In Op-amp datasheets, input offset voltages are specified. Does this input offset voltage remain same (no significant difference) for each IC or can it change significantly each time I use the device? e.g. if the offset is defined to be ±1mV in datasheet and I measure it +0.5mV for one IC. Can I expect it to remain +0.5mV (±some uV) for that particular IC within its lifetime or can it have any value between +1mV to -1mV every time I switch on the device?
Electronic – Does input offset voltage for amplifiers remain almost same
offsetoperational-amplifier
Related Solutions
Couple of theories here:
- How sure are your power supplies are coming up symmetrically?
If one rail comes up before the other, you may have non-zero output voltages from the op-amp for a very short period of time. - Have you implemented all the PCB-layout practices needed for such high-impedances? At minimum, you're going to need guard-rings on all the ultra-high-impedance nodes.
The National (Now TI) LMC6082 datasheet has a good discussion of what is required to get board-leakage currents low enough to not be a problem.
This will likely not address the possibility that you have dielectric soakage issues, as discussed in @RocketSurgeon's answer.
A good and easy way to test his answer would be to desolder one of the caps on a bad board, and reverse it. If the offset is flipped in the other direction, it's a dielectric soakage issue (because the persistant charge in the cap will have a single polarity). If the offset voltage does not change, the problem is not the capacitor.
One thing I don't see the dielectric soakage issue explaining is why the charge seems to come back when the circuit is unpowered, and go away when it is powered. Since the element that discharges the capacitor is connected continuously across the cap, (e.g C1||R2, C2||R1), the contribution of any current leaking out of the cap should be a constant, and not affected by the supply voltage.
The only thing that comes to mind for me would be that there is something hygroscopic somewhere, and it injects an offset current. When you power the board, it warms up, and drives the moisture out over time. Turn the board off, and it begins to resorb moisture.
One comment I do have is I don't see why you have both SW1A, and SW1B. You can entirely dispose of SW1B. Just tie both of the R/C pairs together, and to the output of the op-amp. When one of the cap/resistor sets is selected, the other will just slowly discharge. As long as one end is floating (which is accomplished by SW1A), the voltage on the other end is irrelevant.
The LM324 has a maximum offset voltage of 9mV (worst case, over temperature), according to the datasheet.
With your circuit, with 0V in, you could have a current of 9mV/500m\$\Omega\$/9mV = 18mA below which your pot would not be able to set the current. So it's not a very good design if you need to set it to less than 18mA. It's luck of the draw- the next op-amp (even in the same package) could be 9mV in the opposite polarity, so you'd just move the pot.
Maximum temperature drift of the LM324 is not specified (it's not intended for precision applications, after all), but it might easily be +/-10uV/°C, so if the board changes by (say) 70°C as the MOSFET gets hot, the current will change by 0.7mV or 1.4mA, so you'd have to readjust the pot. Of course the highest power dissipation occurs at high output currents, so the change is relatively small (1.4mA out of 2A is < 0.1%). A 20°C change in ambient temperature means a change of perhaps (no guarantees) of 0.4mA, which is several percent of a 15mA current. If you only care about 5%, and currents above 20mA, probably just okay.
Another difference between a cheap amplifier and a good one is the gain. The LM324 can be as bad as 25,000 gain (and it changes with temperature). A precision op-amp will have a gain in the millions. The difference will show up in how well it compensates for load or line changes (not a big deal in this case).
The bias current of the LM324 can be as bad as 0.5uA (typical 20nA) and it changes with temperature so if you had a high resistance pot, you could see it change with temperature.
The noise of the LM324 is a fairly miserable 35nV/sqrt(Hz), and it has nasty crossover distortion, neither of which affects you much in this case.
A couple of things (other than being extremely cheap) that the LM324 has that a typical precision op-amp may not have- wide supply range (especially on the high end), though it may not do so well at very low supply voltages, and it's single supply (input common mode range includes the minus supply) which you absolutely require for your circuit.
So there are plenty of reasons to use a decent op-amp if it's required by the specifications. Or you can get clever with the circuit- increase the sense resistor to get good accuracy for low currents, but to get wide dynamic range, a good amplifier (and other techniques such as good resistors and good layout) may be worth it. For just hacking around and if your current range not huge (minimum to maximum), an LM324 is certainly acceptable. There's no point in using a $5 op-amp if a 1-cent one will do. On the other hand, there are some requirement for which the best ones are not good enough and one has to resort to discretes and other techniques.
By the way, your circuit may not be stable against oscillation. It can be fixed with some passive components, but loading op-amps with the equivalent of a large capacitance in series with a small resistance is inviting trouble.
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Best Answer
It should be fairly stable for a given chip, when measured at the same temperature, supply voltage and common-mode voltage (and assuming the chip is not damaged by overheating, electrical transients etc.). There will be some drift over time, but it should not be large for most op-amps. Some early CMOS-input op-amps had significant long term drift if they were exposed to large differential voltages, but that's more the exception then the rule.
At different temperatures, the offset voltage will be different, and the limits on that change are usually specified by a parameter such as TCVos, in microvolts per Kelvin.
Here are some specs for a typical precision op-amp (OPA177):
Here they specify the long term drift as typical 0.4uV/month, the maximum Vos over the entire temperature range and the maximum/typical drift with temperature.
The temperature drift spec is usually done using the 'box' method, where an imaginary box drawn around the offset voltage graph from -40 degrees C to 85 degrees C has a height that is not to exceed 150uV, which represents 1.2uV/degree C. The total offset must not exceed +/-100uV at any point in that range and the offset at room temperature must not exceed +/-60uV. In practice the curve will be smooth and often will be monotonic. Note that there is no guarantee that the slope of the curve will not exceed +/-1.2uV degree C, only the average over the whole temperature range is guaranteed.
Cheap general purpose op-amps will have much larger offsets and drifts, and often the drift with temperature and time is not specified, but the principle is the same. Typically the larger the initial offset voltage of an untrimmed op-amp the larger the drift with temperature will be.