If an OP-77 connected as simple inverting summing op-amp stage has N inputs, how could you estimate the practical limit for N ? I think if all but one input is grounded you have the worst case, but where would you go from there?
Thanks, Dave
What's your worry? Offset voltage? Noise? Either way source impedance is a vital factor.
Depends largely on the source impedances driving the summing resistors and the resistor scale used.
For typical input voltages say -10 to +10V, the output of a hypothetical preceding op-amp can probably source several mA to 10mA. So you're looking at summing resistors in the 1K range at the low end. The OP-77 has an offset current in the nA range and offset voltage in the uV range, so few-K resistors for summing is pretty good.
If the summing inputs have radical ratios (one is weighted 1000 times another) then this will become the limiting factor.
Tim.
In addition to the issues mentioned by Mr. Grise and Mr. Shoppa, as N approaches the open loop gain of the op-amp divided by the hoped-for realized gain from each input, the loop gain will approach something near 1, leading to limited accuracy of the actual closed loop gain and increased distortion.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
As my initial concern what happens in my suggested test case when the parallel equivalent resistance to ground gets small?
Just sum up the offset voltage contributions.
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |
We've got a box where we need to precisely sum ten 50-ohm signals with a bandwidth of almost-DC to 1.5 GHz. All sorts of interesting things happen: noise, reflections, crosstalk, oscillation, nonlinearity, heat. Our third iteration looks pretty good, but the inputs do reflect a bit. #4 should do it.
John
The current incarnation is a single common-source PHEMT, with a 45-ohm resistor from each signal (10 MCX connectors in a circle) dumping into the source node, and the summed output from the drain. This is fair, and gain is good, but the fet source impedance is about 10 ohms (1/Gm), so it's not a perfect summing point, so there's a bit of crosstalk. Signal comes in on one line, wiggles the PHEMT source a bit, so some goes out the other nine. They get back to the signal generators on the other end of the cables, bounce a little, and return. It's less than ideal.
If this has more reflection than the customer can tolerate, I guess we'll have to redesign the summer/amp board (again!) and go to 10 mmics, one to receive each signal. Each of them will probably need an input attenuator pad, since it's hard to get a mmic to be a true 50 ohm load (you're lucky to hit 40 on most of them.) So now we're throwing away gain, begging for noise, and we still have to sum the ten mmic outputs. Crowd ten of these in a circle, converging on a summing node in the center, and this starts looking like a microwave ring oscillator.
Oh, MCXs are great coax connectors. Small, easy to mate, cheap, and you can get tons of test cables for the lab on ebay.
John
"Fred Bloggs" wrote in message news: snipped-for-privacy@nospam.com...
(Responding to the question: "If an OP-77 connected as simple inverting summing op-amp stage has N inputs, how could you estimate the practical limit for N ?")
don't you review OpAmps 101 before you post your
You amaze me, Fred. You show signs of some intelligence but seem to have never applied it to understanding your own limitations.
Do you deny that loop gain imposes a practical limit on how many inputs a summing op-amp circuit can have? If so, you are the one that needs remedial training in op-amp theory.
If you are merely suggesting that the limit I have stated is never applicable, why have you not given some hint as to what the real limit on N is? My bet is that you cannot.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
"John Woodgate" wrote in message news: snipped-for-privacy@jmwa.demon.co.uk...
For that topology, the feedback gain is: (1/N) / (1 + 1/N) == 1 / (N + 1) Clearly, as N approaches the open loop gain of the op-amp, the loop gain will be approaching 1. This means that the gain accuracy will be reduced relative to the usual case where loop gains exceed one by quite a bit. Consider the formula for closed-loop gain: Avcl = G / (1 + GH) == 1 / (1/G + H) Usually, we neglect the 1/G term because it is small relative to H. But as N becomes comparable to the open-loop gain (G), that simplifying assumption becomes invalid.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
So the lower the parallel equivalent to ground (unused inputs) the higher the offset voltage... but the noise increases or decreases?
Eh? Does this "summing" amplifier not include a feedback resistor?
If there IS a feedback resistor, the OpAmp maintains a virtual ground at the inverting input, so the gain is simply Rf/Rin.
The unused inputs have no effect on the gain... HOWEVER, they do add _offset_voltage_ terms.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
There is absolutely no limitation whatsoever if you scale the input summing resistors appropriately. For example, an inverting opamp with a a single 1K ohm input resistor is no different than a inverting summing amplifier with one thousand inputs using 1M ohm input resistors and all inputs driven by the single source. There will be no measurement you can make that will be able to distinguish the two if you "black box" the input circuits driving the OA inverting input.
Oh yeah- people are running that check all the time in their calculations. Why don't you review OpAmps 101 before you post your irrelevant pedantic drivel........
The open-loop signal at the input is reduced by a factor of 1/(N+1), so not only is the noise increased by a factor of N+1, but also the DC offset voltage (by the same factor). And the closed-loop DC gain accuracy will be affected-- eg. if (1/(accuracy)) * N starts to approach the open-loop gain of the chosen op-amp. Also the -3dB frequency etc.
The OP77 is a fairly good op-amp- it has a Vos of 0.1mV max, and an open loop gain of 1e6 min, and a GBW of 700kHz (typical), so even if those numbers were effectively knocked down by an order of magnitude by N~=10, it would still be pretty good in most applications.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
"John Woodgate" wrote in message news: snipped-for-privacy@jmwa.demon.co.uk...
I can remember designing a number of op-amp circuits where loop gain was a concern, some fairly recently. Of course, the frequency at which loop gain became a concern was higher than 20 kHz.
In any configuration where loop gain is needed to get accuracy (which includes driving down the natural, open-loop distortion of the amplifier), and where adequate loop gain is not a foregone conclusion, if you add inputs then you have likely degraded loop gain and need to reevaluate its adequacy. This can happen in real situations; you should not defer that kind of analysis for (nominally) unity gain DC summers with hundreds of thousands of inputs.
In an earlier post on this subthread, you wrote: "The voltage at the inverting input is zero for an ideal op-amp and is V/Ao for an open loop gain of Ao." Very true, but if you look at the named subject, and consider the formula for closed-loop gain (which I posted earlier): Avcl = G / (1 + GH) == 1 / (1/G + H) you should be able to see what happens as H (which designates the gain of the feedback network) is made so small that it approaches 1/G. The closed loop gain is no longer well approximated as 1/H, which is the result that obtains with an ideal op-amp.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
So the "Rin" are very small?
Time to write some equations and impress the students ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
See.......
Newsgroups: alt.binaries.schematics.electronic Subject: Re: SED: Summing Amplifier with non-ideal op-amp - GBWwithMultipleInputs.pdf Message-ID:
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Assume all the N summing inputs save one and the output of the op-amp are grounded (the op-amp is doing nothing). With a voltage Vin applied to the remaining summing input, the voltage at the inverting op-amp input is Vin* 1/(N+1). That effectively degrades the Vos, GBW, DC gain characteristics of the op-amp by a factor of ~1/(N+1). Or to look at it from another direction, you have a passive ~1/N attenuator followed by a non-ideal amplifier with gain ~-N (N>>1).
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
When you close the loop, yes.
Yes, -Rf/Rin, with an ideal input. Or -1 for a gain-of-one summing amplifier from a given input to the output.
But shunt that virtual ground to ground through a relatively low-value resistor and you degrade the performance. That's effectively what we have here.
I think they do affect the non-ideal gain (and GBW) as measured from a single input to the output with the other inputs at 0V.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Make that nearly a virtual ground. It is only an actual virtual ground with an infinite gain amplifier. So, when there is finite gain, there is some small voltage (output voltage over open loop gain) at the - input that drives the output, and that small voltage also drives some of the input current through the other input resistors. When the effective parallel resistance of all the other input resistors is so low that it soaks up half of the total input current (from the one input in question), there will be only half of the input current to go through the feedback resistor, and so the gain will have fallen to half.
-- John Popelish
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