Voltage Gain Switch (Design Help)

Well, unconventional in the sense of being a minor variant of the one used in every oscilloscope in the universe, at least until recently....

You're right about the nonlinearity and on-resistance problems with switches--which is a a strong reason to keep the amplifier gains fixed, and do the gain switching at low-to-moderate signal levels. There are also pickup and noise peaking problems with hanging all sorts of gingerbread on summing junctions. Your predecessor used relays to switch fixed-gain amplifiers in and out, which isn't a bad design for a one-off.

What's so hard to understand about the design I posted? You need a gain of 100, 500, 1000, or 5000. Taking out a common factor of 100 for the first stage gain means that the amplifier noise, offset, and drift behaviour will basically depend only on the first stage. This is an important point in instrument design.

A passive attenuator using big honking MOSFETs (2 or 3 ohms ON resistance) gets rid of the switch nonlinearity completely. The ones I show cost 20 cents each, and are available everywhere including Antarctica. Putting the switched attenuator between the two amps means that it can run at the same narrow range of signals at all times, which makes doubly sure that the MOSFETs are not going to cause a linearity problem. That's actually one of the nicer features of this circuit. Choosing a second-stage gain of 50 means a gain of 1.00 for the attenuator's highest gain setting, which is easy to do accurately.

The output amp, with a gain of 50, gets you back up from +-200 mV to +-

Once again, this design doesn't cause excess noise, offset, or drift, because the overall gain up to the attenuator output is always at least

1. This is simple to prove--about 3 lines of algebra. It's simple, cheap, made from jellybean parts, and will work very well. Other than that, it stinks.

Cheers,

Phil Hobbs

In article , Winfield Hill wrote: [....]

His frequency is low so a CD4051, 2 or 3 could be used. If the amplifier is inverting, the swing at the gain switch will be smaller than the output of the op-amp. This makes your suggestion even better.

ASCII art:

R1 R2 R3 ---/\\/\\/--+--/\\/\\--+------\\/\\/---------- ! ! ! ! ---O S1 U1 ! ! --- GND--!+/

U1 is some op-amp S1 is a section of 4053

You can use more resistors in series and a 4052 or 4051 to get more gains.

In article , wrote: [....]

They draw so little current that even a resistor divider can be a good way to do it.

Use three terminal regulators - much better output impedance than zener diodes, less wasted current, and you can usually get the voltage you want, or set it up with an LM317 (or any of its many variants).

The OP-07 is a very nice amplifier, but slow, and doesn't give all that much gain.

The ua725 and PMI's OP-06 had DC gains in excess of 120dB but both parts seem to be obsolete.

If you can't use tony reed relays, then as I think someone else has said probably best to have a fixed gain precision diff amp permanately on the front end, and then a seperate precision amp for each gain you want following that. Then you can use any crude solid state switching device you want on the outputs of the amps.

Dave :)

Winfield Hill ha escrito:

Ok, yes the switch doesn`t need to be able to handle de +-10V output, I`m relocating it to a lower voltage part of the circuit. I`m confused though, you said the gain switch located at the input of the 2nd stage and then you said the switch should be used for switching the feedback attenuators. So should I use the switch to change the feedback attenuators or change the final amplifying stage ?

Phil Hobbs ha escrito:

Well, I understand now, and I must say I really like the idea. It`s kind of unconventional in the sense that you are the only one among the one's who replied that proposed this idea instead of the analog switch or relays ;)

The only problem is that the control signal comes in binary (from a PIC), so I will have to use a decoder to drive the MOSFETs accordingly. That adds another IC to the circuit, yet another thing that can go wrong (but very unlikely). The decoder pops a question: what VGS voltage shoul I use ?. From what I gather, more VGS voltage better linearity and for a wider range of VDS. I was thinking about VGS=10V, but I will need to raise up the voltage coming from the PIC with a BJT, so I think VGS=5V is fine, the VDS voltage is already low enough so it doesn`t matter.

Ken Smith ha escrito:

I must say that I can`t understand the schematic, the connections of the Op-Amp and the switch are all garbled for me. But if I understood correctly your idea doesn`t stray too much from Win's.

snipped-for-privacy@ieee.org ha escrito:

I`m not sure about it, but is an LM317 really neccesary ? I thought Ken's idea of a voltage divider was great :P

David L. Jones ha escrito:

Yep, I`ll use OP-07s, they are cheap and have everything I need, I think. I noted the suggestion of using a faster Op-amp, but it`s unneccesary as the input signal is very slow. Maybe I`ll switch to an OP-27 that is a bit better than the 07.

I'd probably want to use CD4000 series logic, running from whatever your analog supply is (+12 or +15, probably).

There are only 4 gain settings, so it ought to be possible to do this with a few gates.

The gains are not in a binary sequence, so it isn't instantly obvious how they're encoded---but one natural guess would be

A1 A0 Gain G2 G1 G0 0 0 100 1 0 0 0 1 500 0 1 0 1 0 1000 0 0 1 1 1 5000 0 0 0

where we assume positive logic, and calling the gates of the MOSFETs G0, G1, G2 for /5, /10, and /50,

G0 = ~A0 & A1 = ~( A0 + ~A1) G1 = A0 & ~A1 = ~(~A0 + A1) G2 = ~AO & ~A1 = ~( A0 + A1)

The obvious solution needs two CD4002 quad 2-input NOR gate packages, or one CD4002 and a couple of spare inverters. You'll want to use a really slow RC lowpass between the logic and the MOSFETs to make sure you don't couple anything off the supply rail into the signal path. If you're imaginative about it, you can probably do it with one gate package, but this is left as an exercise for the reader, along with the 5V->10V level shifters you'll need. Alternatively you can run the gates at 5V, but don't run them from the logic supply, or you'll be wishing for those nicce well-behaved relays.

Cheers,

Phil Hobbs

A voltage divider can sometimes do the job, with decent by-pass capacitors, but mostly a three-terminal regulator does a better job. The LM317 and LM337 would be an over-kill in most cases - fixed regulators in TO-92 and surface mount packages are available off the shelf for all of the voltages thst you are likely to lead.

You could improve on Phil Hobb's 2N7002 switches with SD214 parts - the "on" resistance isn't as good - at 50R to 75R - but the capacitances are a lot lower.

It does have a lower input noise voltage, but this is only relevant for low impedance sources.

The LT1028 data sheet has a useful tutorial section in the application notes

on page 10 of the pdf. Your OP-07 corresponds to the LT1001. and the OP-27 to the LT1007.

Change the font setting of your news reader. They any fixed spacing font.

My idea is almost entirely Win's. What I was pointing out is that the swing at the switch is smaller than the swing at the op-amps output.

Nice summing up!

robert

Phil Hobbs ha escrito:

I was thinking of maybe using CD4555 Binary to 1 of 4 Decoder instead of simple gates (which might mean more ICs unless I complicate the design for this one-off). I need to connect 3 outputs only, plus the inputs from the PIC. The CD 4002 are actually Dual 4-Input NORs.

I could drive the Gate directly from the 4555's output at 5V, it says it can drive or sink up to 6.8mA, more than enough I guess, but I do not know why you say that's not advisable. If not I would use a PNP transistor to drive the Gate with higher voltage, but I will need to invert the logic from the 4555, or use 4556 Low on select.

I'm interested in knowing what kind RC lowpass filter I will need, I do not have any experience with this kind of coupling, I just assume they are rather isolated parts of the circuit.

Thanks.

snipped-for-privacy@ieee.org ha escrito:

2. > > > > > John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

Ok, I will try to use a voltage regulator, one small and cheap, it`s a rather light load we are talking about. Any suggestions beside the LM317 ?

I do not need to worry about bandwidth for the most part so I don`t think the capacitance is an issue for the moment. I'd rather stick with the low rds ON.

snipped-for-privacy@ieee.org ha escrito:

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Thanks for the Heads-Up on the tutorial, now it's taken into account.

Ken Smith ha escrito:

Changing the font did the trick.

I found this Application Note on signal switching/multiplexing from Texas Instruments: