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I need to buffer an input signal such that I do not load down the source, which is a 100k resistor. However, I need to buffer at least 6 channels, and for more advanced models I'd like to move to 10 channels (for the more advanced one I'd be okay using multiple chips though.)

I'd rather not use op-amps, because they will take up considerable space - just wiring the inverting or noninverting to the output, for example, uses a lot of space for traces - also, half the pins would be unneeded.

I'm not sure what chip or type of chip to be looking for - all the buffers I've found so far only work for digital logic, not analog signals. The buffer should have very low input bias current (nanoamps to hundreds of picoamps), like a JFET op-amp.

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    \$\begingroup\$ @ThomasO, if you want to sell a product please remember that FCC validation is hell to pay with a board that is 2 layer and has higher speed signals. Also remember that your signal speed is determined by your slew rate, not by the clock rate which is normally much lower. \$\endgroup\$ – Kortuk Nov 22 '10 at 17:53
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    \$\begingroup\$ are these signals going to an ADC? If So a simple unity gain buffer is not the proper solution to preprocessing a signal intended for an ADC. At least not if your after resolution greater than maybe 7 or 8 bits. \$\endgroup\$ – Mark Nov 22 '10 at 20:29
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    \$\begingroup\$ @kellenjb, I understand the risk I am taking. I live in Europe (UK), so CE is really the only thing that could come after me. But then how do things like magazines (say Silicon Chip magazine) get away with it...? Surely not all their projects are FCC certified, that's at least $5k each? IIRC, there is an exception for hobbyist things, I need to check it out. \$\endgroup\$ – Thomas O Nov 22 '10 at 21:48
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    \$\begingroup\$ depends on who you market and sell to actually. You don't need FCC certification to sell development kits, parts, modules. That is parts that are not intended for sale to or use by the general public. You are always bound by FCC rules to not knowingly exceed emissions limits (thats on the user's head), however you don't have to get your product certified until you market it or sell it as a contained device to the general public. This is why you'll almost never see FCC stamps on dev boards, i've had a few that i know would not have passed (PICDEM.net 2 for instance) \$\endgroup\$ – Mark Nov 22 '10 at 23:05
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    \$\begingroup\$ How much it costs depends on the device and category it falls into. In both cases if its a "low risk" device you can get by with a Certification of Conformance. Basically you say to FCC/CE "I've tested it, its ok" you just better be able to provide test data if they come asking for it. If its a higher risk device, they will require testing by a known good lab, which is where the higher costs come in. \$\endgroup\$ – Mark Nov 22 '10 at 23:20
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It seems after some discussion that you are looking for a buffer amplifier.

Digikey has a section for this. if you go to the main area for linear amplifiers and then select for buffer type and in stock you get quite a list. I selected for those with an input current of 2nA typical I got a relatively short list(like 10). This however does not give me an easy way to share the links, so you will have to click it yourself.

These are designed to give you the features you want and in a small package, now they normally come in groups of 2^n, so you will have to get a package of 8, but I am sure you can make it work.

Markrages had a bit of extra input to add in a comment:

Cautions about those buffer amps: Most of them are made for video signals and so they are high bandwidth / high current designs. That's a consideration if the circuit is battery powered. Also note that (the ones I looked at) are specialized, single vendor parts. More expensive and more availability risk than op-amps or transistors with standardized footprints. Engineers have a duty to only use oddball parts when their special features are necessary and relevant to the design.

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    \$\begingroup\$ @markrages, I specifically got this part due to conversation on the question about what Thomas needs. He specifically wants a buffer chip to save the traces to use an op-amp. I agree with your feedback. I will add it as a section in the answer if that is okay with you. \$\endgroup\$ – Kortuk Nov 22 '10 at 19:31
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    \$\begingroup\$ go for it. The question specifically excludes op-amps, for specious reasons, but they are exactly the solution to the problem here. In my opinion. \$\endgroup\$ – markrages Nov 22 '10 at 19:43
  • \$\begingroup\$ @markrages, he agreed that he wanted a buffer amplifier in the comments, which he specified he did not want the overhead of mapping the proper pins together. \$\endgroup\$ – Kortuk Nov 22 '10 at 20:16
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    \$\begingroup\$ yes, I saw that. note "the proper pins" are two adjacent pins on the opamp package, so it's not like there are extra traces running across the board to connect them. \$\endgroup\$ – markrages Nov 22 '10 at 20:21
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    \$\begingroup\$ @Thomas O We have students who like to think there boards are low speed and don't need to worry about signal quality, EMC problems, and routing. It is amazing how many of those same students run into weird issues that are extremely hard to debug and happen on random occurrences. As @kortuk was asking, your slew rate has a HUGE effect on the bandwidth of your board. \$\endgroup\$ – Kellenjb Nov 22 '10 at 21:49
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I find it hard to believe that wiring up little SC70 (or smaller) op amps is going to take up too much space on your board. Digikey has 5 pages of SC70 sized buffer amps that would probably do a good sight better job performance-wise than a common-collector BJT, and you could sprinkle them around the board as needed, meaning they'd take up no more space than that BJT.

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  • \$\begingroup\$ I need 6x SC70 packages then. Space is very constrained, I struggled to fit in the voltage divider resistors. Plus don't forget the traces, as I'm dealing with 2-layer. \$\endgroup\$ – Thomas O Nov 22 '10 at 13:04
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    \$\begingroup\$ @Thomas: What voltage divider resistors? For a buffer, you just short the inverting input to the output. \$\endgroup\$ – endolith Nov 22 '10 at 15:55
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    \$\begingroup\$ Think about a pair of quad op-amps. Each package needs one power and one ground connection. And the standard layout puts the output pin next to the (-) input. A gain-of-1 buffer would only save one pin per channel. \$\endgroup\$ – markrages Nov 22 '10 at 16:14
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    \$\begingroup\$ 6x SC70 for the buffers/opamps vs ... 6x SC70 for the transistors and 6x for the emitter resistors? I'm not sure why you think that using parts like this will be larger than the discrete solution. \$\endgroup\$ – akohlsmith Nov 22 '10 at 18:07
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    \$\begingroup\$ @Thomas: I wouldn't focus too much on the presumed big opamps. A quad opamp in TSSOP-14 package takes 32mm2 (5x6.4). I doubt you can do better with discrete components. \$\endgroup\$ – stevenvh Dec 5 '10 at 10:17
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One alternative is to build a common collector with a BJT or a common drain with JFET. It works almost as a buffer: high input impedance, low output impedance and almost a unity gain. Being a simple circuit you can implement without problem. JFET produces a better alternative to low input current needed.

Common Collector Common Drain

Common Collector and Common Drain

If you need more inputs/outputs you can use a common collector array chip such as CA3082. It will save you lots of space. Unfortunately I don't know a chip with common drain array built in.

Common Collector Array Chip

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    \$\begingroup\$ My favorite structure :), need to remember that it will not work with input down to gnd level (below 0.7V) and have gain slightly less than one. Also for higher input impedance you can replace npn win n-mos. \$\endgroup\$ – mazurnification Nov 22 '10 at 12:37
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    \$\begingroup\$ The problem with this configuration is that the BJT draws current (probably greater than the 500nA on the MCU inputs); also, the output will have an offset, reducing the dynamic range... \$\endgroup\$ – Thomas O Nov 22 '10 at 13:06
  • \$\begingroup\$ @Thomas O so you need a buffer that will load less then connecting direct to the MCU would? \$\endgroup\$ – Kellenjb Nov 22 '10 at 15:01
  • \$\begingroup\$ @kellenjb, Ideally, yes. \$\endgroup\$ – Thomas O Nov 22 '10 at 15:02
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You are looking for AD8244 that has 4 channels in a MSOP-10 package (4 inputs, 4 outputs and 2 for gnd and power supply). There are others like that but are more difficult to find. The good thing is power supply can go from 3-36Vdc, the bad thing is the only 3MHz bandwith that is good enough for standard microcontrollers ADC but not enough for other uses. You could use only one opamp but use an analog switch/multiplexer to switch from different sources like the ADG1606 that can switch from 16 inputs AD8244 analog buffer

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  • \$\begingroup\$ I doubt s/he's still looking. The question was asked nine years ago! Welcome to EE.SE. \$\endgroup\$ – Transistor May 18 '19 at 11:47

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