No Trace - High V2 Value

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Posts: 1
Joined: Wed Apr 18, 2018 6:14 am

No Trace - High V2 Value

Post by atomica »

My last multimeeter was dodgy, and I thought my read outs were incorrect, but now that I have a new one I am very confused.
I couldn't find any any troubleshooting steps for V2, besides that someone had a malfunctioning U2.
My board came parcially assembled, which includes U2, so I have no doubt that it is functioning correctly.
I have resoldered and checked values R9/10/11/12, all correct.

The only thing that would make sense to me is that the feedback via R9 is too high, but the value is correct...

What could I be missing?
Thank you in advance.
Measured with CPL set to GND
5V: 4.82V
3.3V: 3.3V
AV-: -5.01V
AV+: 5.00 V

V1: 0V
V2: 3.44V [2x Higher than normal]
V3: 3.58V
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: No Trace - High V2 Value

Post by MNTech »

Since the resistors values are correct there is little else to suspect besides the op amp.

When the feedback on an op amp is working the plus and minus inputs will have the same voltage, zero in this case.
Plus in is pin 10, minus in is pin 9 of U2.

If the op amp is bad and outputting 3.44V pin 9 will have about +0.2V.

If the op amp is good and R9 is open pin 9 will have about -0.2V. The output (V2) would be the maximum the amp delivers, which I think is what you are measuring.
The section of the op amp that is connected to V3 is used as comparator so its output will either be the maximum or minimum.
It is 3.58 volt so that's probably the maximum. V2 has nearly the same voltage.

To summarize, if you see -0.2V on pin 9 check R9 and its connections, otherwise U2 is bad.
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