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Topic: LM324 - Opamp with transducer input. (Read 4280 times) previous topic - next topic

Grumpy_Mike


Get rid of the coupling cap.

No. Read the rest of the thread, they will be needed to remove the DC offset.

The three points marked VG should all be connected together. You will get nothing on the second op amp until you connect the VG to the VG of the first op amp. If you do not want to use that top amplifier as a VG generator then use two resistors and connect them to the VG on the two op amps.

Jiggy-Ninja


No. Read the rest of the thread, they will be needed to remove the DC offset.

If the right op amp configuration was used, certainly, but considering that he was using a coupling cap without proper biasing (he said he copied Dave's circuit exactly) it would kinda work. Still not the best suggestion I've made. The proper way to do things is to follow MarkT's schematic, not Dave Jones's.

Quote
The three points marked VG should all be connected together. You will get nothing on the second op amp until you connect the VG to the VG of the first op amp. If you do not want to use that top amplifier as a VG generator then use two resistors and connect them to the VG on the two op amps.

Given that the first stage of MarkT's schematic is a non-inverting configuration, wouldn't it need an op amp VG generator? Unlike the inverting configuration, there'll be current flowing in and out of the VGND, requiring a buffer for a low impedance output.
Hackaday: https://hackaday.io/MarkRD
Advanced C++ Techniques: https://forum.arduino.cc/index.php?topic=493075.0

MarkT

Indeed, I drew the circuit as I did for a reason.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Johnny010

#18
Jul 21, 2014, 07:45 pm Last Edit: Jul 21, 2014, 08:31 pm by Johnny010 Reason: 1

Something like this in fact:


The unmarked cap sets the cutoff, since you are using 40kHz (not 40MHz as you said!!)
choosing C = 1uF will be fine.



OK I have built up the circuit except for the final inverting amp.

The virtual ground "seems" to work and produce a voltage between the +9 and 0v supply. I have put the on "op-amp 1" of the quad package and taken the output to a separate breadboard rail. I get a voltage of -3.85V when I measure between the 9v (around 8.7V) battery + and the new V.G.

The transducer is giving a ~40mV 20us pulse (25Khz) which is being picked up by the + pin on the 2nd op-amp. I have set the gain to ~10 (1k+75ohm resistors), which would be around 100Khz of available BW. But I am not getting any output on the op-amp compared to the V.G.

The supply for the quad package I am using the 9v+ for the V++ and the V.G for the vcc-.

I have used a 0.1uf ceramic cap for the voltage divider as I dont have any larger ceramics (only electro caps).

What is going on :(?

Oh...this may help explain why. The transducer measures 300K ohms +ve to -ve. That is the impedance right? Does this mean it is only giving out 40mV/300K = 1e-7A ish worth of current so it does not trigger the op-amp?
According to the datasheet, input offset current = 5e-9 typical and 30e-9 max. The current from the transducer should be over these surely?

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