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THS4521-HT: Op-Amp giving negative differential voltage
http://forum.grailtone.com/viewtopic.php?f=11&t=74240
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Author:  duidui [ Fri Dec 08, 2017 3:45 am ]
Post subject:  THS4521-HT: Op-Amp giving negative differential voltage

Hello, for my question I can't find error where in these schematics. So can someone help to explain where I get wrong with this schematics? I use a differential op-amp THS4521 and want to measure the differential signal coming from a pressure sensor (transducer).

The sensor is powered from 5V and it has a range of 0-1000bar. Without connecting the op-amp at 0 bar it gives differential voltage (between P+ and P-) of 0.14mV and 1000bar it is 14mV.

Therefore I used this op-amp to amplify the voltage and measure it with an ADC http://www.kynix.com/Detail/1250161/ADC.html.

The ADC has no internal gain, therefore it must be like this.

Problem is that when the sensor is connected to the op-amp it gets an offset of ca 3mV.

After reaching 25bar the values on the output of op-amp go below 0mV. Since I cannot measure negative differential voltage with the ADC, I have a problem.

I could swap P+ and P- but then I cannot read anything below 25bar.

Sensor is powered from 5V source, the op-amp from 3.3V. Rest is on the attached picture.

Does anyone has experience with this op-amp and could tell me some hints?
Image
I hae got some inputs like apply some voltage to Vocm to get rid of the offset. I tried to apply 2.5V as I had a reference voltage available near the op-amp. I pulled Vocm up with 10k resistor. Unfortunately, the circuit still does not function as it should. As a result, offset is still there, only it occurs in different place, at 37 bar. Vocm, instead 2.5V is going down to 2.4226.

Sensor input (voltage between P+ and P-) is however not going negative, as it was before. So I observe a strange behaviour that the input voltage is rising with rising pressure but output is going negative above 37bar. In the end of testing I have disconnected the 2.5V, leaving Vocm floating, so similar to initial case, only without two 100nF capacitors coupling to ground. Then the voltage on that pin was 1.72V, almost the same as on output ADP+ and ADP-.

Was the 10k pull-up resistor too big? Was the applied voltage too high? I am out of ideas so far. Does anyone here have a suggestion what can I try next?
Thank you!
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