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OP-Amp disable

I have an existing device with low-noise differential amplifier at the front end. It uses two LT6200 op-amps in inverting configuration and has a gain of ten.

Occasionally, that's too much gain, and an attenuator is pugged in in front of the amplifier.

I want to make the gain electronically switchable without hurting the noise performance, so I want to avoid analog switches.

These op-amps have a disable pin. It seems that if I disable the op-amp then the signal goes straight through the input and feedback resistors and on to the next stage without gain. That's handy as I can control the disable pins with a spare uC output.

As long as the signal doesn't exceed the supply at the op-amp pins, that should work fine. Shouldn't it?

Cheers

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Syd
Reply to
Syd Rumpo
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Of course, the polarity will change, but that's no problem.

Cheers

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Syd
Reply to
Syd Rumpo

The 'disable' configuration inverts (or rather doesn't invert) the signal, right?

Reply to
Spehro Pefhany

p

To me, the "shutdown output leakage" spec (100nA, typ) says the output is safely tri-stated in shutdown.

It's not clear for the inputs-the internal schematic isn't detailed enough. You might try it--drive a shut-down part thru 10k, check for input clamping / distorted output.

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Cheers, 
James Arthur
Reply to
dagmargoodboat

A little relay?

What's the source resistance and feed back resistance?

George H.

p
Reply to
George Herold

Yes, that's firmware-fixable.

[Actually, with a gain of 1, you could make a low component count synchronous rectifier. For this chip, the shutdown turn off and on times are 180ns]

Cheers

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Syd
Reply to
Syd Rumpo

Harder to retrofit compared to a track cut and short piece of wire.

This uses 100R source, 1k feedback.

Cheers

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Syd
Reply to
Syd Rumpo

Except if you still want the good noise performance at unity gain. How about a nice relay?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

d

So when the opamp's disabled you've got ~1.1k of source imepdance...

That 'defeats' the nice low noise opamp... what's after the LT6200? Is it low noise too?

George H.

Reply to
George Herold

Noise at unity gain doesn't really matter. There are PGA stages after this which get wound right down when the input is high. Normally, the input is a low level from a very long cable, but a short cable is used for lab testing which then gives too high a level for the fixed gain front end.

The LTspice simulation works - often things like shutdown aren't modelled.

Cheers

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Syd
Reply to
Syd Rumpo

Interesting thought.

One could moderately light up a good LED with the bias current of that op-amp.. 50uA worst-case... wow. There is a heavy price to be paid for the high GBW and

Reply to
Spehro Pefhany

Noise at unity gain doesn't really matter. There are PGA stages after this which get wound right down when the input is high. Normally, the input is a low level from a very long cable, but a short cable is used for lab testing which then gives too high a level for the fixed gain front end.

Cheers

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Syd
Reply to
Syd Rumpo

Good spot, I'd clocked the supply rail diodes but hadn't noticed the diode pair across the inputs. That will kill it, damn.

Now where did I put that relay catalogue?

Cheers

--
Syd
Reply to
Syd Rumpo

Schematic? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Nah, Spehro's pointed out the flaw, namely the internal back-to-back diodes between the inputs :-(

Cheers

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Syd
Reply to
Syd Rumpo

Yep. I just opened the datasheet and saw the same thing. Bipolar doesn't do all that well in shutdown situations... the output is fine, but the input screws you over. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Is your configuration such that the +Input of the OpAmps go to ground, and you're driving thru a resistor to the -Input with a feedback resistor from Output to -Input? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Yes, except single supply, so +ve input to half supply instead of ground, and signal capacitor coupled to input resistor.

Cheers

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Syd
Reply to
Syd Rumpo

How about disconnecting the noninverting inputs - i.e. using an analog switch to connect it to ground 'as needed'?

Reply to
Frank Miles

Yes, Disconnect +Input from (virtual) ground, but also then connect

+Input to -Input to avoid any weird floating effects. ...Jim Thompson 
--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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