Geophone amplification circuit behavior
This is an expansion on Biasing a differential opamp input post. Olin pointed out that gotchas can be in the implementation, so I will provide the circuit and design specifications as well as my research so far.
The geophone is HG-6 B coil, shunt for 0.7 damping is specified to be 9090 ohm.
The opamp I had is INA188.
The goal was to have 0V to 5V single ended output that goes to ADC. Therefore, the plan was to reference the signal to 2.5V so that the common mode input requirements of INA188 would be satisfied. Reading application note of TI Importance of Input Bias Current Return Paths in Instrumentation Amplifier Applications chapter 4 mentions that the reference and bias voltage has to be able to both sink and source current, so for that I selected MAX6225 voltage reference, capable of sinking/sourcing 15mA, albeit drawing current from it reduces stability quite rapidly. Perhaps it would have been wise to buffer it.
The simulation itself:
It does what I expected it to in simulation. Yes, the circuit in the previous post is slightly different, but since we are getting down to details, this is what is actually there. V4 is a charge pump producing -0.23V to achieve true 0V on the output of the INA188.
The actual circuit
The only added extra things are PESD5V0U1BB,115 TVS diodes with 5V working voltage and R33,R42 to protect the opamp in case the geophone is dropped while handled, since it can produce high voltage spikes.
Testing
Before connecting a geophone to the circuit I wanted to verify it by applying a differential signal in a controlled manner first. By what I saw on the scope I feel like this did not work as I had imagined in my mind, there may be a fundamental misunderstanding on my part, so give me some rope here. I used Keithley 3390 function generator. I measured the BNC housing against PE, and it was not connected, so as far as I could figure, if I connected it to GEO+ and GEO-, the circuit would see it as a differential signal. I applied a sine wave to it, left side of the image are the GEO+ and GEO- in reference to circuit ground, and right side is a subtraction of the channels:
So despite the massive noise on input, it seemed there is a voltage produced on the input. However, the output of INA188 was just 2.5V - the reference around which it was centered. I am not sure why this was the case or how it is possible.
What I decided to do next was to connect the actual geophone to the circuit, since I felt that there was something off about how I connected the function generator to the circuit. When shaking the geophone, there was output produced, but only from 2.5V down to 0V:
So far I have no concrete thoughts on why this does not work, I guessed perhaps the 2.5V reference has too low impedance in relation to the geophone input, so I thought perhaps something like this would be more suitable:
Simulation results are the same, and the coil resistance specified is 375Ω, so I don't think that is it. Seeing that the signal never goes towards 5V, I have a thought now that perhaps one of the TVS diodes has a short circuit, but I have not checked that yet (Will update post after I check it).
The questions are:
- Is it reasonable to use function generator as differential input the way I tried?
- If 2.5V is the only reference for the signal, is that sufficient or does there need to be a direct GND reference somewhere?
- What may be the reasons you can see or think of that the circuit does not behave as expected?
I have provided all the information I can think of at the moment, if you need anything extra, let me know.
1 answer
Is it reasonable to use function generator as differential input the way I tried?
In theory, it should have worked, but it's asking for lots of common mode noise. Even with the output apparently not connected to ground at DC, there can be considerable capacitive coupling to ground, power, and other nasty places. Unless the function generator is specified to have a truly differential output, I wouldn't try to use it that way.
What you can do is put a small audio transformer between the function generator output and the input of your circuit.
If 2.5V is the only reference for the signal, is that sufficient or does there need to be a direct GND reference somewhere?
There is a GND reference, through the 2.5 V source. The input as you've shown it is not floating. Other than general low noise cleanliness (see below), your input bias seems reasonable.
What may be the reasons you can see or think of that the circuit does not behave as expected?
The clipping of positive signals seems strange. Your suspicion about a TVS being shorted or installed backwards could be right.
I would go back to testing the circuit with the function generator so that you have a well controlled and known input. Use an audio transformer to guarantee a truly floating differential signal. Remove the clipping diodes to simplify the circuit and get that working with the function generator. Then you can put the clipping circuit back and test with a real geophone.
Why TVSs for clipping? They have rather soft and inaccurate knees. Why not the more usual clipping diodes to power and ground on each input? That will guarantee only one diode drop voltage excursion beyond the rails. With a 5 V TVS, it could be several volts beyond the rails.
I would also address noise in various places more directly. I don't know what the valid frequency range of geophone signals are, so I'll arbitrarily pick 1 Hz to 1 kHz as example.
Starting with the lines from the geophone, you want to squash high frequency pickup from radio stations and other sources. That's easy, since they are much higher than the highest signal frequency.
For example, adding 1 kΩ followed by 16 nF to ground on each line starts filtering out signals past 10 kHz. The filters not matching exactly on each side will cause some imbalance, but mostly at frequencies you can filter out later from the single-ended signal.
I would also add a capacitor to ground at the midpoint of the 9090 Ω between the two inputs. If you use 1 kΩ on each line as described above, that leaves 7090 Ω total between the lines. That would be split into two 3.5 kΩ resistors, with a cap to ground at the center point. Something like a 20 µF electrolytic cap to ground should keep the center point nicely clean at the frequencies of interest. Don't use a ceramic, due to possible microphonics.
The pure DC resistance driving the whole input circuit common mode voltage doesn't need to be very low resistance. It really only needs to be low relative to the diff amp inputs. 10 kΩ between the center point and the 2.5 V reference should be fine. The cap at the center point lowers the impedance at AC, and 10 kΩ at DC is plenty low enough. One downside to this is it will take a few 100 ms for the input voltages to stabilize after startup. I'll assume that doesn't matter for now.
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