![op amp offset trimming op amp offset trimming](https://d2vlcm61l7u1fs.cloudfront.net/media%2F8fe%2F8fe29d31-e641-40bc-9b7b-c523c16d41f7%2Fphp8NZ3W6.png)
If the Hfe (Betas) of the input transistors are not equal, the the base currents will not be equal, and an input offset current will exist as well. The bias current is due to the fact that the Hfe of the input transistors is finite, and therefore, some base current must flow. For example, in a bipolar op amp, the input offset voltage is caused by the Vbe mismatch between the two input transistors. Input offset voltage and input bias current are independent from each other, and have different causes. …in other words……rather than use a resistor from V+ to ground AND the offset-null pot……….-I can simply just use the offset-null pot to deal with both the “input bias current” problem and the “input offset” problem. (?)Īs such, if I want to mitigate (get rid of) these bad effects, then surely I only need to use ONE of the above correcting steps?…… …so in actual fact, “input offset” is just another problem caused by “input bias currents”. ….However, in reality, the op-amps “input offset” is actually caused by the fact that the op-amps “input bias currents” are actually NOT equal…(?). in order to mitigate this problem with an inverting op-amp, a resistor of value R(feedback//R(in) is put from V+ to ground. … I am aware that an op-amps V+ and V- inputs draw input bias currents….and that these bias currents create voltages at the inputs, since they flow through whatever resistors are biasing the op-amp……
![op amp offset trimming op amp offset trimming](https://i.stack.imgur.com/JsIzC.png)
I am aware that this offset is gotten-rid-of by using a pot at dedicated “offset-null” pins….OR by putting some high value resistor between the positive supply of the op-amp and the inverting input.
![op amp offset trimming op amp offset trimming](https://d3i71xaburhd42.cloudfront.net/3dc13df3e6f9649ab1e6a30664073c009bea9876/3-Figure4-1.png)
So, I realise that an op-amp has an input offset that means that when used with negative feedback, its output voltage will not be zero when it has zero volts between its inverting (V-) and non-inverting (V+) inputs. not mentioning the quality of the callibration that will dependent on how a workers mood is and how much pressure there is to produce enough units at a given day.It is known that op-amps give slightly inaccurate output voltages due to …. I think it might be worth checking what the factory calculates for that extra needed test time and then perhaps decide to buy a more expensive opamp as this might in the end be cheaper then the workers needed. This will mean that you need some potmeters to get things exact and then thus need more time in the factory for testing and callibrating. In the first case from the head you can use some additional resistors to reduce the offset from the opamp. In case 2 you will need to always callibrate your system before use as you do not know the temperature at the time of use, nor the supply voltage, nor the drift of the opamp itself. I'm worried about long term drift of the cheaper OAs with high Vos - how much of a problem is that?ġ) you want to limit the initial amount of offset as much as possible and then the offset changes over time will be there, but they are well within the limits you have set George - another solution for us is to just use more expensive OAs with better offsets.
OP AMP OFFSET TRIMMING PLUS
I guess that either solution (an extra trimmer plus the time needed to null the offset, or a costlier part) has its cost. Analog Devices stands out with some of their Zero-Drift amplifiers that have a recovery time of a few microseconds only. The recovery time penalty you will have to pay is in the order of a few tens of milliseconds. Of course, this line of op-amps is more expensive and, the worst part is the extra time the amplifiers will need to re-balance themselves when their input saturates and/or their output overloads. Their Vos can be as little as a few micro-volts, while there are parts with sub-μV input offset. But, the long term and the temperature drifts cannot be avoided.Ī possible solution is the use of "Auto-Zero," "Zero-Drift" or "Chopper-stabilized" amplifiers, which have extra internal circuitry to constantly measure and compensate the input offset, minimising the drift over time and temperature and the effect of 1/f noise. Most old-fashioned op-amps have an extra couple of pins to null their input offset voltage externally, using a trimmer. Giorgos_K wrote: The input offset can either be positive of negative.