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2015 May 7

Lecture in middle of first op-amp lab

Filed under: Circuits course — gasstationwithoutpumps @ 16:47
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The lecture between the halves of the first op-amp lab did not cover much material.  A big chunk of the first part was a discussion with the class about whether we should have a midterm quiz.  After much discussion of the advantages and disadvantages of different approaches, we finally decided that I would give them a take-home, ungraded quiz, so that they could test themselves and later ask questions in class for things they needed more help with. This discussion also brought out some suggestions from students of additional resources that they had found useful (Khan Academy videos, the new edition of Horowitz and Hill, and the All About Circuits web textbook). I also got a chance to give them some reassurance that they are doing well, since some are getting discouraged. I’m packing a lot into the class, and it is easy for the students to get overwhelmed—especially since some are just now getting to capacitors in their algebra-based physics classes.

Because most of the class had working audio amps in the Tuesday lab, I made a number of suggestions for a tinkering lab on Thursday.  In addition to the ones I already mentioned in

  • Switching from a symmetric dual power supply to a single power supply.
  • Paralleling two op-amp chips to get twice the current capability.
  • Adding a potentiometer for variable gain.
  • Adding a unity-gain buffer to separate the loudspeaker driver from the gain amplifier.
  • Adding a tone-control circuit, like the Baxandall tone control on http://www.learnabout-electronics.org/Amplifiers/amplifiers42.  They can’t use exactly that circuit, as they have only 10kΩ potentiometers, not 100kΩ ones.  The idea can be adapted, or the students could do simple treble-cut or bass-cut circuits.
  • Using a loudspeaker as a microphone. I think that should work, as I get about a 500µV signal from my loudspeaker when I talk into it.  The don’t need any DC bias for the loudspeaker mic, and they may even be able to eliminate their high-pass filter, as the loudspeaker mic can be set up to have its output already centered at 0V.

I also suggested hooking up a plug to the output of a music device, investigating how the amplifier clips, and hooking up a function generator (with a voltage divider) to replace the input and high-pass filter so that gain can be measured without the difficulty of varying sound level in the room.  The point of the lab (after they’ve done a good job of explaining how they designed the basic amplifier) is to get them to play with the design—to do something they see as fun.

I also talked about why some student had been seeing asymmetric clipping when they hooked up their loudspeakers.  The key concept (which they had not had) is that the input-referenced voltage offset could be as large as ±4.5mV for the MCP6004 op amps that they are using. With a gain of 50, that makes an output offset of up to ±225mV, but with an 8Ω loudspeaker the current limits cause clipping at about 200mV, so the output signal could be shifted far enough so that half of it is clipped, even it all looks like it should be in range.  I talked a little about the possibility of doing offset nulling, but didn’t really give them a circuit that they could use.

In lab today, people did seem to be having a lot of fun, and both morning and afternoon sessions ended early.  I’m looking forward to reading the design reports this weekend, because they should be different in interesting ways, as different students chose different directions to explore.  I helped a few students debug their circuits (as usual, the most common problems were loose wires, power supply not providing power, and scope probes set differently from what the oscilloscope thought).

2014 June 26

Instrumentation amp from op amps fine for EKG

Filed under: Circuits course — gasstationwithoutpumps @ 22:55
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As I mentioned in Instrumentation amp from op amps still fails, I’ve been trying to decide whether to have students build an instrumentation amp out of op amps in the circuits course.  I decided that it wouldn’t work for the pressure-sensor lab, because of the large DC offset.  One could calibrate each amplifier, either in software (by recording a a few seconds of 0 pressure difference, and subtracting a constant fit to that region from the data) or in hardware, but I’d rather they had a more straightforward experience where the DC offset was small enough to be ignored.

I conjectured that instrumentation amp built from discrete op amps would work ok for the EKG lab, though, as the EKG already has to deal with much larger input voltage offsets due to differing electrode-skin contact.  So I added a second stage  with a gain of 81 to the instrumentation amp in the previous post with a gain of 19, to get a combined gain of 1539.  I put in the high-pass filter needed to eliminate the DC offset, and a low-pass filter to reduce noise slightly (and make aliasing less of a problem).  The corner frequency is a bit high (60Hz noise is not going to be reduced much), but that may allow a better view of the fast R spike in the EKG waveform.

    The EKG circuit has four modules: a virtual ground (here set to 0.5v), an instrumentation amp, a high-pass filter to eliminate DC bias, and a second-stage non-inverting amplifier with some low-pass filtering.

The EKG circuit has four modules: a virtual ground (here set to 0.5v), an instrumentation amp, a high-pass filter to eliminate DC bias, and a second-stage non-inverting amplifier with some low-pass filtering.

The amplifier worked surprisingly well. I did sometimes have trouble with 60Hz noise, but it did not seem to be any worse than the amplifier based on the INA126P. I can remove the noise by digital filtering, though I’ve only played with that by post-processing the data files, not by designing a notch filter to run in realtime on the KL25Z (something to do when I have more time).

Here are a few traces made with EKG circuit above, feeding into the PTE20-PTE21 differential input on the KL25Z board, recorded using PteroDAQ.

This is lead I, without filtering, showing a rather disturbingly large 60Hz noise signal.

This is lead I (LA–RA), without filtering, showing a rather disturbingly large 60Hz noise signal.

This is lead I (LA-RA), showing how the digital filter cleans up the signal. This was Bessel bandpass filtered to 0.3Hz to 100Hz, followed by notch 57Hz–63Hz, followed by notch 117Hz–123Hz. Each filter was a 5th-order Bessel filter, applied first forward in time then backward in time (using scipy's filtfilt function).

This is lead I (LA–RA), showing how the digital filter cleans up the signal. This was Bessel bandpass filtered to 0.3Hz to 100Hz, followed by notch 57Hz–63Hz, followed by notch 117Hz–123Hz. Each filter was a 5th-order Bessel filter, applied first forward in time then backward in time (using scipy’s filtfilt function).

This is lead II (LL-RA), which for some reason had rather low noise even without filtering.

This is lead II (LL–RA), which for some reason had rather low noise even without filtering.

I noticed that sampling at 360Hz allowed me to see a bit more of the structure of the S and T complex than I’ve seen previously, particularly in lead II, and I can even make out a little bump of a U wave just after the T wave.

I now have to decide whether to have students do the EKG amplifier without an INA126P chip, using only op amps. The design will be fairly heavily constrained, as they’ll need to get it all working on a single MCP6004 chip, but it will justify my spending a bit more time on how instrumentation amps work.

I may redesign the blinky EKG to use a single MCP6004 chip also, which would reduce the price of that substantially.

Instrumentation amp from op amps still fails

Filed under: Circuits course — gasstationwithoutpumps @ 17:02
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I’ve been trying to decide whether to have students build an instrumentation amp out of op amps in the circuits course.  Currently the INA126P instrumentation amp chip that I have them use is a black box to them, even though I include an explanation on the lab handout showing how it is internally a pair of op amps and 4 resistors:

Internally, the INS126P instrumentation amp is two op amps and 4 resistors.

Internally, the INS126P instrumentation amp is two op amps and 4 resistors.

I won’t repeat that presentation here (there’s a condensed, early version of it in a previous blog post).  I’ve not actually lectured on the 2-op-amp design before the instrumentation-amp lab, in class, though I did manage to talk about the 3-op-amp instrumentation amp this year (a waste of time, since they did not really process the ideas).

What I was interested in today was whether the pressure-sensor lab could be done entirely with op amps, rather than with the more expensive INA126P chip.

I decided to design an amplifier with a gain of around 200 and an output reference voltage around 0.5 v (based on a 3.3v supply), using the 2-op-amp design and MCP6004 op amps. Here is what I came up with:

This is the design I came up with and built.  It works, sort of.

This is the design I came up with and built. It works, sort of.

The amplifier amplifies and seems to have about the right gain, but there is a large DC offset on the output: about 0.24V, which translates to an input offset of about 1.2mV. I checked with a multimeter, and the negative-feedback voltages are indeed about that far apart, while the inputs from the pressure-sensor bridge are less than 40µV apart. The pressure sensor sensitivity is about 80µV/kPa/V, or 264µV/kPa with a 3.3V supply. If I use the pressure sensor with a blood-pressure cuff, I’ll want to go up to about 180mmHg or 24kPa, so the sensor output should be in the range 0–6.3mV. An offset of 1.2mV is huge!

If I remove Rgain from the circuit, the output offset drops to 20.88mV, which is 1.1mV referenced to the input (close to the 1.22mV measured at the negative feedback inputs).  Further removing R2 or R4 does not change the voltage difference between the negative-feedback inputs.  In fact removing all three of Rgain, R2, and R4, so that we have two unity-gain buffers (with 180kΩ and 10kΩ feedback resistors), still leaves the negative feedback points 1.22mV apart.  Each seems to be about 0.6mV from the corresponding positive input.

The problem is that the input offset voltage of the MCP6004 op amps is only guaranteed to be between –4.5mV and +4.5mV:  I’m lucky that the input offset voltage is under 1mV!  Even the INA126P instrumentation amps that we’ve been using have an input voltage offset of up to 250µV (150µV typical). One can obviously get better instrumentation amps, but the selection in through-hole parts is limited, and I’d have to go to an instrumentation amp costing $4.25  (LT1167CN8#PBF) instead of $2.68 to get the input offset voltage down to 20µV.

I’m going to have to rewrite the section of the book on instrumentation amps, to discuss (at least briefly) offset voltages.  I had originally thought that that the signals we were looking at were big enough that the offset voltages didn’t matter. For the INA126P, a 150µV offset would be about 0.6kPa, while the 1.22mV offset I was seeing in my homemade instrumentation amp would be about 4.6kPa.

I wonder also whether I can make an EKG circuit using this 2-op-amp instrumentation amp circuit.  The EKG already has to deal with potentially large input voltage offsets due to differing electrode-skin contacts.  In fact those offsets may be over100mV, far larger than the 1.2mV from the amplifier.  I’ll have to add another stage of amplification (after a high-pass filter), but that shouldn’t be a problem. I looked at this problem a year ago in 2-op-amp instrumentation amp and Common-mode noise in EKG, and concluded then that common-mode noise would be too large, but I’m tempted to try again, using the design here with gain 19 and a second stage with a gain of around 80 (for a combined gain around 1520), as last year I rejected the idea before actually building the circuit.

 

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