New OPA891 op-amp - bias current - Page 1
Forum section: electronics / general

author: floobydust
author_rank: Super Contributor
timestamp: August 11, 2024, 12:12:59 am
content: New op-amp from TIOPA891has some really good specs for ultrasound and audio applications:0.95nV/√Hz voltage noise180MHz unity gain bandwidth105V/μs slew rate200mA output current driveBut I'm puzzled by the input bias current spec. is 9uA typ. and 20uA max. - which seems huge. NPN BJT input stage.Am I missing something or is the datasheet out to lunch?

author: coppercone2
author_rank: Super Contributor
timestamp: August 11, 2024, 01:49:39 am
content: it looks like they don't care about DC too much for this op amp on the applications.

author: Weston
author_rank: Regular Contributor
timestamp: August 11, 2024, 02:12:44 am
content: Ooh, a nice part. Thanks for sharing.Looks like it would also be good for a rogowski coil integrator. I was using the OPA2822 before, but this part has lower voltage noise and a lower 1/F corner frequency while being able to more effectively drive coax cable.

author: magic
author_rank: Super Contributor
timestamp: August 11, 2024, 07:45:50 am
content: It's only twice worse than uncancelled bias current of LT1028, according to the published schematic of the latter. Could be explained by lower β on this high-speed process, or higher Rbb (maybe the transistors had to use less area to keep parasitics at bay) necessitating even more ridiculous input stage bias current.This part clearly omits bias cancellation and most likely doesn't have a bootstrapped input stage cascode (wide common mode input range, compare with OPA211) so no superbeta trickery this time.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2024, 10:12:43 am
content: 0.95 nV/√Hzf > 10kHzso not specified at DC.The, large, low beta, high GBWP, input transistors must require a decent current to drive.

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2024, 03:45:54 am
content: TI OPA891 datasheet: "These amplifiers are built using a 36V, complementary bipolar process with NPN and PNP transistors that possess an fTof several GHz."You can sort by IBat Digi-Key, it appears speed is not a factor though. Bias-cancellation is there or not I guess.Previous TI datasheet rev had an "oops nA not uA" edit lol thought slim chance it might apply.I was pitting the part against anLT1115plusLT1010buffer, as in Walt Jung's excellent solutions in the LT1115 datasheet. IB50nA typ. (at room temp) so the bias cancellation circuit is there. Same value forLT1028/1128.

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2024, 10:33:05 am
content: Quote from: floobydust on August 11, 2024, 12:12:59 amThe input bias current of 9 microamps and input offset current of 30 nanoamps, which means that it lacks input bias current cancellation. If it had input bias current cancellation, then the input bias current would be more like +/-30 nanoamps, which is comparable to other similar parts from Analog Devices and Linear Technology.Input bias current cancellation adds to input noise so is left out when low noise is more important, like with audio applications.