Differential amplifier problem

Question

I'm working with a DAC and experimenting with removing the zero scale error - the actual output at a zero input code. I'm on a breadboard so it isn't particularly accurate but the ZSE is 5mV to 11mV. I set up a differential amplifier as follows - image is copied from a website, but I've confirmed and re-confirmed that my setup matches: So, I have the following inputs: V1 = 0.005mV V2 = DACoutput R = 100K. All of them. Opamp is an LM741 which is all I have on-hand, V+ = 5V, V- = ground . It has a max offset voltage of 6mV. Given that, I would expect Vout = V2 - V1. Vout is connected to a 1K resistor to ground. So the reality is that the R values are not perfectly 100K but are within the 10% spec. Thus I wouldn't expect Vout to be an exact difference, but 'more-or-less'. This is what I'm measuring: DACout (V2) = 4.06V, Vout = 4.04V DACout (V2) = 0.011V, Vout = 3.6V I've tried working out why with a low DACout I'm still getting a large Vout. Measuring the resistors and plugging them into the gain formula: Vout = -0.005 * (99.03/99.5) + 0.011 * (99.21/(99.05+99.21)) * ((99.5+99.03)/99.5) Vout = (-0.005 * 0.995) + (0.011 * 0.500 * 1.995) Vout = -0.005 + 0.011 = 0.006 So, essentially, my calculation confirms what I would expect the amplifier to do with a -0.005mV and 0.011mV input. I can't figure out where I've gone wrong, can anyone give me some pointers? 
 Attachments: 2477.LM741.asc.zip

michaelkellett · Accepted Answer

What power do you have connected to the 741 - it's not rail to rail capable it needs positive and negative supplies, +/- 10V is the minimum recommended.

The input common mode range is +/- 12V when running with +/- 15V supplies - that means that the amp won't work properly if either input is closer to a supply rail than 3V.

The LM741 was designed (by David Fullagar at Fairchild) in 1968 - 52 years ago - we have better parts now

Just seen your photo - single rail supply ?

The poor old 741 can't work like that, but you won't get any (even rail to rail IO types) op amp to do get the output right down to zero.

MK

michaelkellett · Answer

If you want to sink any current at 0V you need a negative supply. You could consider using +/- 5V supplies (+/- 10 if you stick with the 741 )

You can do a lot of this without spending money by using a simulator - LTSpice is free and good - (from www.analog.com)

The OP388 parts are interesting but they are chopper amps which means that they bring their own special problems and they are not that cheap.

One (of many) things to be concerned about is that the DAC will be non linear at low outputs if its a single rail part !

If you really want to play with actual parts rather than simulate then get some Microchip op amps (because some come in DIL packages).

MCP6002 are dirt cheap, rail to rail IO, not too fast so won't be too hard to handle on bread board. Single rail max supply 6V.

Offsets not that good but bias current is tiny.

1292245 from Farnell at £0.26 for a dual op amp means you won't be too sad when you blow one.

I'm not sure what your goal with this work is - if you can explain a bit more I might be able to offer more useful suggestions.

MK

Jan Cumps · Answer

Yes, we're doing it the naive way. That's why I hope that analog / DAC experts chime in and show the shining path to real world solutions. In the eLoad we made, things like OpAmp offset, quiescent current and other attributes played a significant role. Particular at the near 0 current point.... The 741 is not the best in class for input current specs. More modern OpAmps dealt with that problem. But as you've noticed, there's more to solve. I kind of like that these things don't solve easily in firmware with calibration and corrections. They are real electronics problems. That's what I'm here for ...

wolfgangfriedrich · Answer

I was going the same route as Michael, he already gave the correct hint in his question. This is the relevant datasheet section for a +-15V supply: And the 741 could not be run with +5V/0V at all:

michaelkellett · Answer

The problem is this:

Suppose you use an MCP4822 dual 12 bit DAC, the output offset max at zero code is +/- 1% of full scale = 20.48mV or 41 bits.

A typical rail to rail input amplifier will have a common mode range that goes down to maybe 0.3V below ground (although performance may not be well specified there.)

Your diff amp will be OK - the voltage on the positive input will be max 10mV to -10mV. To null that you'll need + or - 10mV on the other pin but there is a flaw in this reasoning !

It won't work - think about that weasel spec, the code is zero and the offset is -20mV or -41 bits . But the DAC only has 0 - 5V supply rails - its output can't go negative !!

So if you set any code in between 0 and 41 the DAC output just sits at 0V - so you can't null the zero offset of a single rail DAC.

MK

Andrew J · Answer

I'd followed that bit of your implementation and was trying to do something without an extra DAC - in theory it ought to work. Interestingly (to me at least), I'd realised that with 5 DACs one could output a linear scale from 0V to Full Scale: One to output the linear region with software compensated offset and gain One to output the pre-linear region (0 to start-of-linear, say 12mV) compensated by the output of a second DAC to limit to 0V to 12mV. One to output the post-linear region (say, 4V to 4.096V) compensated by the output of a second DAC. Software directs the required code to the right DAC and you'd only need 20 DACs to output 4 accurate DAC channels Using an Op Amp, at least in theory, should allow me to drive from 0mV to full scale - zero scale error. That, of course, widens the full scale error so it's not perfect and I guess you decide which end of the scale is most important. What I'd really like is an OpAmp configuration that would stretch zero scale error to 0V whilst also stretch full scale error to full scale whilst keeping it all monotonic. But I'll settle for understanding what I'm doing wrong here first! It might help with a photo of the breadboard (without the 1K load):

Andrew J · Answer

Basically, I'm having a play around with DACs and ADCs creating my own version of Jan' and Peter's Boost board for their eload. Ultimately, I'll probably add a DC load onto this as well but right now, I'm just trying to get a better understanding of these devices and how I might improve the output. That's fairly simplistic because I am trying to make a control board with other things as well, but here is what I'm trying to do specifically with the DAC / ADC side of things.

The 12-bit DAC, with a 4.096 external reference, I have is 'ok' - pretty cheap, not highly accurate and I will change it for another shortly - and I can measure it's output as 5mV to around 4.080V. So there is a Zero Scale Error of around 5mV and a Full Scale Error of 26mV. Sticking with the ZCE: what this means is that if I try and output codes 0 to 5 I get 5mV, 6 I get 6mV etc. My idea was to use a differential amplifier with a 0.005mV input on the inverting side to 'subtract' this from the output of the DAC on the non-inverting side.

The 16-bit ADC isn't too bad and I know that will come with its own errors. I had thought I would buffer the ADC inputs with a simple RC filter on the buffer output to filter any noise. For this, I had thought to use an OP388 as TI promote it for such a purpose:

"The OPAx388 (OPA388, OPA2388, and OPA4388) series of precision operational amplifiers are ultra-low noise, fast-settling, zero-drift, zero-crossover devices that provide rail-to-rail input and output operation. These features and excellent ac performance, combined with only 0.25 μV of offset and 0.005 μV/°C of drift over temperature, makes the OPAx388 a great choice for driving high-precision, analog-to-digital converters (ADCs) or buffering the output of high- resolution, digital-to-analog converters (DACs). This design results in excellent performance when driving analog-to-digital converters (ADCs) without degradation of linearity."

There are things that I don't yet understand but that's why I'm doing it - for fun and learning, so I don't mind spending a bit of money to work things out. I'm trying some things I can do on a breadboard first and then reading around potential solutions for what I'm seeing. At some point I want to actually produce a PCB because it's as cheap to do that as to get DIL adapters for the ICs!

Jan Cumps · Answer

Using a non-ideal OpAmp is the best way to learn. Solving it by using a better OpAmp (as I did ) is a cop-out. You learn nothing. And the problems are still there when you care about the full "close to 0 range". I think that if you can solve the apparent OpAmp pitfalls with a 741, you've upped your OpAmp understanding in a way that will never be achieved by just replacing it with a newer one ... That said, I'd never use a 741 in a design anymore. The newer ones are vastly better...

Andrew J · Answer

Yep, I confirmed early on that with or without the 1K resistor it makes no difference.

Andrew J · Answer

Wolfgang/Michael,

it's using 5V+, 0V- so no -ve supply rail; I could run it from a 12V+ which would keep DACout max well below that limit. I did wonder about a -Ve supply rail originally, but as I wasn't asking it to output a -ve voltage I didn't think it would matter.

The LM741 is all I have to hand and was something I got hold of when I first started out learning this stuff. I'm prototyping ideas at the moment before getting a prototype PCB made up. One of my ideas involves buffering an ADC input so I had sort of settled on getting a OP388 for that and I'd thought if this zero code error compensation worked I would get another (OP4388) for that purpose.

michaelkellett · Answer

If I understand correctly what you are trying to do it might be a lot simpler and cheaper to use a singe DAC with 2 more bits and a deliberate fixed offset on the output and do the compensation entirely in software. You could still make it fully automatic by adding an ADC but this needs to be good compared with the DAC. Don't forget the voltage reference - it's quite tricky to get even 12 bit accuracy over a modest temperature range. MK

Jan Cumps · Answer

This is the offset circuit I designed : It uses a decicated OpAmp and an available programmable DAC to "offset the offset" of another DAC, by injecting the voltage in an OpAmp subtract configuration. It didn't correct the 0 point errors I have in my DC load, but it fixes the DAC offset. A potentiometer and manual trimming would fix that too, but this solution allows to use calibration firmware to offset the offset. My advise here is of a generic OpAmp user. It would be great to see comments from designers hat used precision DACs and precision OpAmps in a measurement design ... That's the secret sauce.

Andrew J · Answer

Agreed, that's why I'm making my control board a little more complex by looking at a variety of potential issues. I'm limited to a LM471 right now because that's all I have and it fits a breadboard - I had thought of an OP388 for actual use. So I'm testing the theory right now before committing to a PCB for better prototyping. Good job too it seems

Jan Cumps · Answer

calling in jc2048 - opamp expert. Vout is connected to a 1K resistor to ground. This should not have impact - the output of an opamp is low impedance and as long as it can drive the 1K load, this is fine... So we should focus on the input and feedback resistors ...

Jan Cumps · Answer

So it's actually working: the photo shows the meters in volt range. The measurements below are done with all in mV range. Initially, I get about 2 mV out off the subtractor I've set the DAC1 to 200. That's the region where mine is in the linear part. Under that, it doesn't move below the offset. DEVE:DAC1 200 DAC 1 outputs 4.92 mV DAC 2 1.64 mV OpAmp is still at 2 mV At this point, any change in DAC1 increases the output. This is the range I want to work from - starting DAC1 from 200 - and consider that 0. So I have to work away the 2 mV offset with DAC 2. Starting with value 10: DEVE:DAC2 10 DAC2 is now 2.04 mV OpAmp is at 1.55 mV DEVE:DAC2 20 DAC2 is now 2.42 mV OpAmp 1.19 mV DEVE:DAC2 30 DAC2 is now 2.85 mV OpAmp 0.75 mV DEVE:DAC2 40 DAC2 is now 3.24 mV OpAmp 0.35 mV DEVE:DAC2 45 DAC2 is now 3.46 mV OpAmp 0.13 mV DEVE:DAC2 50 DAC2 is now 3.65 mV OpAmp -0.06 mV <- overcompensated The following two are the closest I can get to 0... DEVE:DAC2 48 DAC2 is now 3.56 mV OpAmp 0.02 mV DEVE:DAC2 49 DAC2 is now 3.62 mV OpAmp -0.02 mV In theory, I could automate this by connecting the last uncommitted ADC I have on the board to the output of the subtractor and let the system run through the paces itself until it settles for the lowest linear point, then 0 it out ...

Jan Cumps · Answer

Jan Cumps wrote: I can detach the power MOSFET gate via software. That mechanism is described here: Programmable Electronic Load - Input Enable Functionality

Differential amplifier problem

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