Voltage regulator for analog circuits

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I've been looking into making a pH sensor circuit with an atmega168. I have learned that I needed to use op amps to amplify the voltage from the pH and have an offset in order to ensure I have no negative values.

I finished the circuit and now I am looking for parts.

I need a +/- voltage regulator for the op amps right?
I haven't found any for +/-3.3V as I would hope. With 5V I haven't found any parts I could solder.

Anyone work with op amps in low voltage devices that can give me any suggestions?

Thanks in advance.

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If your output is always positive, you MAY be able to work with a single supply. To get a negative supply voltage, you need more than a negative regulator - you also need a negative voltage source. Alternatively, you COULD use a switching inverter to generate you negative supply voltage from a positive input voltage.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Hi Emmannuel,

If your current consumption is low, you could use a MAX232CPE RS232 DIL-packaged chip to generate the negative voltages from a +5 volts rail. It can give you about -10 volts. Then regulate to your required rail value.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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Also consider the classic capacitive charge pump inverter. Those work nicely but are a bit noisy.

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If your circuit uses ac transformer with rectifier is easy to modify it to output a negative votage.
In a simple rectifier with 1 diode and the capacitor according how the diode is connected the output is
0-+xxV or 0 - -xxV.
Having one more diode connected with the opposite polarity in the point where the other diode connects with the transformer and a capacitor connected the circuit outputs a negative voltage also.

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If you are only making a few of these you might look for a DC to DC converter module that is ready made building block. All you do is supply the input power such as +5 or 12V and get your +/- rails out.

EDIT: Here is an example PN at Mouser for $8

580-NKA0503SC

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I don’t see why you need a negative supply (unless you have built your circuit that way – this means you have to shift it before you drive the ADC in the micro).
It's been a while since I worked with pH electrodes but here goes ...

A pH electrode gives you about 60mV per pH unit, and you can go 7 units up or down from pH=7 where the voltage is 0.

Electrodes are very high impedance (say 500Meg) so you need to buffer the output with an op amp before you drive the ADC input. A JFET or CMOS input op-amp will have bias currents in the pA or fA range and if you use it in non-inverting mode you get the very high input impedance.

Considering that you need 7x60mV for half the range and may have an offset of say 1/2 a pH unit you will only get about 0.5V above and below midrange.

So you can use any single rail op-amp with a gain of 3. Use a virtual ground at 1.6V to offset the pH probe. You then drive the ADC and subtract the offset in software.

pH is not that accurate unless you run a full calibration, but even then I imagine the 10bit ADC is more than adequate for a meter without gain. It will still measure about 1V from 0 to 14 which will become 310 lsb counts full scale, a resolution of 0.3%. Frequent calibration also removes the effect of op-amp offset voltage (this is done in software of course – what’s the point of the micro if it doesn’t do some work). It could also measure the temperature.

OK, there’s more, like what your reference electrode is and whether you need low power but in principle and from your description I don’t see you needing any more than a single 3.3V supply for the whole circuit.

Cheers
Klave