No, but I'm much more comfortable with digital circuits than analog, so that doesn't mean there are no issues. At least we can see the full circuit now, and the digital parts seem ok to me.
Thank you for the complete schematic.
To be clear, you have 1V65 on pin 6 of the AD623 but 1V2 on pin 1 of the MCP6002, have I got that right?
I am OK with analogue circuits but there are definitely people here who know more than I do. With that said I have looked at your circuit and the data-sheets for each device and the circuit looks OK to me. While I don't think it's the cause of your problem the datasheet for the AD623 does show 0μ1 capacitors on the power pins, I suggest you add them. I would generally put 0μ1 ceramic capacitors on the power pins of all ICs anyway.
Assuming both mine and @PaulRB 's assessment of the circuit is correct and there is nothing wrong with it then that leaves faulty components or the circuit not being as shown. Here are some suggestions:
- Check for shorts, particularly on the output of the AD623 and around the MCP6002
- Check all power supply and reference voltages are as they should be
- Check the circuit really is as you think it is
- Check resistor values are correct
- Check for devices getting hot, ideally with an infra red camera if you have one, or your finger if not
It might be helpful to post photos of your circuit, photos should be very clear and well lit. If a wire disappears of the edge of one photo then it should be clearly visible on the next so that all wires can be traced.
@PaulRB @PerryBebbington Great, thank you both for taking a look.
To be clear, you have 1V65 on pin 6 of the AD623 but 1V2 on pin 1 of the MCP6002, have I got that right?
Yes that’s right. I will double check everything as you suggest and add those decoupling caps this evening. I may experiment increasing the resistor values in the RC filter as the AD623 drive high impedance loads but then again, I think MCP6021 has high impedance anyways but worth a go.
Thanks for your thoughts 
That is correct so the filter input impedance at DC would be the op-amp input impedance so changing the resistors should have no effect.
How are you making these DC measurements?
What type of capacitors are you using in the filter circuit?
Is this build on a PCB or a breadboard?
Hi Jim, the DC measurements are made by connecting the output of the AD623 or the output of the filter directly to the ADC of the SAMD21. I am using ceramic capacitors ±10% and this is currently on a breadboard.
I did increase the resistor values and reduces the caps just to experiment but no change in behaviour. I will try configuring the op amp just as a voltage follower and see if it’s the IC but my understanding is this is unlikely? The other thing I could try is applying mid supply bias at the geo phone inputs - no idea why that would make a difference but worth a shot 
Is there a buffer and filter on the ADC input?
Not the best way to debug, a scope or multimeter would be better and provide more useful information.
currently on a breadboard.
That can be very problematic, especially if you are using one of the cheap ones.
I will try configuring the op amp just as a voltage follower and see if it’s the IC but my understanding is this is unlikely?
It a good debugging step since you don't know what is actually
going on.
Also is the other half of the opamp configured correctly? You can't leave inputs floating.
I suggest that you measure with an independent device, such as a multimeter. If you don't have one please buy one. Even a cheap on is worth having, (£$€)100 will get you a decent one.
Suspect the breadboard first. Check every connection really carefully, breadboard is not reliable.
Thanks both, preferring a multimeter over the ADC read out allowed me to find the issue pretty quickly. Looks like i was missing the output cap on the DC converter so it was outputing a negative rail but no where near what it should have been outputting. I reconnected everything, double checked recommended applications in the datasheets, and all looks good now with a solid bias of 1.65V.
Thanks again for all your help, I've learnt a lot 
One academic question if you don't mind so I better understand how diff inputs should be coupled to DC. In the original schematic, I was routing floating inputs into the in-amp and then the in-amp was applying mid-supply bias - is this too late in the signal chain to couple the signals to DC? Does it need to be done as the first step always?
Depends on what is connected to the inamp.
Is it a bipolar signal or does it already have a bias?
Thanks Jim, I was thinking of the current application so the bipolar differential inputs from a geophone, could I feed these floating inputs directly into the in-amp if the in-amp couples them to midsupply?
If I were doing it, I'd make everything single supply and bias up the geophone signal like shown in the second diagram in post #5
Gotcha, i will try adding the midsupply at the geophone input. Thanks for your help
Analog has a nice online tool for simulating their Inamps
I can't give you a generic answer that will always be right in every situation, however, here are some things to think about:
- You originally suggested that the capacitors were to remove DC bias, but there was no DC bias to remove. You need to know what the requirements are of the 2 parts of the circuit, if there's no DC to remove then don't try removing it.
- Check the input requirements of the amplifier to see if it needs external bias circuitry, I'd expect any kind of opamp to need DC bias on the inputs from somewhere.
- Make sure there are no floating inputs, including inputs you are not using. Check the datasheet for the manufacturer's recommendation.