Voltage measurements in circuit with virtual ground.  - Page 1
Forum section: electronics / general

author: raff5184
author_rank: Regular Contributor
timestamp: October 01, 2019, 03:38:23 pm
content: Hi all,I am measuring the input/output voltages of an opamp using a standard oscilloscope.The circuit is attached. I first connect a probe with the "-" on Vref and the "+" on Vin+ and I can see the AC input signal correctly.Then I want to measure the output. I switch the "+" of the probe to Vo but I can't see the amplified AC output voltage. I only see a DC offset on the output. The DC offset is expected but I don't know why I don't see AC signal.Now, if I use 2 probes connected as the two previous cases, one on Vin+ and the other on Vo, I can see both the AC input and the AC amplified output, but this time without DC offset.But I don't understand what's going on. Can somebody explain?Also is the scope changing the ground of my circuit?Thank you

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 01, 2019, 04:19:42 pm
content: If your transducer is a PZT type device with no DC resistance, the + input of the opamp has no return. Put a large value resistor from the + input to Vref, maybe 1 megohm, or higher if the opamp is high impedance. Then the circuit should work as expected.

author: raff5184
author_rank: Regular Contributor
timestamp: October 01, 2019, 05:57:58 pm
content: thank you, I included the 10 Mohm resistor (tried with 1MOhm as well), but still doesn't work. However I can measure everything properly if the input is from the waveform generator of the oscilloscope

author: Damianos
author_rank: Frequent Contributor
timestamp: October 01, 2019, 07:29:08 pm
content: If the triangle, at the bottom of the circuit, is ground then you are shorting the Battery1. Consider the consequences...What is the Transducer? What are you measuring on this (level, shape, frequency)?What are you measuring at the output in each case?What are the values of circuit components?What type of probe are you using?What is Vo without input signal?The circuit is amplifying the DC offset also, is it needed?

author: raff5184
author_rank: Regular Contributor
timestamp: October 01, 2019, 08:01:18 pm
content: I think I found the problem I basically connected Rg to Vref using a capacitor and now everything seems to work.To answer your questions:The triangle is just the ground of my circuit (the negative sign of battery1), not the earth.This is the transducer used to receive a wireless wave 50 mVpp - 200kHzhttps://www.m-b-t.com/fileadmin/redakteur/Ozeanographie/Hydrophone/Produktblaetter/TC4013.pdfAlso I did't include the values of the components because I think the problem doesn't depend on their values as much as on some configuration mistake.Probe is just a standard oscilloscope probe like the picture attached.Vout without signal is about 1.5VDC with 1 probe and 0 with 2.No, I don't need to amplify DC, only ACThank you

author: AG7CK
author_rank: Regular Contributor
timestamp: October 02, 2019, 04:35:36 am
content: Quote from: raff5184 on October 01, 2019, 08:01:18 pmI think you have not found the problem, and it only seems to work. It seems that you are using the probes / generator as bias paths.The answer of Conrad Hoffman is the key. Search for "galvanically isolated sensor bias current resistor" or similar. It even returns posts from this forum:https://www.eevblog.com/forum/beginners/isolated-current-sense-instrumentation/All the things you do not understand are probably caused by your lack of understanding op amp biasing.I might be wrong. If so, I will also learn from this thread.

author: AG7CK
author_rank: Regular Contributor
timestamp: October 02, 2019, 06:18:52 am
content: https://www.researchgate.net/publication/265036087_Analog_Front-End_Circuitry_in_Piezoelectric_and_Microphone_Detection_of_Photoacoustic_Signals

author: Damianos
author_rank: Frequent Contributor
timestamp: October 02, 2019, 09:40:19 am
content: Quote from: raff5184 on October 01, 2019, 08:01:18 pmSo the transducer does not pass the bias, as already mentioned byConrad HoffmanandAG7CK.Do a study to the links provided by AG7CK.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 02, 2019, 11:49:27 am
content: I'll just add that probing opamp inputs very often messes up the circuit and gives misleading results. Sometimes it's better to measure everywhere else and infer what's going on at the input. You can also make up a network to replace the transducer and drive the circuit with a signal generator, again probing the output.