2DW23x and parallel op amps - schematic - Page 1
Forum section: electronics / beginners

author: analityk
author_rank: Regular Contributor
timestamp: August 04, 2022, 04:34:50 pm
content: Hi all,after i make new mistake i will ask you confirm this attached schematic. I have never before geting two op amps parallel and it is possible place to learn new lesson.Please comments or linking additional info if you can.Sincerely.Analitykedit: resistors value are not proper due to i will do it for 2dw232 diodes individualy, there is more places for easiest geting precision resistance. I will remove pot after starting-up.

author: iMo
author_rank: Super Contributor
timestamp: August 04, 2022, 07:03:55 pm
content: Why would you wire those 2 opamps in parallel? To decrease the amplifier noise 1.41x ?The noise of the OPA189 is 15nV_rms (0.1-10Hz), what is the noise of the 2DW23x ?Also I would not directly wire two opamps in parallel as you did, usually you have to wire 2 complete amplifiers in parallel.. The question is whether the OPA2189 is using the single clock for both opamps in the package (chopping freq beating)..

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2022, 07:15:14 pm
content: This is not the proper way to use 2 OP amps in parallel. Even though these are AZ OPs with a relatively small residual offset, the 2 OPs would still be a litte different and fight each other. So both OPs would need an individual feedback network and only do averaging after that, outside feedback loop.The 2DW23x zeners may be low noise, but I would still expect them to be higher noise than 1 OPA189. So I don't think one would need the 2nd parallel amplifier.It would be a good idea to have the capacitor for direct feedback directly from the OP and not directly from the output. If the output is to drive a higher load, it would be good to have an extra driver (e.g. extra OP as buffer or a tansistor). With already some 7-15 mA for the Zener the precision OP would get uncompftable warm. Variable capacitove loading the OPA189 may have an effect too.The way the trim is made is not good, as the contact resistance of the viper has quite some influence. The more practical way for fine trimming the current is with selected resistors and possibly add a few optional in parallel. If one absolutely wants a trimmer, the trimmer (e.g. 1-10K) should be in series with a rather large resistor (e.g. 10-100 K) and that pair than in parallel to a small part of the full resistance (e.g. 1 the 1 K or maybe less).The current adjustment would be more like to some 5% and maybe 1% to get the zero TC point at a given temperature. To get a more stable votlage it would be more like also stabilizing the temperature. The idea would be to about trim the linear TC and fight the 2nd and higher order parts with a regulated temperaure - even of the regulation is not very stable.

author: iMo
author_rank: Super Contributor
timestamp: August 04, 2022, 07:40:50 pm
content: Below the simplest wiring with a buffer transistor and a single opamp..An example only..PS: you may use the second opamp in the package for the controlling of the oven..

author: analityk
author_rank: Regular Contributor
timestamp: August 04, 2022, 08:38:39 pm
content: Thanks.After i read your post i remembered how to do it, example was in lt1028 and propably in many other ds. Pic rel. Assume i do it right (like on pic) - is there any reason for not to do it?As you sugest above i will use single op to drive zener diode and second for thermal stabilisation.This npn tranistor can be any generic npn? BC817-40 will be good?Quote from: imo on August 04, 2022, 07:40:50 pmWhat is role of D2?Sincerely.Analityk

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2022, 08:45:23 pm
content: Combining 2 AZ OPs can be tricky, as they use some modulation and produce some extra spikes. Most of the time nothing bad happens, but there is a small chance to get some interference.OP amps in parallel reduces the voltage noise but increases the current noise. So it can be an issue with a high impedance source, e.g. if there is an additional low pass filter between the zener and the amplifier.The zener D2 is there, so that the OP does not need to go all the way to the positive rail. AFAIK the OPA189 is a RR type and could thus get away without the diode + resistor at the base. With a classic OP like the OP07 the required supply voltage would be lower.The transistor can be a generic type like BC847 or BC817.

author: iMo
author_rank: Super Contributor
timestamp: August 04, 2022, 08:46:51 pm
content: Yep, it could be any generic NPN transistor (BC817-40 is great for it)..The D2 zener lowers the opamp's output such it stays around the middle of the Vcc..And - no, the D2 with the transistor do not introduce any additional noise..@Kleinstein: do the dual/quad AZ opamps use a single internal clock to avoid beating?