1. Home
  2. Electronics Design
  3. OP-Amp disable
  4. Page 2

OP-Amp disable

And also use the \SHUTDN\ pin to tri-state the OpAmp Output. ...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
Loading thread data ...

That's worth a try, thanks.

Cheers

--
Syd
Reply to
Syd Rumpo

I've done games like that on some of my custom chip designs.

My favorite stunt to change gains without the (variable) resistance of the analog switch getting into the act...

formatting link

Sometimes I've been known to have multiple diff-pairs to do such a task, controlling via turning their current sources on and off, like I did in ancient times...

formatting link
...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

Or use a resistor from the input {it must have been low impedance from your other comments} to the noninverting input, avoiding a second analog switch.

Reply to
Frank Miles

Yes. Except that resistor may "gain-up" the offset voltage, if too small. ...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

The ADA4899 is similar, but has 100-nA bias current typical, 1 uA max. A fave of mine.

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 USA 
+1 845 480 2058 

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

    ...Jim Thompson

That's what I was thinking. 100k should be enough to stop the input flapping around. Say, if used as a +1/-1 gain stage for a lockin, I don't really care about a 1% |gain| error, do I?

Or, to say in another way, if I had a +1/0 amp, it'd still work fine in a lockin, I'd just give up 1/2 the signal.

George H.

Reply to
George Herold

t the

on and

n in

the

he

ck

as I

s,

          ...Jim Thompson
k

og

      ...Jim Thompson
       ...Jim Thompson

Hmm, scratches head... The input is through R5, I assume.

George H.

    ...Jim Thompson
      |    mens     |
    |     et      |
 |
      |

ide quoted text -

Reply to
George Herold

the

For Syd's application, yes.

Pick your druthers. I inadvertently grounded both possible inputs in my haste to make a drawing ;-)

[snip] ...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

If this for an opamp that is turned off - don't see how. Either the amp is OFF, and the NI input is at the same potential as the Inv input (and small input offsets shouldn't matter) _OR_ the amp is ON, the NI input is grounded via analog switch - so the resistor doesn't matter. Of course this assumes that the Ron*Coff product is low enough to justify these assumptions.

The other technique for switching between inverting and noninverting uses the technique that you describe, akin to the well-known 1-pole all-pass configuration. A SPDT analog switch would be ideal for this, either connecting the NI input to the overall input (+1 gain) or ground (gain=- Rf/Ri). I hadn't thought of that before - it could well be better than the 'shutdown' method as it would result in reduced offset changes and maintain impedance buffering, not to mention avoiding using an opamp in ways not fully specified on the data sheet.

Reply to
Frank Miles

You missed my point. Resistor is to "drag" +Input tracking -Input when OpAmp is OFF.

When OpAmp is ON, the resistor is still there and offset will be "gained". (Null is NOT zero, it's the offset voltage.)

Good point. (I've also used that scheme as a synchronous detector/rectifier.) ...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

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.