thermal consquences of offset voltages for BJT type amplifiers with null pins - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 16, 2013, 12:48:50 am
content: So, I am looking at offset nulling and I read in a Analog Devices app note that there is a rule of thumb for FET op-amps that for every milivolt of nulling there is a consequential 4uV/ degree C offset drift, they stress that this means that using the null-pins on a FET device should be used to cure only the offset of the device, not the offset of the system..(less offset is better in this case)How about BJT input op-amps? Are there any consequences for using NULL to offset a large offset?I have two op-amps in a row, and I would like to null them both using one potentiometer...Here is the article talking about FET op amps:http://www.analog.com/static/imported-files/tutorials/MT-037.pdf(specifically page 5)************I am specifically interested in the NULL pins. I know I can deal with a system offset by injecting current into an amplifier, but this is not my motive.**************The op amp I am interested in, the lt1028, specifies an equation(Vos/300) = uV/Degree C, meaning that the drift is only "zeroed" when the offset is zeroed, but the data sheet lists this equation while specifying a 1000 ohm trim resistor, giving an offset nulling range of 1mV. Does this equation still hold true if the offset range is greater, like 10mV.

author: free_electron
author_rank: Super Contributor
timestamp: December 16, 2013, 01:04:35 am
content: Since the transistors in the opamp are thermally matched (they sit on the same substrate) your biggest worry is going to be the thermal drift of the adjusting resistor itself.There will be a residual thermal gradient inside the package but it is going to be a constant for any particular device.Trimpots in general have very bad thermal properties...

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 16, 2013, 02:04:15 am
content: Quote from: free_electron on December 16, 2013, 01:04:35 amBut is there any correlation between increased thermal offset in the device vs the NULL offset voltage, pretending that the trimpot is perfect.Just a rule of thumb for a BJT like the rule of thumb that AD gave for the FET input op amp.

author: SeanB
author_rank: Super Contributor
timestamp: December 16, 2013, 06:27:58 am
content: Nulling is best done per opamp, and best done with select on test resistors, as then you can have low TC resistors ( using 1% resistors or 2% resistors will mean you are getting a cheap low TC device) to adjust the offset, best according to the manufacturers best offset circuit, which typically uses a 10k pot with series resistors to the pins, typically between 2-5k. Thus your offset will need to be trimmed by E96 resistors between 2k and 10k, not a big number of resistors, but which will probably need a test jig and a selector switch bank to choose them.As well it will be best to chose devices selected for a better null out of the box, or you will have to buy in a larger number and test and bin them into grades according to null voltage.Using an opamp null to remove non opamp errors makes the bias currents through the input stage non equal, and this then will have an affect on all parameters that are depending on that current being equal, like CMRR, Vos, TC and such. Nulling is to ensure input stage currents are as close to equal as possible, nothing else.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 16, 2013, 08:12:08 am
content: AH, I see.That is a good point about the bias currents. Unbalancing them is bad.