2 opamp as bridge output. - Page 1
Forum section: electronics / projects

author: GigaJoe
author_rank: Frequent Contributor
timestamp: December 18, 2018, 05:39:21 am
content: Hi everyone,so i got a thing beyond my knowledge , hope someone able to explain it , where is my mistake .I take dual opamp and feed 2x10Vfirst non-inverting ; second opamp - inverting ; K=1 for both ; feed 20Khz meandr .that are result: A - no-inv, B -inv ( measure between ground (neutral) and output of each opamp )Then, I measure output between both opams, (like a bridge)here is output between both opamp:I assumed that result would be a simple add on , as a flat meander (in fact amplitude doubled), then I able do feed 2 TDA opamps as the bridge.Why it not flat? and getting much worse when F increasing, like 60-70K it not even close to meander, some chaotic thingi .Where is my mistake ?

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2018, 09:51:45 am
content: It seems like you're measuring it incorrectly.Have you got the oscilloscope ground on one of the outputs? If so, then this is the problem. Either use a differential probe, or two channels and the x-y maths function.

author: spec
author_rank: Frequent Contributor
timestamp: December 18, 2018, 03:56:20 pm
content: Hi GigaJoeWe would need a full schematic, including the opamp type, and an indication of how your scope is connected to the circuit, before we could even start to help you.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: December 18, 2018, 05:42:29 pm
content: Hero999,I'mLike a primitive error, in measurement , a ground has to be at ground. Otherwise it begin an antenna to accepting allhere is a picture when I hold the ground cable ( short black path with crocodile) by fingers ...that a frequency comb produced

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 18, 2018, 06:08:03 pm
content: Use a differential probe or show the difference using two probes and a math channel on your scope.Connect the scope's ground to your circuit ground and certainly not to any of the opamps output except if your circuit is completely isolated...You may also insert a load between the two outputs (if you haven't) as there will probably be one in the end if you're using this as a bridged amplifier and you need to check whether both opamps will be able to source and sink enough current.

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2018, 06:31:31 pm
content: Quote from: SiliconWizard on December 18, 2018, 06:08:03 pmYes, that's the correct way to do it.QuoteEven if the circuit is isolated from the oscilloscope's ground, it's unlikely to be sufficiently isolated at AC, unless the frequency and output impedance is really low and capable of driving a relatively large capacitance. The circuit will be capacitively coupled to the oscilloscope's ground, to some extent, even if the circuit and/or the oscilloscope are battery powered. The amount of capacitive coupling will depend on the physical size of the circuit and battery. If the op-amp's output is connected to the oscilloscope's ground it will see a large capacitance a few hundred pF or more which can pick up mains hum and cause oscillation. The situation will be worse if the circuit and oscilloscope are both powered from the mains, even if the circuit's PSU is floating.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: December 18, 2018, 07:21:37 pm
content: Ok ... great ... so I need to have a diff probe ... to use a scope in multimeter - mode ...I thought to use Smart APC with enough battery to feed the scope for many hours, but it seems not an option .... (or it does ? )

author: Nerull
author_rank: Frequent Contributor
timestamp: December 18, 2018, 11:58:18 pm
content: Two probes + a math function doesn't work quite a well as a proper diff probe, but it typically gets the job done.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: December 19, 2018, 03:59:01 am
content: true, especially on my cheap Siglent, it not a solid resultant line, but something ladder like white dashes, especially if signals close to eliminate each other.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: December 21, 2018, 02:00:28 pm
content: As others have said, you already have the two traces on the screen and all you need to do is turn on the math function to compute the difference.In addition you also should use DC coupling, not AC coupling as in your screen shots, to be able to compute the true differential voltage.Most scopes, including yours, have (earth) grounded probe connectors which happily short out your DUT if connected to the wrong nodes.