USB changing 5v voltage and its effects on zero offset with ratiometric pressure transducers

Other Hardware 3rd Party Boards
1

I have just built up a DIY UNO (OrangePip Segments 328)

This came with a LM7805 regulator which I have launched into the bin and replaced it with a Tracopower TSR-1-2450 stepdown switching regulator.

I am using a 9v external power supply and I'm measuring a very stable 5.022v ( no load / sensors) however once I plug in the usb cable the voltage increases and fluctuates between 5.079v - 5.080v.

For info, the OrangePip board uses a TO-92 7133A-1 LDO.

Could somebody please explain what I'm seeing here so I can try to figure out how to stop the USB port affecting the 5v but still have a functioning USB port ?

I was intending on using this board with 4off 5v ratiometric pressure transducers which would be connected to Excel Datastreamer so USB and a stable 5V is needed.

2

Are you seriously worried about about such a small change of voltage and a fluctuation of 0.001V ?

How are the sensors interfaced to the Uno ?
What exactly is providing this 9V supply ?

3

Hi,

A 9v 2A switching DC adapter is supplying the power.

My HUBA 4 wire transducers are fed from the 5v rail and read off the analog inputs.

The formula I'm using is basically:

Pressure =AnalogRead - Offset * 16 / (fullscale - offset)

I'm not sure if this is to be expected with the code I'm using but if the USB is plugged in, my zero points changes on my current UNO that uses a standard linear regulator. I don't have this UNO to hand so I can't measure any values until the weekend.

I haven't actually tested the one I have built up tonight, I just wondered if there was a way to make it stable. But as you've rightly pointed out, we are talking 0.058v

4

Have you considered using the internal voltage reference of the "Uno" ?

5

The problem I have Bob is that the transducers are ratiometric and need to be supplied via the Arduino 5v rail and if I'm correct, need to be read using the default 5v reference to maintain "ratiometricity" for want of a better word.

Where it's becoming unstuck for me is the 5v voltage variance if USB is plugged in and how that affects the zero offset value i.e zero will no longer be zero.

It might have helped if I'd went in with this question to start with rather than clutter up the forum. It may be a better idea to start another post in reference to ratiometricity and any suggestions on how to achieve stability.

Thanks for your replies

6

Please don't do that. Add to this topic and by all means change its title but don't start another one

You say that the sensors are ratiometric which to me means that their output is proportional to an input. If that is so, then does the reference voltage need to be 5V ?

7

Yes they are ratiometric

I'm just assuming I need to use the default 5v reference as the sensor output is 0.5-4.5v?

8

That is an important detail

The internal reference is 1.1V on a 328 based board

9

Indeed Bob, apologies but I should have mentioned that from the get-go.,

Given that a change in 5v voltage affects zero offset calibration, is the only feasible way to obtain stability is to say, use a stable, precision 5v source and supply the Arduino 5v pin & sensor 5v supply voltage and use something like an ADS1115 external ADC ?

Basically I'd like to know what are the best practices regarding the use of ratiometric pressure transducers.

I do intend on using CAT6 cable for the transducers and keeping the sensor supply voltages well away from the analog pins on a piggyback proto-board. I also intend on using a smoothing filter.

10

If the change is ratiometric, as is extremely likely, the effect is canceled by using the sensor supply voltage as the ADC reference.

That is the entire point of using a ratiometric ADC with a ratiometric sensor.

The output of the sensor ranges from 10% of the ADC maximum to 90% of the ADC maximum, regardless of the supply voltage.

11

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.