I need to design an amplifier that takes in a differential signal (call these inputs A and B), adds a bias C (where this signal is referenced to amplifier's ground), and applies a high gain (G ~= 100). In other words:
Output = ((A-B) + C) * G
I need this operation performed for two channels, and I want to minimize the number of ICs and components required. High CMRR is a plus. In an earlier generation of this design, I was performing this operation using an instrumentation amplifier (where bias signal C was added after the gain, i.e.):
Output = (A-B)*G + C
But I've realized that I definitely need to be biasing before the gain. Any creative ideas?
Thank you for your response. I assume you are suggesting that I use an op amp 'adder' in conjunction with my original instrumentation amplifer. This will work, but I was hoping that a more compact solution existed... Maybe even an integrated circuit with such behavior?
Twisted from EDN-Design Ideas, October 1, 1992 (I save everything I run across that amuses me ;-)
...Jim Thompson
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| 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 |
It\'s what you learn, after you know it all, that counts.
I greatly appreciate you taking the time to help me, but this solution seems to produce an unamplified C, although I am hoping that the output would include an amplified version of C, i.e. G*C. The solution (A-B)*G + C is what my current instrumentation amplifier gives (with C being the reference input).
I interpreted your request to mean that the offset needed to be inside the common-mode loop to prevent amplifiers hitting the rails.
Is your need really a fixed offset with VARIABLE gain?
...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 |
It\'s what you learn, after you know it all, that counts.
That is very similar to the old thermocouple into a diff-amp problem. A-B is the t/c signal and C is the cold-junction compensation. I tried the scheme below, with a sort-of success, but later dropped it and went back to injecting C after the front end. _ A+------------------------------|+ \\ | >---+-->
C is converted to a push-pull current (dual OTA) and I(C) injected into the common source resistor Rs.
I used a dual current to try and keep the stray loading to 0v on Rs balanced, for minimum impact on the CMRR. DC CMRR was acceptable, but AC CMRR (even at 50Hz) was sensitive to unbalances in the layout Cstray.
The tempco of the dual OTA was integrated into the overall cold-junction t/c compensation.
You've already been shown that you need to add +/- C/2 to the A and B inputs and then IA the two. You don't mention anything about voltage ranges, power supplies, input impedance requirements, or CMRR.
Thank you all for your responses. Fred: your solution is probably what I'll end up using (that or Rich's). These are solutions that I had considered, but will require at least 4 ICs for 2 channels (in other words, two sets of amplified differential pairs). My application is so space constrained, that these 4 ICs will be difficult to fit in. But... I guess I'll have to.
I assume you're using single-IC amplifiers? One instrumentation amplifier gives you A-B and a second G=1 instrumentation amplifier adds C to A, what's so hard about that? You can get them in very small packages. If B and C are from low-Z sources, you can use a difference amp to subtract C from B, before the instrumentation amp subtracts these from A, (A-(B-C))G = (A-B+C)G This is useful because difference amps come in duals, so your added functionality costs only one small IC, without discrete parts, for two channels.
BTW, there are single IC's that'll let you amplify C by G, or nearly so. For example, the TI Burr-Brown INA322 amplifies A-B by G+1 and lets you add -C*G to that. Perhaps the discrepancy between G and G+1 would be OK for you? Here C is referenced to the output ground or reference, compared to the input ground in the paragraph above and most suggestions. Did you make clear which was your situation for C?
And I notice the similar INA332 is available in a dual, INA2332. that could be one IC for both channels, with the added function, instead of two ICs, without added function, that you have now.
Why do you need the biasing before the gain? Is G adjustable?
Best regards, Spehro Pefhany
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"it\'s the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
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P.S., I'm thinking that if the problem you're trying to solve is saturation of the amplifier(s), and the gain is fixed, you might be able to use two equal relatively high value resistors either side of the gain-setting resistors in the conventional 3-opamp inamp configuration. One resistor to bias voltage C', one to the negative voltage -C'. The gain will be affected (increased) a bit by the resistors (a fixed amount) but the CMRR should be good, since everything is symmetrical.
My solution addressed the amplifier saturation issue, but didn't have "C" inside the gain equation.
...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 |
It\'s what you learn, after you know it all, that counts.
Winfield, I took a look at the datasheet for these amplifiers, and it appears that they both have an default internal gain of 5, meaning that the equation defining the amplifier would be:
Vout = (A-B)*(G+1)*5 - C*G
which is close, but still not ideal. I'm guessing that someone else out there may make a similar instrumentation amplifier that has a default internal gain of 1, which I'll search for today. If anyone happens to know of such an amplifier, please let me know.
I know several people on this thread have asked what I'm trying to do, so I'll elaborate for a moment: I have a differential input signal that consists of a DC component and an AC component:
Vin = AC(t) + DC
I'm trying to amplify the AC component and strip away the DC component, a task I've seen mentioned in several threads in this newsgroups before. However, the trick in my case is that the frequency of the AC component varies dramatically, to the point that the output signal will appear to stop for sometimes hours. In other words, filter-based solutions won't work. Winfields idea of latching the DC component
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looks attractive, but it's too large for my current design. I have a specialized digital algorithm that is can estimate this DC component, but it's a little bit awkward. It requires iteratively sampling the signal from an ADC, making adjustments to a DAC channel that's buffered into the instrumentation amplifier's REF line, then repeating. It works, but it's awkward and I'm looking for improvements.
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