What is the best way to provide +/- 12VDC to opamps?

plankton

I'm working on several sensors circuits (pH, ORP, Conducitivity) and all the low-input bias FET opamps I've seen so far al require a minimum of +/- 7VDC supply.

What is the best way to provide +/- 9 to 12 VDC?

I've played with ICL7660 but they max out at 10VDC, don't drive much and the voltage drops off fast under load...even 20ma.

Thanks.

Scott

Leon

I've used a bridge rectifier with a centre-tapped transformer to give +/- 15V, for powering op amps.

Leon

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plankton

Thanks leon. I should have metioned that I'm familiar with the classic dual-supply circuits and don't want to bother getting and wiring in a center-tapped transformer and all that. If possible I'd like to leveral the the single +5VDC and +12VDC voltage sources I already have to my controller.

Mike Green

You can use a MAX232 to generate roughly +9V/-9V if the current drain is low enough. You can also buy or build your own switching regulator to produce the voltages needed.

Phil Pilgrim (PhiPi)

Here's an app note that may help: pdfserv.maxim-ic.com/en/an/AN974.pdf.

-Phil

Tracy Allen

For many purposes there is no need to use +/- 12 volts. These days there are lots of high impedance mosfet input op-amps from Maxim, National, Linear Tech, TI Intersil and others that will work on much lower supplies like +/-5 volts, or even a single supply as low at +3.3 volts, or lower. The cell phone industry depends on those and mixed signal (analog + digital) chips. They are not exotic or hard to find.

An example that I keep around in my own shop is the dual LMC6062 from National. It has a 16 femtoAmp typical input bias current and other characteristics that make it suitable for a pH probe interface. All the op-amps in the National LMC series are CMOS devices, and have trade-offs of accuracy, bandwidth, power consumption and all the other characteristics that you have to sort through when choosing an op-amp, but CMOS input generically is often the best choice for high impedance pH type interface. At Linear Tech, the same applies for op-amps with the LTC prefix. At TI it is TLC.

There are times when a +/- 12 or 15 volt supply is a good idea, but that has to do more with the common mode noise one might run into in an industrial environment, and in those environments it is often necessary to take one big additional step to provide a galvanically isolated interface to the pH probe. That means an isolated power supply and an isolation barrier for the data, either digital or analog.

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Tracy Allen
www.emesystems.com

plankton

Thanks all. Looks like I have some more homework on my hands!

Scott

plankton

Tracy - I've been experimenting with an OPA-129 'Ultra-Low Bias Current' op amp. Bias current is +/- 30 fA, but what I've noticed even when I run the opamp input pin directly to the BNC connector that the voltage drifts like crazy.

I'm considering running a differencial design with an opamp on both ends of the BNC then feed those to a comparitor opamp. I've also tried adding a 'guard' circuit around pins 2/3 and tie that to the substrate ground, but the voltage still drifts.

Back to the drawing board...

SCott

Tracy Allen

"drifts like crazy"? Is there a pH sensor as the input to the BNC?

You have to establish whether the drift is within your circuit, or coming from outside. Try a steady known voltage source at the input, say, a lithium coin cell with a voltage divider made with MOhm range resistors.

Is the op-amp hooked up as a voltage follower? If so, the guard voltage should come from the output and surround the non-inverting input pin. But that is not the problem if the input pin is lifted away from the circuit board and connects directly to the BNC center pin.

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Tracy Allen
www.emesystems.com

plankton

Tracy,

For now, I'm using a DC to DC converter (NMH0509S) that converts 5vdc to +/- 9VDC (111ma) to power my opamps because ... well ... I already have them...

If I disconnect the probe, the output voltage drops to V-.· If I connect the probe and leave everything alone the voltage will stablize. However, if I bring my hands within about 2 inches of the board the voltage drifts again.· Very, very sensitive circuit.

I'll try your suggestion of replacing the probe with a fixed voltage like 1.5 or 3VDC battery and see if that makes any difference.

I'm playing with the circuit from OPA129 datasheet 'FIGURE 5. High Impedance (1015&#937[noparse];)[/noparse] Amplifier.'· See attachment.

Thanks.

PS Since I have four of these, I'll try a differencial circuit to see if that makes it less sensitive.

Scott

Tracy Allen

If you have an oscilloscope or an AC voltmeter, you can look for AC on the output signal as you bring your hand near.. AC pickup or oscillation (instability) can look like drift on a DC voltmeter. It is most likely 60Hz AC pickup, but it might be high frequency oscillation due to feedback.

How is the wiring, especially to the high impedance inputs? It should be short and direct. Shielding (a metal box) might help too. The same thing with the OPA129 circuit.

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Tracy Allen
www.emesystems.com

plankton

Tracy,

I finally figured it out. For the 500 ohm resistor, I mistakenly put a 51 (green brown black) instead of green/brown/brown. So, instead of a gain of about 20x my gain was 200x. No wonder there was so much 'drift'.

After replacing the resistor, I did a quick and dirty check with ph solutions (non standard) of ph8 and pH 11 and received voltages of ph8=-2.0; ph11=-5.1; which is consistent. It also looks like I need to adjust and invert the output because the voltage is going negative.

Thanks again guys. Looks like I have a working circuit I can develop now.

Scott

dev/null

I have been looking for an easy solution to +/- supply for a voltmeter I am building. I finally came acroess this circuit. Works great for my meter. Good up to 100mA or so.


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dev/null

Beware that the CD-1 version of 7905 have the ground pin on the left, instead of middle. Check the datasheet.

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kwinn

Ah, the good old 555. Never has something so simple been used for so many circuits. Has to be the most versatile non programmable chip ever made. Used for more functions than anything I know of. Timer, pulse generator, motor run sensor, power supply inverter, PMT hv supply, cassette tape data modulator/demodulator, missing pulse detector, and many many more that I just can not recall at the moment.

dev/null

It's the coolest chip ever. On a note, the above circuit can't be used with a CMOS 555, it has to be the standard one, as the CMOS one is too slow.
Oh, and you could of course use 7812 and 7912 to get +/- 12V.

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