I don't know loads about electronics but am struggling to find a simple way to get a low negative voltage

im trying to make a handheld device which is used to measure bite force, so I have some Flexiforce A301 piezoresistive sensors connected to the circuit below and then Im reading the voltage through an arduino chip


Vref should be around -1V but ideally I would like -0.6V as I need a larger range of force inputs (the chip has a max input voltage of 5V)

I would ideally like to use a single AA 1.5V battery which I was thinking I could connect the + to ground to get a negative voltage, and then somehow regulate it to achieve the desired -0.6V. It does not need to be exactly -0.6, it can be between -0.5 and -1V but Once it is set to a value I need it to stay fixed at that value in order to reliably calibrate the sensor through the arduino code.

I'm in the UK and have a small budget - any ideas would be appreciated, thanks!

 

Hello,

Take the AA battery a resistor and a diode:


Bertus

 

The simplest way around the problem will be to raise the negative supply terminal of the opamp above "ground"(zero volts.use one or two silicon diodes, 1N4001 or 1N4000in series between that and the supply negative.. Add the diode so that it is biased into conduction. Then the terminal of the sensor can connect to the supply negative ("ground") and it will be the correct voltage lower than the signal side connection of the amplifier. A cheap and easy way to gain the same effect.
Certainly there is a small loss of efficiency from the voltage drop across the diodes, but it is not a big deal.

Now on May 1, I realize that there are two conflicting requirements at play. First, not all op-amps work well with input voltages close to their negative supply voltage. But the other requirement is the desire to have the output voltage be zero while the input voltage is not zero. On top of that, the signal is feeding the inverting input.
So a bit of re-thinking is required. I will post that correction later.

 

My first thought was to move everything up by 0.5V. Then I found an application note saying the same thing. Here "Vref" is moved up some small amount. In the example they say 15%. This causes an offset that can be compensated for.

Here you can see the 15% offset.

If the output goes to a microcomputer them the offset can be removed in software. If you need it done in hardware we can show you how to fix the offset with the addition of a second op-amp.

 

My suggestion is based on the reality that most op-amps are not able to perform as well with the input voltage near the negative supply voltage.

 

most op-amps are not able to perform as well with the input voltage near ........... supply voltage.

Agree but "The MCP6004 is a quad general purpose op amp offering rail-to-rail input and output over the 1.8 to 6V operating range."

 

The LM324/LM358 have PNP input transistors and work very well with their inputs around ground. The main drawbacks are that the designs are very old (and well proven) making the bandwidth low (no problem for a DC reference) and with input offset voltages not as good as you can get with more modern op amps.