reference voltage as ADC reference, if the sensor output don't scale with Vc, I suggest you scale the range to [0, 65536] with software.

6. How you get 64435 in your formula?

WeiguoLu
Posts: 9
Registered: ‎08-01-2017

Re: How to achieve constant voltage output

1.) I'm using the thunderboard-react (RD-0057-0201)

2.) Yes

3.) Yes, however I can't find the current required in the sensor datasheet.

4.) Yes. I'm using a pressure sensor that is ratiometric to the supply voltage. With 0 pressure, the voltage should read 10% of Vc and with the maximum pressure, the voltage should read 90% of Vc.

5.) I receive an integer from the ADC scaled between [0, 65335] (16 bit range)

6.) That was an error, the correct number should be 65535 (16 bit range).

Posts: 2,326
Registered: ‎10-14-2014

Re: How to achieve constant voltage output

Then I am wondering whether VDD could work to power the sensor. At the same time choose VDD as ADC reference. Then 0 pressure to full pressure would get 10% to 90% ADC code.
The VDD may power sensor with enough crrent.
if you use other peripheral like IDAC you may need to consider if it could provide enough current and external buffer needed. you should also consider if the output voltage would change with load change if no buffer was added.
WeiguoLu
Posts: 9
Registered: ‎08-01-2017

Re: How to achieve constant voltage output

Yes, VDD works to power the sensor. So you're saying that 10% of my max ADC value will correspond with 0 pressure and 90% of my max ADC value will correspond to full pressure? This way I don't have to worry about what the voltage is. I can simply calibrate pressure to the ADC values and this will be accurate even if voltage slowly drops over time.

 

I may lose resolution with lower voltages. Do you know how low the battery voltage can go before the mcu won't turn on? I may need to monitor voltage and determine a threshold for minimum voltage I can run the sensor at. 

 

Thank you!

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Posts: 3,100
Registered: ‎02-07-2002

Re: How to achieve constant voltage output

Maybe you can open up the datasheets for the MCU and for the sensor yourself and find out.