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production spread of opamp GBW

T

Terry Given

Jan 1, 1970
0
What level of GBW production spread should I assume for worst-case
testing? the opamp is a TLV274, GBW 3MHz or so.

Cheers
Terry
 
P

Pooh Bear

Jan 1, 1970
0
Terry said:
What level of GBW production spread should I assume for worst-case
testing? the opamp is a TLV274, GBW 3MHz or so.

I came across an entertaining production problem once where a batch of
bifet audio spec op-amps with a typical 4 MHz GBW had dropped to 3MHz or
thereabouts.

Caused the product to fail a specific test. We swapped the part out for
its second source cousin. The affected parts were fine elsewhere.

Graham
 
C

Chris Jones

Jan 1, 1970
0
Terry said:
What level of GBW production spread should I assume for worst-case
testing? the opamp is a TLV274, GBW 3MHz or so.

Cheers
Terry

I don't know what process that part is on, but on-chip resistors and
capacitors typically have +/- 20% or so tolerance for each type of
component. In practice the variation is likely to be much less unless they
switch to a different fab or shrink the die etc. Whether you would believe
that it is worth planning for a worst case variation in both R and C
simultaneously is up to you.

Chris Jones
 
J

John Larkin

Jan 1, 1970
0
What level of GBW production spread should I assume for worst-case
testing? the opamp is a TLV274, GBW 3MHz or so.

Cheers
Terry


I read somewhere recently (LTC appnote on thermocouple acquisition, I
think) that opamp gbw and open-loop gain can have bad tc's, so it's
best to way overkill on gain when precision matters.

John
 
T

Terry Given

Jan 1, 1970
0
John said:
I read somewhere recently (LTC appnote on thermocouple acquisition, I
think) that opamp gbw and open-loop gain can have bad tc's, so it's
best to way overkill on gain when precision matters.

John

I dont care about Aol, only GBW. Its easy to do the simulations, but I
still kind of need a guesstimate for GBW range. 2-6MHz is perhaps enough?

Cheers
Terry
 
P

Pooh Bear

Jan 1, 1970
0
Terry said:
I dont care about Aol, only GBW. Its easy to do the simulations, but I
still kind of need a guesstimate for GBW range. 2-6MHz is perhaps enough?

Easily I would say. +/- 1MHz would be my guess.

Nice to have that data though. Just been using an Infineon CoolMOS part and
the data sheet shows both typical Ron and another 'worst case' curve for 98%
of all devices. They don't say how bad the other 2% are though !

Graham
 
T

Terry Given

Jan 1, 1970
0
Pooh said:
Terry Given wrote:




Easily I would say. +/- 1MHz would be my guess.

Nice to have that data though. Just been using an Infineon CoolMOS part and
the data sheet shows both typical Ron and another 'worst case' curve for 98%
of all devices. They don't say how bad the other 2% are though !

Graham

FWIW I was once privy to "real" Hitachi electrolytic cap data. The
lifetime figure they quoted on the datasheet was 3 sigma *below* the
production mean, which was IIRC twice rated.

Cheers
Terry
 
J

Jim Thompson

Jan 1, 1970
0
What level of GBW production spread should I assume for worst-case
testing? the opamp is a TLV274, GBW 3MHz or so.

Cheers
Terry

The compensation corner is sufficiently below the transistor effects
that they can be ignored.

Resistors are typically +/- 20%

Capacitors are typically +/- 15%

GBW is proportional to 1/RC

So you're looking at 0.725x < GBW < 1.47x

BUT, The data sheet will usually specify a minimum, so max GBW would
be ~2x the specification minimum.

(TC's ignored, but typically inconsequential.)

...Jim Thompson
 
T

Terry Given

Jan 1, 1970
0
Jim said:
The compensation corner is sufficiently below the transistor effects
that they can be ignored.

Resistors are typically +/- 20%

Capacitors are typically +/- 15%

GBW is proportional to 1/RC

So you're looking at 0.725x < GBW < 1.47x

BUT, The data sheet will usually specify a minimum, so max GBW would
be ~2x the specification minimum.

(TC's ignored, but typically inconsequential.)

...Jim Thompson

Hi Jim,

thanks for that - exactly what I/m looking for. Archived with thanks :)

Cheers
Terry
 
K

Kevin Aylward

Jan 1, 1970
0
Terry said:
Hi Jim,

thanks for that - exactly what I/m looking for. Archived with thanks
:)

Note that GBW is usually determined by:

Av=gm.Xc

where, gm is 40Ic, Xc is the capaciter reactance. Gm is not a resister.

However, this current is ultimately set by a resister, somewhere.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
 
W

Winfield Hill

Jan 1, 1970
0
Kevin Aylward wrote...
Note that GBW is usually determined by: Av=gm.Xc where,
gm is 40Ic, Xc is the capaciter reactance. Gm is not a resister.
However, this current is ultimately set by a resister, somewhere.

Jim's simplification, GBW proportional to 1/RC, still holds.
 
J

Jim Thompson

Jan 1, 1970
0
Kevin Aylward wrote...

"Somewhere" = the bias setup. The better designs have the tail
current set by a PTAT, keeping gm relatively flat.
Jim's simplification, GBW proportional to 1/RC, still holds.


...Jim Thompson
 
K

Kevin Aylward

Jan 1, 1970
0
Winfield said:
Kevin Aylward wrote...

Jim's simplification, GBW proportional to 1/RC, still holds.

Yes, but thats what I meant by "this current is ultimately set by a
resister".

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
 
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