OPA388 gain / noise question when set as instrument amplifier ? - Page 1
Forum section: electronics / projects

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 05, 2024, 07:30:58 pm
content: HelloIn my quest of other solutions for dynamic strain gage measurement the OP388 dual set as an instrument amplifier looks interesting as:The OAP388 is cheap so that nice for trailThe performance are quite highThat very low offset voltageThat a single power amplifier so easy to supplyquestion : I need a gain of 200 base on the available ''simple'' circuit that seems ok ? or there is possible improvements regarding noise ?RegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2024, 04:48:11 am
content: The circuit is not very good noise wise. The inverting input of the right OP-amp sees the noise of the two 25 K resistors. So one would see the resistor noise as additional noise source, that would be slightly higher than the OP-amp noise. The classical 3 OP-amp solution could be lower noise.The minimal fix would be using smaller resistors, like 1/10 the values for the 100 K and 25 K.One way to check is a simulation.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 06, 2024, 07:28:16 am
content: HelloThanks as PCBWAY deliver by 10 pieces and as the IC are cheap even the voltage source is cheap that possible to built several board and compare side by sideRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2024, 07:43:05 am
content: Modern ADCs often come with differential inputs. The DMS signal usually has a limited common mode swing and one can than use just the first part of the 3 OP-amp INA. So 2 OP-amps with than a differential output. The ADC is usually good enough handling the little common mode left.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 06, 2024, 05:14:56 pm
content: Hellothanks , my goal is to make experiment on dynamic strain gage strain at the maximum possible frequency so in the 200 KHz ( frequency response close to the maximum which is in the 250 KHz following some mechanical experiments on ring impacts ) so I need a minimum 1 MHz amplifierI have design some dual amp circuit with OP182 OP189 and now OP388 compare themI use separate PCB for the 1/4 bridge , the amplifier and the filter in the aim to save time in PCB assembly and make a puzzle assemblyFor first output tests I will use my 12 bits oscilloscopeRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2024, 09:01:18 pm
content: If it is about the dynamic part, one may get better performance from non AZ amplifiers, espcially BJT based ones. The DC drift is a little worse, but still not bad. There are also BJT based INAs for this case.To get a BW of some 250 kHz with a high gain (e.g. 200 fold) it needs rather fast amplifiers. This would be 50 MHz of GBW, maybe half if the 2 amplifiers share the burden in parallel. For symmetry reasons the 3 OP-amp circuit may be preferred and definitely look at lower resistance. The 3rd amplifier could provide some of the gain, to reduce the need for very high speed at the input. Because of the high frequency an integrated solution can also be interesting, as with this speed it is not just resistor matching but also capacitive matching, which could be even more tricky.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 07, 2024, 06:39:50 pm
content: HelloThanks for answerS , that really help I am not very skill in electronic i work on mechanical test amd material scienceI use normally AD8421 as that an easy amplifier , even the AD8429 but more costlyThey advertise in their specs that they are at 1MHz bandwidth with gain of 100 , can you explain why I need a 50 MHz amplifier to reach 250 KHz ? except if i keep technology as the OPA 388 where bandwidth is 100 KHz with 40 db ( if my reading is OK )ThanksRegardsOS