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OP-Amp disable

S

Syd Rumpo

Jan 1, 1970
0
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
 
S

Syd Rumpo

Jan 1, 1970
0
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?

Of course, the polarity will change, but that's no problem.

Cheers
 
S

Spehro Pefhany

Jan 1, 1970
0
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

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

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.
 
G

George Herold

Jan 1, 1970
0
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.

A little relay?

What's the source resistance and feed back resistance?

George H.
 
S

Syd Rumpo

Jan 1, 1970
0
The 'disable' configuration inverts (or rather doesn't invert) the
signal, right?

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
 
S

Syd Rumpo

Jan 1, 1970
0
A little relay?

Harder to retrofit compared to a track cut and short piece of wire.
What's the source resistance and feed back resistance?

This uses 100R source, 1k feedback.

<snip>

Cheers
 
G

George Herold

Jan 1, 1970
0
Harder to retrofit compared to a track cut and short piece of wire.




This uses 100R source, 1k feedback.
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.
 
S

Syd Rumpo

Jan 1, 1970
0
Except if you still want the good noise performance at unity gain. How
about a nice relay?

Cheers

Phil Hobbs

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
 
S

Spehro Pefhany

Jan 1, 1970
0
[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]

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 <1nV/sqrt(Hz) noise performance.

You might want to have a close look at figure 1-- the /SHDN pin
clearly is shown to affect (probably kill) the front end bias.. there
is also a back-to-back diode pair effectively across the inputs. The
latter could cause issues if the signal level is high enough.

http://cds.linear.com/docs/en/datasheet/62001ff.pdf
 
S

Syd Rumpo

Jan 1, 1970
0
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.

<cut and paste reply to Phil Hobbs>

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
 
S

Syd Rumpo

Jan 1, 1970
0
[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]

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 <1nV/sqrt(Hz) noise performance.

You might want to have a close look at figure 1-- the /SHDN pin
clearly is shown to affect (probably kill) the front end bias.. there
is also a back-to-back diode pair effectively across the inputs. The
latter could cause issues if the signal level is high enough.

http://cds.linear.com/docs/en/datasheet/62001ff.pdf
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
 
F

Frank Miles

Jan 1, 1970
0
Yes, Disconnect +Input from (virtual) ground, but also then connect
+Input to -Input to avoid any weird floating effects.

...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.
 
G

George Herold

Jan 1, 1970
0
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.

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.
 
F

Frank Miles

Jan 1, 1970
0
Yes. Except that resistor may "gain-up" the offset voltage, if too
small.

...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.
 
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