OPA277 offset voltage issue

Matt,

The output offset you are seeing is very large indeed.  Also, this device has a very high PSRR (power supply rejection ratio) and it does not make sense that you would see a large variation at the output due to power supply induced offset.  Also, the sensitivity to "air currents" is also troublesome.

If I understand what you are saying correctly, you have the circuit shown below.  If this is the case, this is a very benign configuration and should not cause these types of offsets to appear at the output.  Without more detail the only thing that I can suggest could cause these problems would be the following:

(1)  An ground loop (i.e. potential difference) between the 100k ground and your non-inverting ground.

(2)  Rectified noise off the inputs , insufficient supply decoupling

(3)  Does this happen on more than 1 device?

 If the 100k resistor is not tied to ground and you are using a separate signal source or generator, I think you should check the voltage across the first 100k resistor to see if the current corresponds to what you are seeing at the output.

Matt

I would take a voltmeter and measure the voltage between "+In" and "-In" terminals. This voltage will be the offset of  the Op. Amp.

Thanks for the recommendations. Yes, my circuit is correctly depicted in your schematic. Here are a few further observations based on your suggestions:

1. There doesn't appear to be a ground loop issue - I have physically connected the non-inverting ground to the R2 100k ground with a short piece of wire, and there is no measurable potential difference.

2. Supply rails look clean when viewed on a scope - they are decoupled with 0.1uF ceramic to ground.

3. I tried several other OPA277s, and they all yield similar results.

Based on my measurements, the voltage at the output is dropped entirely across resistor R1; there is no voltage drop across R2.

I ran a few further experiments by reducing the resistances of R1 and R2. With R1/R2 ~10k, the output voltage drops to about -0.8mV. When I reduce R1/R2 to ~1k, the output drops further to -0.06mV, which is within spec. Could it be that the op-amp is happier driving smaller impedances or seeing a smaller impedance at its input? I noticed that the data sheet is based on Rl = 2k.

Matthew

Dmitron,

The voltage I measured across the input terminals was 0V - not fully sure what this implies, as one would normally expect this voltage to be the output-referred offset divided by the amplifier gain. Please see my previous post for some further observations. Thanks.

Matthew

This configuration for the OPA277 is very straightforward; there should be no issue driving 100k ohms.  The fact that you can change the value of the resistors and keep the ratio the same and the voltage at the output decreases leads to the following conclusions:

(1)  All of the OPA277 devices you tested are either damaged or have an IB that is grossly out of specification

(2)  You have a parasitic leakage path on your board or socket that is injecting current into the summing node.  This would effectively look like a couple hundred ohms of parallel shunt impedance on R2 which makes your effective gain much higher than you think it is.

Did you try this in a different socket or test configuration?  One quick check would be to remove R2, keeping 100k ohms in the feedback.  if you still get ~ -20mV you probably have a leakage path to ground and should check your solder connections or socket.  If you are using a dual device (OPA2277) the summing node (vin-) is located adjacent to the V+.  If this is the case it is also possible that there is a parasitic leakage path from V+ to Vin- that is causing the output to be much different than expected.

I hope this helps.

Matt

I ordered some extra OPA277 samples, and they yielded similar results as before. Per your second suggestion, I removed R2 and kept 100kohm in the feedback path. Unfortunately, this didn't improve things much, so I tried installing the OPA277 in a couple of other types of sockets. On the third socket that I tried, the output voltage finally dropped down to a more satisfactory value. I then re-installed R2, and measured the output voltage to be approximately -0.6mV to -1.0mV on several different samples, which is much closer to the in-spec voltage (some of it could perhaps be due to measurement uncertainties). At this point my conclusion is that there may have been a flaky socket connection or solder joint. Is it also possible that this behavior is due to variations in the contact resistance from socket to socket? Thanks for your help.

Matthew

"Improve" was probably poor word choice on my part - thank you for the further clarification. Looking back at my notes, I was still seeing anywhere from about -4mV to -25mV with R2 removed (depending on the socket and op-amp sample), pointing to the parasitic leakage path you suggested.

Matthew