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:
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:
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.