Original question:
And what is R1, R15 and R16.
We are moving very slowly here, because we have to tear nearly every answer out of you. This is no fun 
Why is it so difficult to post a complete schematic, Atul?
Kai
Hi Atul,
Unfortunately, again it is difficult to pinpoint the issue without knowing more about the circuit. A number of factors could be at play. Assuming units of mA, a fluctuation of 0.01 to 0.05 mA could correspond to a fluctuation of 0.3413mV to 1.7064mV at Vin, due to your gain of 29.3 mA/V ( ([10k/412]+1) V/V * 2 * 40/69 mA/V ).
Take for example this circuit, essentially Figure 5 from the datasheet modified to more closely resemble your circuit. Note that the way your R15 potentiometer is set up, without a resistor between the bridge leg and the wiper, the potentiometer position has a serious, nonlinear effect on the offset of the bridge measurement. As you haven't provided an R15 value I just used 10k for demonstration purposes (ideally this would be significantly larger). In the below sim, the output should be in the mA range (you can think of the VCVS as performing the 40/69 mA/V conversion) but depending on the wiper position it can shift significantly. If there is instability in the wiper setting (very common with potentiometers) then it stands to reason this could result in instability of Vin and thus what looks like instability at the output.
Note too that you'd typically expect the potentiometer to be connected in parallel with the bridge, with the bottom terminal shorted to the bottom leg of the bridge rather than to the IRET. Again please see the design Kai did here - click me
From your description it sounds as though there may be a noisy environment at play. I'm assuming the 5th pin on the J2 connector is a connection to a shield on the load cell. It could be that the bridge is picking up noise and this is appearing at the XTR/OPA inputs. You could try increasing the size of the C3 cap to 0.03uF, and may want to consider adding the additional 0.01uF bypass caps shown in Figure 6 of the datasheet.
Unknown or parasitic capacitances on the circuit connected through J4 could theoretically be playing a role here as well, causing some instability or ringing behavior that will result in an AC current riding on your steady-state output, but it's hard to speculate without knowing more details.
Cheers,
Jon
this is complete ckt with values. right now I'm checking it without trim pot.
I'll add it externally with suggested corrections.
Hi Atul,
please wire the XTR106 exactly as shown in the datasheet:
So use "Vref5" and "Vreg", "Vref5" for the load cell and "Vreg" for the OPA2277.
Remove the pot R15 because the way you have connected it to the circuit is completely wrong.
The load cell, the OPA2277 and the XTR106 must be mounted close together. Don't allow any cabling here.
If you still have unexplainable noise at the output of XTR106, add 10nF low pass filter caps from both inputs of OPA2277 to the ground pin of OPA2277 (which is "Iret"). And mount these two caps close to the OPA2277.
If you still notice noise at the output of XTR106, add some shielding of load cell and input circuitry and connect the shield to "Iret". But be careful with all these measures because Iret is not true signal ground but is a floating auxiliary potential. So if anything of the input circuitry (load cell, shielding) could be touched during normal operation, then make an isolation arround the circuit and the shield. Otherwise you can get trouble with ESD, Surge and Burst (EFT).
The same is valid when you are doing measurements and must touch Iret with the ground terminal of DMM. Simply by this you can inject so much EMI and noise that the circuit appears to be noisy, although it isn't noisy at all.
If you need to touch the outputs of OPA2277 with the DMM, don't do it directly but insert a 100R isolation resistor which you mount close to the output of OPA2277. This is necessary to avoid instability.
Kai
