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2015 March 18

Freshman design projects moderately successful

I just finished grading this year’s freshman design projects. I think that the projects were more successful this year than last year, in part because I kept the students focussed on electronics and programming (for which they had lab access and which I could help them debug), and in part because the projects were somewhat less ambitious.

There were two groups doing EKGs and 4 groups doing blood pressure meters.  Both EKG groups managed to demonstrate their projects working, as did one of the blood-pressure groups.  (I’m being fairly generous here about what “working” means—they had to get their electronics to work, capture the data, and plot the waveforms, but further interpretation or software was not required.)  The other three blood pressure groups did not manage to demonstrate their projects, but one of them managed to plot waveforms for the pressure measurements (without getting their high-pass filter and amplifier working for the pulse measurements).

Some things I learned for next year:

  • Tell the students what op amp to get.  A number of students picked op amps that turned out to be rather old-fashioned ones with very low input impedance (as low as 2MΩ), rather limited output ranges, and external nulling circuits. The cheap MCP6002 or MCP6004 chips would have worked better at lower cost.  In fact, I gave one group that seemed to have a good schematic (but couldn’t get their circuit to work) an MCP6002 chip, which they wired in place of the op amp they had been using, and their circuit worked immediately.  I would have done the same for other groups, but the others with poorly chosen op amps were about a week behind and did not have circuits that were that close to being functional.
  • Warn students sooner not to use FedEx.  My son’s and my experience with FedEx this year has been that they are ludicrously slow. At least one group was burned by a ridiculously long delivery time, having ordered with FedEx delivery just hours before I warned the class about them.  (The US Post Office is faster and cheaper for lightweight electronics orders from Digi-Key.)
  • Students who never ask questions in class probably don’t understand much that is going on—all the groups that successfully demonstrated their projects had at least one active participant in class.
  • Students who fail to turn in their progress report are almost certainly not going to complete the project on time—I need to be more assertive in getting them moving and demanding that they show me their schematics.  Almost everyone had errors in their schematics on their first design (and one of the successful groups went through 4 incorrect designs before getting to one that worked).  Students that are afraid to show me incorrect or incomplete work don’t get the feedback they need to correct the problems—I need to normalize errors more and insist on seeing stuff, even if it is wrong.
  • The MXP5050DP pressure sensors are very easy for students to use, though a bit pricey at $16 each.  The built-in amplifier makes doing pressure measurements with an Arduino fairly trivial (hook up the three wires of the sensor to A0, +5V, and GND).  They were a good choice for the freshman design seminar, though I’ll continue to use MPX2053DP sensors without an integrated amplifier for the applied circuits class—that assignment is intended to get students to design with an instrumentation amp and to understand a bit about strain gauges.
  • Get the students to plot stuff earlier in the quarter. One group tried installing gnuplot on a Mac in the lab in the last few hours, which did not go well for them.  They did eventually find a plotting program that they could install and run, but then did not have time to run the data they collected through the filtering program I’d written for the class.  Their signals were pretty clean, though, and the plots they produced were good even with just the RC high-pass filter in their amplifier, without digital filtering.
  • The students seemed (for the most part) pretty excited about the projects—even those whose projects didn’t quite work seem to have gotten a lot out of the lab times.  I should look in a couple of years to see how many have stuck with engineering majors (I suspect that some might switch to computer science or computer engineering, rather than sticking with bioengineering, but that’s ok).

2014 June 26

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.


2014 June 1

Blood pressure monitor

I thought of a new variant on the pressure sensor lab for the circuits course: a blood pressure monitor.  I happen to have a home blood pressure monitor with a cuff and squeeze bulb that can be detached from the monitor and hooked up to the MPS2053 pressure sensor instead.  With this setup and an instrumentation amp, I can easily record the pressure in the cuff and observe the oscillations in the cuff pressure that are used for oscillometric blood pressure measurement.

Cuff pressure measurements using an MPX2053DP sensor, and instrumentation amp, and a KL25Z microcontroller board running PteroDAQ software.

Cuff pressure measurements using an MPX2053DP sensor, and instrumentation amp, and a KL25Z microcontroller board running PteroDAQ software.

The fluctuations can be observed by removing a baseline (fitting an exponential decay to the dropping pressure, for example, and the subtracting it out) or by using some sort of digital filter. I tried using a 0.3Hz–6Hz bandpass filter (4th order Bessel filter, applied using scipy.signal.filtfilt):

Oscillations corresponding to the pulse are very visible when the slow pressure decay is filtered out.  I've zoomed in on just the time of the dropping pressure, marked with lines on the previous plot.

Oscillations corresponding to the pulse are very visible when the slow pressure decay is filtered out. I’ve zoomed in on just the time of the dropping pressure, marked with lines on the previous plot.

The pulse is very easy to see (about 40.4bpm in this sample—low even for me), but figuring out the systolic and diastolic pressure from the fluctuations is a bit messy:

The oscillometric method of measuring blood pressure with an automated cuff yields valid estimates of mean pressure but questionable estimates of systolic and diastolic pressures. Existing algorithms are sensitive to differences in pulse pressure and artery stiffness. Some are closely guarded trade secrets. Accurate extraction of systolic and diastolic pressures from the envelope of cuff pressure oscillations remains an open problem in biomedical engineering.  
[Charles F Babbs, Oscillometric measurement of systolic and diastolic blood pressures validated in a physiologic mathematical model, BioMedical Engineering OnLine 2012, 11:56 doi:10.1186/1475-925X-11-56]

One shortcut is to find the maximum amplitude of the envelope of the oscillations, and look at the pressures at fractions of the amplitude:

However, it has been shown that the pressure, Pm, at which the oscillations have the maximum amplitude, Am, is the mean arterial pressure (MAP). Empirical and theoretical work has shown that the systolic and diastolic pressures, Ps and Pd respectively, occur when the amplitudes of oscillation, As and Ad respectively, are a certain fraction of Am:

  • Ps is the pressure above Pm at which As/Am = 0.55
  • Pd is the pressure below Pm at which Ad/Am = 0.85

[Dr. Neil Townsend, Medical Electronics, Michaelmas Term, 2001,]

I’m too lazy right now to try to come up with a good envelope follower and find the times for 55% and 85% of peak. The peak seems to be around 48.3s in this plot with magnitude of 0.336kPa and a predicted MAP of 16.28kPa (122mm Hg).  I based the MAP on low-pass filtering the signal to remove the fluctuations and make a good smooth curve for finding the systolic and diastolic pressure, once times on the envelope are picked.  Again, a 4th order Bessel filter applied with filtfilt looks good:

Low-pass filtering removes the fluctuations, so that picking two time points can give clean pressure readings for the systolic and diastolic pressure.

Low-pass filtering removes the fluctuations, so that picking two time points can give clean pressure readings for the systolic and diastolic pressure.

From the standpoint of the course, the filtering to get a good signal is probably too difficult, but students could record the cuff pressure and observe the fluctuations. They might even be able to do some crude RC filtering, though this is really an application that calls out for digital filtering.

2014 May 22

Class-D lab revision didn’t work

In Long weekend, I discussed what I was planning to do about anticipated problems with the class-D amplifier lab, specifically

  • Replace the AOI518 nFET with one that has a lower input capacitance, such as the PSMN022-30PL,127.  The gate charge at VGS of 4.5v is 4.4nC, about half that of the AOI518.
  • Replace the open-collector comparator with one that has push-pull output, like the TLC3072, which can provide ±20mA current (more than the LM2903, even before we allow for the current through the pullup resistor).

I did a neat version of the schematics last night using the TLC3072 comparators and the AOI518 nFET. This year I remembered to include an adjustable gain stage in the preamp, so that I could more easily control the volume. Today in the lab, while the students were soldering up their instrumentation amps for the pressure sensor, I wired up the class-D amplifier, one stage at a time, confirming that each stage worked using the oscilloscope before moving on to the next. The build took me longer than I had expected—almost 2 hours.

Everything worked fine until I connected the drains of the two FETs together.  Initially it worked ok, but after about 20 seconds the shoot-through current increased, causing the current limitation of the bench power supply to kick in.  Then the voltage on the lower power rail moved up  close to ground, and the input voltage on the comparator was swinging below the negative rail.  I think that this damaged a couple of my TLC3072 chips—I’ve marked them, and I’ll have to test them before using them.

Replacing the AOI518 transistor with the smaller  PSMN022-30PL,127 did not help.

I finally borrowed an LM2093P chip from one of the students (I’d left mine at home, by mistake) and tried replacing the TLC3072 chip with the LM2093P. They have the same pinout, but the LM2093P is an open-collector output, so I had to add pullup resistors.  I guessed a couple of values, based on vague recollections of last year’s design, and the amplifier worked.

Initially I could only run the amplifier up to ±7v on the power supply, without the FETs getting too warm—there was still too much shoot-through current during transitions.  I switched to a lower resistance for the pullup on the pFET gate, to make the voltage swing less and the turn-off faster.  At that point the amplifier worked quite happily with a ±8v swing without the transistors getting warm.The circuit worked with either of the nFET transistors, so I’ll just have the students stick with the AOI518 in their parts kit.

I couldn’t crank up the volume on the speaker, though, because I got feedback squeal whenever the gain got too high.  Perhaps I should make a long speaker extension cable for students to do testing next week.  I seem to be out of speaker wire, though.

The class-D amplifier design will be a tough one for the students, and I’ll need to do a supplemental handout on open-collector outputs (I’d cut that material from the handout when I thought that we would be using the TLC3072 comparators).

Last week I thought that the students could start on the class-D amplifier in lab today, having finished the soldering for the pressure-sensor amp on Tuesday, but it took almost the whole lab time today for students to finish the soldering, even though everyone had working breadboards on Tuesday.  The lab ran over by almost 2 hours for one group, taking a total of 8 hours instead of 6 (last year the same lab took only 4 hours for the slowest group, probably because last year’s class came to lab more prepared).

The layout took longer than students expected, as did the soldering.  Everyone did (eventually) get working soldered instrumentation amps, though for a couple of groups I had to point out that their wiring did not match their schematic (they had called me over to help debugging).

In one case they had connected a resistor to the wrong point in their circuit.  I found the bug by tracing where their virtual ground was connected, and asking them to identify each component. Even after I showed them both resistors connected to their virtual ground, where only one was supposed to be, it took them a long time to realize what the discrepancy was. They had wired exactly what they had laid out, but the bug was in their layout, and they had not done a thorough enough job of checking against their schematic.

Another group had a working circuit but with too little gain. After checking a few of the DC voltage levels with them, I compared each of their resistors to the schematic.  In one place they had wired in a 1kΩ resistor where the schematic called for a 10kΩ resistor.  They unsoldered the incorrect resistor and soldered in the resistor from the design, which salvaged the circuit.

I also returned Wednesday’s quiz in lab today—pretty much like last year, the scores were much better on the second quiz than the first one, though still only half what I think the students should be able to do at this point of the quarter.  I’m once again assigning the students to redo the quiz as homework.  I need to decide soon whether to give them another quiz during the final exam time.

2014 May 19

Long weekend

In Problems rewriting the Class-D amplifier lab I discussed the problems I was having with rewriting the class-D amplifier lab, and came up with the following ideas:

  • Try the same basic design as last year, and see if I can get a power supply voltage and pullup resistor settings that work.  I’ll have to do this in the lab on campus, because I don’t have a power supply at home that provides the same voltage ranges.
  • Replace the AOI518 nFET with one that has a lower input capacitance, such as the PSMN022-30PL,127.  The gate charge at VGS of 4.5v is 4.4nC, about half that of the AOI518.
  • Replace the open-collector comparator with one that has push-pull output, like the TLC3072, which can provide ±20mA current (more than the LM2903, even before we allow for the current through the pullup resistor).
  • Add a driver chip that is intended for driving a pair of FET transistors, like the FAN7382, which can provide very fast rise and fall times even into 1000pF loads. But the driver chips are intended for two nFET transistors, and the charge pump for the high-side driver would be difficult to explain in this course and requires external components (diodes, resistors, and capacitors) that the students would have no idea how to choose.
  • Use an H-bridge or half-H-bridge chip instead of having students design the power stage with FETs.  Most are designed for controlling motors, and don’t give timing specs that would tell me whether they could handle a high enough PWM frequency for audio output.

Yesterday I ordered some TLC3072 comparator chips and some PSMN022-30PL,127 nFETs, also some MCP6004 op-amp chips, since I seem to have run out, and they are quite cheap.  (I probably need more instrumentation amp chips also, but they are more expensive so I’ll probably wait until I really need them). I hope that I can get the TLC3072 comparator chips before lab on Thursday, so that I can test out the circuit before the students need to build it.  Otherwise I’ll have to cycle into work over the weekend to test the lab. Digi-key shipped it this evening by first-class mail, which probably means I won’t get it until Friday—when they’ve shipped early Monday morning, I ‘ve gotten things by Thursday.

This weekend had me spending a lot of time on the circuits course—I estimate that I spent about 35 hours on the course from Friday night until Monday morning. In addition to figuring out how to rescue and simplify the class-D amplifier lab, I also had to rewrite all the tutorial material for the lab handout, create a quiz for Wednesday’s class, and grade their optical pulse monitor lab reports.

The grading is still a bit painful, as some of the students write nearly incomprehensible English and some are still making trivial mistakes on the schematics (like leaving out component values and shorting out components).  One error on this round was particularly worth commenting on to the class: consistently using mΩ when MΩ was meant.  What is a factor of a billion between friends?  About half the class still can’t explain sensitivity and gain computations, even when they get the right answer.  In some cases, they probably had not come up with the answer themselves, as they made mistakes that magically got cancelled and disappeared when they reported their final results. They keep such awful lab notebooks though, that it is quite possible that they had done the correct computation at one point, but could not reconstruct their work when they went to write it up a few hours later, so I won’t accuse anyone of cheating.

Today I collected prelab homework at the end of class.  The assignment of prelab homework to be turned in did get students to read the assignment and make a stab at doing the design work, and (for the first time) I got questions about the design on Monday rather than halfway through lab on Tuesday.  Most of the designs were incomplete and had serious errors, but at least an attempt had been made to understand the problem, and I was able to clear up several misunderstandings in class:

  • Several students tried to put the op amps before the instrumentation amp.  I obviously had not made it clear what an instrumentation amp was for.
  • Many of the students had not gotten the idea that an instrumentation amp is a single symbol in a schematic not a conglomeration of op amps and resistors.  This was probably my fault for trying to explain how an instrumentation amp works, rather than how to use one.  It doesn’t help that the INA126P data sheet shows only the innards, and not the external view of the amplifier. I think that next year I may skip explaining how an instrumentation amp works internally, though I think it is kind of cool.

    Instrumentation amplifier symbol.  I prefer putting Vref on the bottom, rather than the top of the amplifier, but either way works.

    Instrumentation amplifier symbol. I prefer putting Vref on the bottom, rather than the top of the amplifier, but either way works.

  • I spent a fair amount of time trying to help the students understand the notion of output voltages being referenced to a specific other voltage. I think that next year I will make a point of always writing the gain equations as V_{out} - V_{ref} = g(V_{p} - V_{m}), so that the output is always a difference between two voltages, rather than as V_{out} = V_{ref} + g (V_{p} - V_{m}), like I’ve been doing this year.  Although the two statements are algebraically the same, I think that they’ll be better able to do multistage gain computations and less likely to forget to connect up Vref if they have the output expressed as a difference.
  • I cleared up some small misunderstandings of how to do layout on the protoboard PDF drawings.

At the end of class I collected all the homeworks, and I “graded” them tonight.  I’m providing feedback and a done/not-done check, not a grade (which is lucky for them, since most were such sloppily done notes that I doubt that most of the students could reconstruct their thoughts from the notes).

I did get a chance to introduce the students to pulse-width modulation and FETs today, which are essential topics for the class-D power amp next week, but I’m not happy with the presentation I gave—I did a better job in the handout, which I suspect most of the students will never read.  The problem with working so much this weekend was that I was very tired all afternoon today, and was not able to put together as coherent a presentation as I wanted to.

I still have to do a design for the class-D power amp with the new comparators, but I’m too tired to do it tonight.  I’ll probably sketch it on paper tomorrow and do a neat schematic in SchemeIt tomorrow night. I’ll wire up all I can at home either tomorrow or Wednesday night, while waiting for the comparator chips and MCP6004 op amp chips to arrive.  (Actually, I’ll probably just use an MCP6002 chip, since I don’t need 4 op amps, and I still have a couple of MCP6002 chips left.)

Tomorrow I need to remember to bring in the cheap aquarium air pump, some tubing, and a clamp so that students can observe back pressure and the fluctuations in pressure from the pump (assuming that they get their amplifiers working).

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