Opa4227

msh5686

Hello everyone,

I am working with the OPA4227 on a project that requires an op amp that will read extremely small voltages.... typical input voltage from the particular sensor going to it is 20 mV. I have it wired in a typical non-inverting negative feedback configuration (use this all the time so I'm 99% sure it isn't wired incorrectly. The issue I am having is that the op amp when fed a voltage below ~ 0.5 V only puts out 0.07 V. As soon as I get to ~0.5 V, the op amp jumps to the positive rail (4.25 V). I have the circuit set for a gain of 11. Also, something strange in that the two inputs, which should be pulled to the same voltage (right?) never reach the same level. I haven't only tested the one chip, but several different ones, so I know it is not just a defective unit. I am completely stuck on this one as I've never had op amp issues before so any advice is greatly appreciated!

Tracy Allen

What power supplies for the op-amp? It is designed for +/- 5V to +/-15 V supplies, and would not work on logic supplies or in many single-supply circuits. The input and output voltages can't be closer than about 2V from either supply rail.

msh5686

I have tried powering it with both 5 V then 12 V, but nothing seemed to change. As for the input voltage, it will typical be around 20 mV... given a gain of 11 that should ideally give an output of 0.22 V

Tracy Allen

What I mean to say is that you need a dual supply, with negative 5 volts to pin 11 and positive 5 volts to pin 4. The small signal hovers in the middle near a ground connection at zero volts.

msh5686

Ok... I have been using a variable power supply that goes from zero to twelve volts. The output are one positive and the other ground. So this wont work? Can you recommend a supply that will? Thank you for the help so far!

Tracy Allen

Are you wanting to DIY a power supply, or go out and buy one? It's funny, dual output power bench supplies used to be very common, because analog circuits using op-amps usually perform best when biased away from the power supply rails. But now, it is hard to find dual power supplies at the usual hobby outlets.

Some DIY options:
-- 2 battery packs hooked together (+) to (-), with ground in the middle.
-- DIY from a center tapped transformer with a couple of diodes and capacitors to make the (+) and (-) with respect to the center tap.
-- Use the single ended supply along with something like an ICL7660 voltage inverter. You feed +5V in on one pin, and -5V comes out on another pin, with respect to a common ground. There are many ICs available for that purpose.
-- Buy a DC-DC power supply. Many are available and many different power ratings. For example, you could buy one in a DIP14 package that takes 5V at the input and provides +/- 12V at the output at 1 watt. E.g. DCP010512 from Texas Instruments/Burr Brown, about $10.

msh5686

Actually, I would really prefer to not have to mess with dual supply...add a lot of extras I would only need for the one op amp. I am in the process of trying to find a similar op amp, but with power supplies from ground to say 12 V. Most important is a very low offset voltage as my voltage going into the op amp from the sensor is on the order of 20 mV. Any suggestions?

Tracy Allen

There are a lot of single supply op-amps that have accuracy and offset specs at least as good as the OPA4227. From TI, Burr-Brown, National, Linear Tech, Analog Devices, Intersil, Maxim, Microchip, etc. etc. If you go to their web sites for op-amps, in the parametric search you are looking for precision and an offset voltage in the microvolts, and input common mode range includes V-, and output swings to V-. You did not mention a bandwidth requirement, but you would need to include that if you really need the 8MHz bandwidth of the OPA4227. Also watch out for the power supply limits. The output on a single supply op-amp does not go to identically zero, but it will swing down to within a few millivolts of V-.

The single-supply op-amps I use primarily for DC precision are the LTC1051 or the LTC2055. They are "autozeroing" op-amps meant for precision at DC and low frequencies, and the offset is less than 5 microvolts.

Another good option I forgot to mention is a virtual ground. A separate op-amp is DC biased from a single supply so that it puts out say V/2 at its output, and you use that as the common reference point for your circuit. I keep on hand a few dedicated virtual ground chips (3-lead TO92), the TLE2426 and TLE2425.

msh5686

Thanks for all the help. At last, I think I have all I need to finish my project now!