Question about op-amp balancing or offset adjustment - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: February 06, 2012, 01:50:49 am
content: Its the jelly bean jfet amp LF 411 , datasheet ->http://www.national.com/ds/LF/LF411.pdfQuestions about the balance/offset adjustment feature :I can't find what is the range of the offset adjustment using that 100K trimpot in the datasheet ? Did I miss it ? Appreciate if you could point out where is it in the datasheet.The op-amp Vos = 2 mV (max) with the tempco of 20 uV/C, now if I use a 100K trimpot that has +/- 100 ppm/C, what is the final overall tempco for the Vos ?If I paired an op-amp with a trimpot, and then adjust the Vos say to within +/- 10 uV at the breadboard, then move both to the circuit board using ic socket and also the same trimpot is also using socket,assumingthe socket contact resistance is equal at the breadboard vs at the board, will I get the same or really close to the Vos that I've adjusted at the breadboard ?Power supply level for Vcc and Vee are identical at both locations, and the reason is I don't want to do the offset re-adjustment again at the circuit board.

author: amspire
author_rank: Super Contributor
timestamp: February 06, 2012, 02:33:16 am
content: Quote from: BravoV on February 06, 2012, 01:50:49 amThey don't specify the offset adjustment range, but it is very likely 100K will not be enough to zero offset for all IC's. It will work for some. You should use a 10K pot.QuoteIf the pot is near the center, it will have very little effect on the temp coefficient. Even with a 10K pot, I think the worse case when the pot is at one end will only cause about 1uV/C. A 100K pot would be a tenth of this.QuoteIt is hardly practicable to adjust to 10uV, when a 5 deg ambient change causes a shift up to 100uV, and the moment you put the lid on the circuit's case, you will probably have a 10 deg C temp rise in the box anyway.The adjustment feature is really to improve the offset by an order of magnitude, so for the 411, 100uV offset after adjustment is a good result. Change of socket resistance between the test circuit and the final board is not an issue, but thermal differences between the test circuit and the final board will be a big issue.But it really sounds like you are using the wrong opamp. Nowadays, design practice is to choose parts that eliminate any calibration if at all possible. You can buy opamps easily with guaranteed offsets in the uV region, and if you go for the auto-zero opamps, you will have offsets in the 0.5uV to 5uV range. Why are you using the LF411 if you really need better then the specified 2mV offset?Richard

author: BravoV
author_rank: Super Contributor
timestamp: February 06, 2012, 02:57:19 am
content: Hey Richard, thanks for prompt response.Quote from: amspire on February 06, 2012, 02:33:16 amHmm.. sorry, I'm lost, why 100K as stated by the datasheet will not be enough ? While 10 K will cover better range ? Please elaborate further.Quote from: amspire on February 06, 2012, 02:33:16 amSo what are you saying is that the trimpot's TC doesn't matter too much (trivial) to overall op-amp 's Vos TC ?Quote from: amspire on February 06, 2012, 02:33:16 amI'm aware of abundant of op-amp choices out there, especially like the chopper-amp, its just the reason using this 411 is availability and its dirt cheap.Actually I'm helping my friend and he is asking me about this matter, and the targeted Vos is actually just about 200 to 400 uV, so I think using this still within safe zone, and also it has a nice slew rate (which is needed) and also high input impedance is favourable as well.About the thermal difference, the targeted circuit working temp will be quite stable and sort of guarded, it will be housed inside a vented case without any high dissipating components around it like psu or high current devices.Btw, correct me if I'm wrong, I "assume" those common popular chopper-amp (not those exotic hard to get one) is slower and also has it's input switches at certain frequency compared to this jfet input which is considered less noisier, again, correct me here.Now, is this an adequate choice ? or you have any other suggestion ?

author: amspire
author_rank: Super Contributor
timestamp: February 06, 2012, 03:41:19 am
content: Quote from: BravoV on February 06, 2012, 02:57:19 am

author: BravoV
author_rank: Super Contributor
timestamp: February 06, 2012, 06:44:24 am
content: Richard, I misread the trimpot value.Btw, the price of a 411 non A version is just $0.5/pcs locally here, thats why I called it dirt cheap, while MCP6021 is only available thru online purchase which is sometimes cost too much because of the shipping charge. Like digikey will charge $75 minimum for s/h.I guess I will have to stick with this 411.