** The output buffer is crude with no forward bias - the 5532 can swing +/-0.6 volts with only 6mA going to the coil.
More swing suddenly produces much large currents.
Is such a discontinuity acceptable ?
.... Phil
How much resistance is in the coil? Hopefully the 200 ohms of series resistance is much less than the coil.
If you could make the coil yourself, I would center tap it and then you don't need the resistors to set the common mode.
The buffer is junk. Where did you dig it up?
Unless you need the whole 0 to 10K swing for R5, you might use a fixed resistor in series with a smaller variable resistor (like R10 and R11). This will improve the adjustment range.
As drawn, R11 isn't variable at all. R10+R11 will always be 510 ohms, no matter how you turn the knob on R11.
As Phil noted, there isn't any bias on the two output transistors. At low output signal levels, U3.1 is providing all the output by itself.
What are you going to power this with? If you are using lots and lots of gain on U1, the power supply noise might show up in the output.
The calibration coil should be shielded or live a long way away from the input coil or both. Otherwise you'll make an oscillator.
Matt Roberds
I have no idea what the deal is with the output coil. My comment regarding series resistance is for the input coil. Resistors are a noise source. Less is more if you don't need them. A center tapped coil would not require resistors to establish a common mode voltage.
Your output stage is still class B. Rather than trying to roll your own buffer for the op amp, why not just use a beefy op amp or use a buffer chip with the op amp. LTC has examples of this.
The home brew buffer, even if you made it class AB, has phase shift. You need extra compensation in the op amp to make up for the phase shift.
I really don't like the coil on the output since as the other poster mentioned, it is likely to cause oscillation if the input coil picks it up .
Those diodes are upside down, and are shown as zeners which they aren't. Also, you should use the correct symbol for non-polarised capacitors.
I'd steer away from pots for gain control, particularly on the first stage.
Cheers
-- Syd
U1's SEN pin should be connected to its OUT pin. Connecting it as shown will negate the gain of U2.
The AMP01 needs a dc path from its output to its sense pin.C4 will allow the U1's OUT pin to drift to one of the supply rails. Which rail depends on U2's offset polarity.
The resistors don't help, and the diodes short the power supplies.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Hi Robert, Well you've gotten an earful. What's the purpose of the coil? At 24 ohms with 24 V drive I might be tempted to run it class A with only one output transistor.
The other thing about coils is that if you want a constant field you are better driving them with a current source. (With a voltage drive the current (and B-field) decreases as the coil warms up.
George H.
Your best approach is to use LTspice and post the ASC circuit instead of a JPEG. Without SPICE, you cannot possible visualize all the voltages and currents and the interactions as you modify the circuit, so your understanding of what the circuit is doing will be very limited.
When you are able to model your circuit in LTspice, you will find it much easier to find flaws and fix them yourself instead of having to depend on the time and patience of volunteers to help. There is no need to bother people with elementary mistakes that you can solve yourself. What you really want is the elegant solutions that comes from experts who are willing to invest the time and effort on real challenges instead of simple mistakes.
If you want people to suggest improvements, it would be much easier for them to modify your LTspice file than to describe changes to a static image.
Learning SPICE is not easy. The learning curve is very steep. But you will find the effort can repay itself many fold as you gain a much deeper insight and understanding of how circuits really work.
It's battery powered, so class A might waste too much power. Lower battery voltages might be helpful.
Or use a power opamp. Or at least put an opamp inside the loop.
The differential input stage bothers me. With 5 megs to ground, I can imagine huge common-mode problems. Less so if the pickup coil is center-tap grounded, or at least well shielded.
A single-ended version could be done with just two opamps.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
This is a high gain instrumentation amplifier, based upon the Analog Devices AMP01, I designed to detect low intensity ELF's in real time and amplify them to drive a small calibration coil.
The signal input is from a large induction coil of the type used for geomagnetic surveys.
Can anyone see any faults in the linked schematic, or ways which its performance could be improved?
Any advice would be much appreciated.
Robert Stevens
To improve the performance, don't use an instrumentation amp; you do not care about common-mode rejection, really. Instead, use an op amp connected in transimpedance fashion, because that gives you low input impedance.
The low-frequency performance depends on the time constant of the inductor and input impedance, which is L/R. It's nice to have high inductance, but vital to have low R.
Beware batteries, they can have gas-pressure sensitivity, and have been known to make waveforms at very low frequency when their seals burp...
Please refer to my revised diagram below for my corrections.
I now have a 5K 10T pot with 5K series.
Sorry. Obvious mistake.
I added 10K bias resistors and diodes. I am not sure if the lower leg of R20 should go to ground or -V. I want the current in the coil to be unidirectional, e.g. not to reverse each half cycle.
Please clarify.
We are intending to keep it simple and use batteries.
Agreed.
Thank you for your suggetions.
Robert Stevens
Can you please explain why this is a problem? In the new diagram I show that the 24R load is actually the coil itself.
It came with the AMP01 ciruit which I sourced online.
Robert Stevens
Thank you Phil. I have revised according to the new diagram below.
I would like unidirectional current through the coil. Not sure where to connect the lower leg of R20, but I had a go.
Robert Stevens
Is that at DC? That would be a lot of wire.
** Really?
Surely the pickup coil is ES shielded with metal foil ?
AC supply frequency magnetic injection will be in differential mode.
... Phil
For picking up VLF? Why did you do that -- a loop (with all the wire at the rim) intercepts the most flux from the outside world. You can salvage it a little if you can find a few feet of thick ferrite rods, but that's not a very cheap solution.
Tim
-- Seven Transistor Labs Electrical Eng>
The trouble is that stray magnetic fields from your mains wiring will couple to the sensor coil so CMRR will not help you with that. You will need to be a long way away from any mains wiring for it to be clean.
I expect then that it will oscillate wildly at 10Hz.
Why is the output driving another coil? I could understand detecting the geomagnetic signal, amplifying it to a voltage and feeding that into a digitiser to make a time series and or realtime FFT display but what purpose does the output coil serve apart from causing feedback?
-- Regards, Martin Brown
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