OPA376 output noise

Hi Marek

Thanks for reply

biasing voltage is mid rail (2.5V) and with virtually no signal applied I dont think it should be going into non linear region.

Yes I agree in my schematic OPA132 is not appropriate choice.

See the attachment for relevant part of schematic, its not up to date and there are few errors, like U17/U22 are OPA376, R112/R113 are 220K and R60/R67 is 100K but as i mentioned in last email that i have tried quite a few and found that it is this resistor controlling output noise (and also effecting gain which is logical, because of decoupling cap and biasing resistor corner frequency). U14 and U15 are used to provide three gain settings, control from uC. AVDD/2 is created with standard OpAmp mid rail configuration (also used in almost every other circuit).  U31 is a servo loop to nullify any dc offset.

One observation, in a very quite envirnoment (AirCon Off) and  input wire removed noise reduces by atleast the factor of 10.

7827.OpAmp Noise Problem.pdf

Mujahid;

There are a number of things that can cause the output to be influenced by the ambient air-- as you said "Air Con Off". One is simply the thermoelectric potentials caused by dissimilar metals in the input portion of your circuit. Very high gain and air turbulence can cause delta T across a PCB and this shows up as 1/f noise. I characterized this in the original Burr-Brown data sheet for the OPA27. The actual OPA27 noise was far lower if the high gain circuit was shielded from ambient air.

Another source of problems can relate to the TC matching of the resistors & switches controlling the gain of the difference amplifier. These resistances must match very well if the diifference amplifier is to achieve high CMR.

Building your own instrumentation amplifier is not a good idea. Using a good IC instrumentation amplifier in as high gain a fixed gain as possible and following it with a switched-gain op amp will be far more sucessful. The IC has on-chip laser-trimmed thin film resistors that have extremely good matching and TC tracking-- far better than you can achieve with disctete components..

Can you put the notch filter at the output of the INA? That would allow you to lower the source impedance seen by the INA on the non-inverting inputs which would be at a minimum the value of the bias resistors (R60 and R67) shown to be 3K9 each in the attached sch. There may be so much 50/60Hz pollution this may not be possible but there's a lot of series R in the notch networks that's going to hurt your 1/f noise performance. Are R112 and R113 in series with the inverting inputs really necessary? Noise current will develop across them and my uneducated hunch is they don't help the DC performance that much. I'd shunt them to see what happens.

Does it do it with the input coupling capacitors completely removed? There's DC bias (AVDD) on the right-hand side of those 4X 10uF per leg and a 1M pull down on the left-hand side. Though small, noisy leakage current does flow across those capacitors.

The leakage current of the input coupling caps, if electrolytic, is many orders of magnitude greater than the pA-range bias current of the input op amps. Typically you'll see 1-3 uA per cap. The leakage current also has a large 1/f component. There's a lot to be said for film caps but in 40 uF x2 I doubt that you want something D-cell sized.

If the input is floating, the discharge time constant is very long (40 seconds) due to the 1M at the input. You may be seeing some dielectric absorption effects from the capacitors when they discharge. They will appear to discharge, then "rebound" due to DA and then discharge some more. You can spend hours watching DA effects because its so, so slow.

I've fought capacitor leakage and DA in low noise mic preamps for pro audio. Take the 1/f noise from high source impedance, capacitor leakage current with dielectric absorption add 80 dB of gain and things get interesting. You see a lot of component effects from stuff we normally take for granted: Like caps blocking DC.

That's my 2 cents. Good luck.

I just got to the bottom of the problem, it is actually ac coupling capacitor causing problem. I was using ceramic X7R capacitor for ac coupling. ceramic capacitors have piezoelectric and may be some other effect that makes them very sensitive to any vibration around and temperature change.

Moral of the story, with low voltage signals and precision required, never use ceramic capacitors for ac coupling.

Although it has been very frustrating but i learned a lot

thanks for everyone's help

Mujahid

Mujahid;

Congratulations on tracking down this obscure problem! Few people realize that high-K ceramic capacitors are piezoelectric. Just tap on one in a circuit like yours with a pencil and see what happens! As an additional warning, they are also pyroelectric-- nice little sensors that respond to temperature changes-- delta T. You may have ben seeing some of that as well.

"I just got to the bottom of the problem, it is actually ac coupling capacitor causing problem. I was using ceramic X7R capacitor for ac coupling. ceramic capacitors have piezoelectric and may be some other effect that makes them very sensitive to any vibration around and temperature change.

Good job! Guess you know how to make a ceramic microphone now...

What type of capacitor are you going to use?

Hi -

     Try the MAX4238. This is the lowest noise chopper op amp available. In addition, the MAX4238 has an input bias current of only 2pA so the input is of electrometer quality. The noise of the 4238 is about 100nV/Rt-Hz at 10Hz and drops to about 20nV/Rt-Hz at 0.001Hz.

     You may may need to control temperature in order to reduce noise to the levels you are working at. For a metal film resistor, a temperature difference of 1 Deg-C between the end caps will generate an offset voltage of about 10uV, which is changing constantly with temperature; so if temperature has 1/f, then the resistor offset will show this also.

     Circuit connections are actually thermocouples, so if everything is not gold flashed you wiill also measure the noise of many thermocouples in the op-amp input circuit.

Remember when measuring noise, it is impossible not to measure the noise of about 20 contributors in the op-amp input !!!

I have also simulated noise using LTSPICE and it seems quite close to measured noise. This designed could have been better if i had realised all this before. anyway i probably have to live with it.

I'm going to try tantalum capacitor with low leakage current of 0.6uV, farnell part no  1457412.

Please share if someone have any suggestion about replacement capacitor

mujahid

Mujahid;

You might be able to find some film capacitors that would be high enough in value to work-- they will have very low leakage and won't exhibit the problems that you've observed in high-K ceramic types. They will be pretty large physically, even though your required voltage rating won't be very high.

You can analyze noise with TINA as well. With any software, simulations are no better than the device models, so beware of old macromodels that are too basic to include things like noise.

The OPA378 is a low noise 5V chopper amplifier.  It has 20nV/rtHz across the entire frequency spectrum.  Also, see attached noise article on the topic of zero drift amplifiers and noise.