Voltage Gain Switch (Design Help)

(snip)

What's wrong with the relays? For that job, relays aren't a dumb idea at all. The analogue switches will have some nonlinearity and some temperature coefficient of resistance, whereas relays are nearly perfect--their only drawbacks are cost, which you don't care about for a one-off, and slow switching speed, which you also don't care about.

Cheers,

Phil Hobbs

In message , dated Thu, 14 Sep 2006, snipped-for-privacy@gmail.com writes

That data is CRUCIAL for us to recommend a solution. 5000 is too much to get reliably from one op-amp, even at 200 Hz.

It's probable that you can put the switching elsewhere than at the +/-10 V level, so you can use an ordinary switch.

I've given you a new idea, but we need to know the exact gains you need.

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of 2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

What is the minimum gain required?

As others have said, 5000 is too much for a single opamp, so maybe a fixed gain preamplifer, at the lowest gain may be adviseable

martin

Phil Hobbs ha escrito:

Yes you are right, but my task was to convert the circuit to solid-state. It`s kind of a test. There's no way around that, plus there's those old-fashioned switching noises, the bulkiness,ect. Although I admire the ease of use of the relays :P

John Woodgate ha escrito:

The Gains are: 100, 500, 1000 and 5000 The other circuit has a pre-amplifier with a gain of 100, and the the other four amplifiers with gains of 1, 5, 10 and 50 respectively. It uses standard Op-Amps I don`t remember which model, all I know is that there are no calculation's done about them, except for the negative-feedback gain.

The overall output of the circuit must be able to swing to +-10V, it was a requirement, I haven`t talked to my "Boss" about it, and probably won`t until Monday. The input is from an LVPD transducer, I don`t know much about those.

Let me know if I missed something else.

Thanks.

yep

Relays can be very small, sealed, and offer much better performance generally than fet type switches, but not as fast, and many companies make them

martin

In message , dated Thu, 14 Sep 2006, snipped-for-privacy@gmail.com writes

Doing it that way, no input signal must overload your high-gain preamplifier. I would reduce that gain to 20, and follow it with four op-amps as you have already. Then I would finish with an output stage that has a gain of 5 and will produce +/- 10 V output. That way, your switch never has to see more than 2 V.

That uses one more op-amp, but they are cheap. Almost all 'cooking' op-amps will produce +/-10 V up to 200 Hz if you have +/- 15 V supplies (maybe less) and a load on the output greater than 1 kohm. But avoid LM324 and LM358, which need subtle treatment that will bother you.

>

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of 2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

martin griffith ha escrito:

If you tell me that I can find those here in Argentina at an affordable cost, you may have a point. Still I would have trouble convincing my "Boss". It`s not a solution, but an alternative if no solution is to be found, which I find hard to believe.

By the way they told they bought some instrumentation Amplifiers to replace some old Amps they had (similar requirements), but they are expensive (I`m talking about a working circuit not an IC). So yes it can be done, and the cost is an issue.

Dont have a clue about Argentina's electronic distribution network, what about online suppliers?

and ,yes, Cost is usually an issue

martin

Ah, okay, a job security issue, gotcha. We can all understand that, except for the old retired farts cluttering up the place. ;)

I'd probably start with a gain-of-100 preamp, followed by a high impedance passive attenuator, followed by a gain-of-50 output stage. The input errors of the second stage won't be a problem, because the net gain from the preamp input to the output of the attenuator will always be at least 2. For an instrument, you probably want a nice quiet JFET amp like an OPA656, or if your impedance levels are low, a LT1028 bipolar.

The voltage swing at the output of the attenuator will never be more than 0.2V, which makes the linearity problem easier to deal with. There'll be no bandwidth change with gain, because both amps are running at constant gain.

I might use a voltage divider something like this:

0-----RRRRRRR------*--------*-------*-------0 | | | 30k R R R R R R 7k50 R 3.33k R 610 R R R R R R R | | | 0 0 0 2N7002 2N7002 2N7002

gnd gnd gnd

You could probably get away with some DMOS peripheral driver chip for all 3 switches, because none of them will ever get further than 0.2V from ground in normal operation, so none of the protection diodes will turn on. The 2N7002 gets down to 2 or 3 ohms on resistance, with some very small nonlinearity at the current levels we're talking about. Their capacitances are in the 100-pF range at low voltage, giving a 3 dB frequency of something like 18 kHz for three of them.

Cheers,

Phil Hobbs

Leo,

The thing you need is an analog switch and op-amps that can handle

+/-10 volt swings as I read it.

I have used an Analog Devices, others make it, which is old, over 15 years, but still avialable. It is a ADG509 which is a dual 4 to 1 analog switch controlled with binary 2 bit TTL logic inputs. It can be powered from +/- 15 V.

You would use with gain op-amp with the inputs having a resistor/pot (pot to set exact gain) combinations driving neg input. Will need to drive inverting buffer so output follows input. Could use with voltage divider combination, one on each input and one on output of switch and drive the + input of op-amp. Select the voltage divider combinations to set the gain.

For +/- 10 volts will need to power all analog on +/-12 or more.

You could also use op-amp with this same analog switch selecting different feed back resistors for the op-amp to adjust gain.

The ADG509 is wide band DC to over 5 MHz. Have used in video switchers also.

Good luck to you and your co-worker about to graduate with EE degree. I still remember the butter flys when I graduated in 1975.

73, ron, n9ee/r

You would drive mart> >

Oops. There *is* a bandwidth change with gain, because as soon as you turn on one of the switches, the capacitance of the others ceases to matter so much, and the bandwidth goes way up. Add a capacitor in parallel with the feedback resistor of the second stage to drop the bandwidth down to 1 kHz or so.

Cheers,

Phil Hobbs

Be careful, Phil. "The old farts" hold the long term lease for this newsgroup.

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Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

The LT1028 has been a favourite of mine for a very long time, but the more recent Analog Devices AD797 is a little bit nicer.

The main difference seems to be in the output stage, which is a bit less cranky than the LT1028's.

--
Bill Sloman, Nijmegen

Nice part, thanks.

Cheers,

Phil Hobbs

Use a high precision differential amplifier whose gain can be controlled with a single resistor (0.1% or better depending on your requirements). Use relays to switch the gain resistor. The AD620 is a good starting point:

datasheet:

The AD620 can do gains up to 10000, and should be able to do 200Hz at a gain of 5000, but I don't think there is a Gain vs Freq curve above

1000 if memory serves me correctly.

Of course you'll have to watch your error budget very closely at such high gains.

Dave :)

Phil Hobbs ha escrito:

I`m going to answer the designs one by one.

About this one, I understood the general idea, which I believe is to switch gains through the MOSFETs. But, I have no idea about the second stage, the attenuator. First the signal gets amplified then attenuated, the overall gain of the two stages being at least 2. Then it goes to the third stage which has a fixed gain of 50 ? Which of the stages has the selectable gain (voltage divider)? You probably should mention a standard DMOS driver because I have never used one before.

It`s unlikely that I will use this design, but I am interested in how it works, seems rather unconventional :)

LT ha escrito:

I`ve seen the ADG509 for sale here in Argentina, but I still don`t know the cost. Seems like the way to go if it is affordable.

About the design, the guy before had the idea of switching the feedback resistor, but my co-worker (the guy who's in charge of the department, meaning he gets paid and I don`t) argued that the on resistance of the switch would interfere with the presicion of the gain, but now that if think about it, given a device it can be adjusted by trimpots only once and should remain stable enough, but don`t know about that.

Thanks for the luck, I think I`ll be needing it much longer than he does since I`ve just started and he's about to graduate ;)

David L. Jones ha escrito:

Thanks Dave, the high-precision amplifier is a good idea, but I still have that question about the feedback resistor switching.

John Woodgate ha escrito:

The Op-Amps I`m gonna stick with are OP07s, unless there is a major problem with them. I like the idea that the switch doesn`t have to be the final stage, somehow I took that idea off my mind. Still I have a problem using 4051s or 4066 because none of those would allow +-12V power supplys, so I will have to lower the voltage to power them, I will probably use some zenner diode.