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Choosing Op Amps for DC Servo

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saint gillis

Member
Brussels - Belgium

Posts:
170

Choosing Op Amps for DC Servo

« on: June 09, 2014, 02:55:30 PM »

What  are the factors to consider
for a good Op Amp to use as DC Servo, input offset voltage I guess, slew
rate?

- SSL used TL052, 0,6mV input offset (0,35mV for TL052AC),
quite fast, slew rate 20V/µs

- John Hardy in the M1/M2 uses OP97,
very low input offset 0,02mV max (+ low t° drift), slow, slew rate
0,2V/µs

- Fred Forsell uses OPA604, 1mV input offset (typ, 5mV
max), fast, slew rate 25V/µs

Each design is different, I guess for
some reasons. Do we need a very fast Op Amp for DC Servo, DC offset is not
something that is always changing, it is more like a constant factor that we
need to correct right?

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VictorQ

Member

Posts:
52

Re: Choosing Op Amps for DC Servo

« Reply #1 on: June 09, 2014, 03:59:18
PM »

Quote
from: saint gillis on June 09, 2014, 02:55:30 PM

Do we need a very fast Op Amp
for DC Servo, DC offset is not something that is always changing, it is more
like a constant factor that we need to correct right?

Right.

I suspect that the
most desirable characteristic is plentiful supply... Offset voltage and input
bias current shouldn't be too high, though, since they show up as residual
offset.

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JohnRoberts

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Hickory, MS

Posts:
8911

Re: Choosing Op Amps for DC Servo

« Reply #2 on: June 09, 2014, 04:43:01
PM »

This has been discussed a lot and I would
like to add a subtle distinction or qualification to explain a topology
dependent factor.

There are more than one way to implement a servo and
that will affect the non-obvious factors. The obvious ones are things like DC
offset, input noise, DC stability, etc.  Less obvious are the AC
performance (slew rate, gain bandwidth, etc).  If the input, and output of
the servo opamp are LPF by passive real poles, the opamp does not need to be
very fast. If the input, and/or output is exposed to wide-band signals, the OA
needs to be fast enough to handle those without causing artifacts or errors.

The NI servo topology generally involves a passive LPF in it's input so
that path is rise-time limited. Where the output connects matters too but it is
sometimes possible to embed a passive real pole in series with the output too
making the AC demands for the opamp much relaxed vs, non rate-limited
connections.

JR

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saint gillis

Member
Brussels - Belgium

Posts:
170

Re: Choosing Op Amps for DC Servo

« Reply #3 on: June 09, 2014, 05:41:58
PM »

Quote
from: JohnRoberts on June 09, 2014, 04:43:01 PM

This has been discussed a lot and I
would like to add a subtle distinction or qualification to explain a topology
dependent factor.

There are more than one way to implement a servo and
that will affect the non-obvious factors. The obvious ones are things like DC
offset, input noise, DC stability, etc.  Less obvious are the AC
performance (slew rate, gain bandwidth, etc).  If the input, and output
of the servo opamp are LPF by passive real poles, the opamp does not need to
be very fast. If the input, and/or output is exposed to wide-band signals, the
OA needs to be fast enough to handle those without causing artifacts or
errors.

Great! now seems obvious...

Quote

The NI servo topology generally
involves a passive LPF in it's input so that path is rise-time limited.

What is "NI servo topology"?

Quote

Where the output connects matters too
but it is sometimes possible to embed a passive real pole in series with the
output too making the AC demands for the opamp much relaxed vs, non
rate-limited connections.

I suppose you are talking about the
servo's output... I m sorry I don't really understand what is the AC demands for
the opamp...

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JohnRoberts

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8911

Re: Choosing Op Amps for DC Servo

« Reply #4 on: June 09, 2014, 06:48:30
PM »

Quote
from: saint gillis on June 09, 2014, 05:41:58 PM

What is "NI servo topology"?

Non-inverting.

Quote

I suppose you are talking about the
servo's output... I m sorry I don't really understand what is the AC demands
for the opamp...

Opamps must respond to counter act
signals coming in their output. Negative feedback will define what the output
voltage "should" be, but significant current coming into the output must be
opposed by the amplifier. In the extreme this can cause errors. My point is that
a slow opamp will have difficulty resisting fast signal currents at their
output.

opamps can also be sensitive to AC signals on their PS pins too
but that is not any different for servos vs other circuitry.

JR

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saint gillis

Member
Brussels - Belgium

Posts:
170

Re: Choosing Op Amps for DC Servo

« Reply #5 on: June 09, 2014, 08:26:47
PM »

Quote
from: JohnRoberts on June 09, 2014, 06:48:30 PM

Opamps must respond to counter act
signals coming in their output. Negative feedback will define what the output
voltage "should" be, but significant current coming into the output must be
opposed by the amplifier. In the extreme this can cause errors. My point is
that a slow opamp will have difficulty resisting fast signal currents at their
output.

Interesting! Is this the reason why we
find a quite big resistor at the servo's output when slow Op Amps are used, and
quite smaller resistor when faster Op Amps?

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JohnRoberts

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Re: Choosing Op Amps for DC Servo

« Reply #6 on: June 09, 2014, 09:04:44
PM »

Quote
from: saint gillis on June 09, 2014, 08:26:47 PM

Quote
from: JohnRoberts on June 09, 2014, 06:48:30 PM

Opamps must respond to counter act
signals coming in their output. Negative feedback will define what the
output voltage "should" be, but significant current coming into the output
must be opposed by the amplifier. In the extreme this can cause errors. My
point is that a slow opamp will have difficulty resisting fast signal
currents at their output.

Interesting! Is this the reason why we
find a quite big resistor at the servo's output when slow Op Amps are used,
and quite smaller resistor when faster Op Amps?

Probably not... the resistor in
series with output is more related to the circuit it connects to, to establish a
reasonable servo range.

JR

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joaquins

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Argentina

Posts:
1135

Re: Choosing Op Amps for DC Servo

« Reply #7 on: June 09, 2014, 09:08:11
PM »

The DC offset could be managed because you
ussualy use an attenuator at the output, because you don't need such a wide
range, normally with a couple of mV you are fine there, in that case you have
the offset error divided by the atten, and when you use the john hardy/jensen
aproach with an opamp with such a low offset less than 200µV for sure and a
200k/5k (worst case) divider you get less than 5µV for the final error wich
seems so low than calibration only could get things worse, unless you could
measure 1µV with confidence.

Slew rate may behave as was told before,
they use the NI aproach, I've used the inverting aproach with OP27 (2.8V/µs,
<100µV offset) and for some reason worked much better than NI, I couldn't get
the NI to work properly, the inverting at first try. Never knew why, seems as
some stability problem...

JS

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Pip

Member
New York City

Posts:
255

Re: Choosing Op Amps for DC Servo

« Reply #8 on: June 09, 2014, 09:40:23
PM »

See if this helps. In this microphone example
which is fast slew rate stuff they used TLO62, the TLO72 is probably the next
better choice for this type of use. But as others pointed to the use of the
servo is directly tied to the purpose of the circuit.

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City

Samuel
Groner

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Zürich, Switzerland

Posts:
2841

Re: Choosing Op Amps for DC Servo

« Reply #9 on: June 10, 2014, 02:35:28
AM »

Quote
from: JohnRoberts on June 09, 2014, 04:43:01 PM

Less obvious are the AC performance
(slew rate, gain bandwidth, etc).  If the input, and output of the servo
opamp are LPF by passive real poles, the opamp does not need to be very fast.
If the input, and/or output is exposed to wide-band signals, the OA needs to
be fast enough to handle those without causing artifacts or errors.

Even in the case of the pole being
an "active" one (i.e. inverting servo), the servo opamp does not see significant
voltage rate-of-change at its output. Its output does have to source/sink audio
rate currents, but there is (to my best knowledge) no correspondence to voltage
slew-rate limiting in the output current behaviour in common opamps. Also the
typical magnitude of the AC current is in the low uA region, so probably not
very significant (lower than the quiscent current of the output stage, so class
A-like low distortion behaviour can be expected).

But surely your advice
to carefully look at the AC behaviour is very
valid!

Samuel

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abbey road d enfer

Global Moderator

Member
Marcelland

Posts:
4353

Re: Choosing Op Amps for DC Servo

« Reply #10 on: June 10, 2014, 07:14:54
AM »

Quote
from: joaquins on June 09, 2014, 09:08:11 PM

The DC offset could be managed because
you ussualy use an attenuator at the output, because you don't need such a
wide range, normally with a couple of mV you are fine there, in that case you
have the offset error divided by the atten,

Are you sure? Seems to me like,
whatever the DC loop, the input offset of the servo-opamp is the absolute
limit.

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joaquins

Member
Argentina

Posts:
1135

Re: Choosing Op Amps for DC Servo

« Reply #11 on: June 10, 2014, 07:52:07
AM »

Quote
from: abbey road d enfer on June 10, 2014, 07:14:54 AM

Quote
from: joaquins on June 09, 2014, 09:08:11 PM

The DC offset could be managed
because you ussualy use an attenuator at the output, because you don't need
such a wide range, normally with a couple of mV you are fine there, in that
case you have the offset error divided by the atten,

Are you sure? Seems to me like,
whatever the DC loop, the input offset of the servo-opamp is the absolute
limit.

Yes, maybe I'm speakin s**t...
tonight I do de math and get mind peace.

JS

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JohnRoberts

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Hickory, MS

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8911

Re: Choosing Op Amps for DC Servo

« Reply #12 on: June 10, 2014, 10:21:32
AM »

Quote
from: Samuel Groner on June 10, 2014, 02:35:28 AM

Quote
from: JohnRoberts on June 09, 2014, 04:43:01 PM

Less obvious are the AC performance
(slew rate, gain bandwidth, etc).  If the input, and output of the
servo opamp are LPF by passive real poles, the opamp does not need to be
very fast. If the input, and/or output is exposed to wide-band signals, the
OA needs to be fast enough to handle those without causing artifacts or
errors.

Even in the case of the pole being
an "active" one (i.e. inverting servo), the servo opamp does not see
significant voltage rate-of-change at its output.

Agreed

Quote

Its output does have to source/sink
audio rate currents,

If the output of the servo connects to a
wide band circuit node, like connected to the outside world, that path could
contain RF energy.

Quote

but there is (to my best knowledge) no
correspondence to voltage slew-rate limiting in the output current behaviour
in common opamps. Also the typical magnitude of the AC current is in the low
uA region, so probably not very significant (lower than the quiscent current
of the output stage, so class A-like low distortion behaviour can be
expected).

I realize this borders on over-design.
At the point you experience a serious HF signal (current) at the output of the
servo the behavior of the rest of the circuit is probably a larger concern.

Quote

But surely your advice to carefully
look at the AC behaviour is very valid!

Samuel

I prefaced my original post that
this is subtle but whenever an opamp has AC current driving back into it's
output the forward AC performance matters. As you know an opamps output
impedance is a function of the open loop output impedance, negative feedback,
and available forward gain. If the opamp is very slow and lacks adequate gain at
high frequency that will compromise the ability of the negative feedback to
lower output impedance at high frequency.

When looking at DC
servos there can be a temptation to underestimate the demands on the active
circuitry.  With the remedy of passive LPF coming and going, the opamp
never sees HF so can be selected based on optimal DC performance alone.

Note: for many DC servo applications the edge rates or rise time of the
signals seen by the servo opamp are already band limited by the rest of the
audio path so this is not really a concern.

JR

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saint gillis

Member
Brussels - Belgium

Posts:
170

Re: Choosing Op Amps for DC Servo

« Reply #13 on: June 12, 2014, 06:41:40
AM »

Quote
from: JohnRoberts on June 10, 2014, 10:21:32 AM

When looking at DC servos there can be
a temptation to underestimate the demands on the active circuitry.  With
the remedy of passive LPF coming and going, the opamp never sees HF so can be
selected based on optimal DC performance alone.

So if we take the care of adding
an output LPF it doesn't need to be as low as the servo's input LPF (RF
precaution)  ?

Now let's take this exemple (but could have
taken any other)
http://www.johnhardyco.com/Schematics/MPC-1_Schematic.jpg

I have a naive novice question: when we are in high gain, -in of the 990C at 20r
from the ground, isn't the servo's output going through a 200k/20r voltage
divider and being divided by a factor of 10000? I'm sure it is here my lack of
understanding Op Amps...

I also have a last question, why
R13 and R14 have to be the same value?

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abbey road d enfer

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Posts:
4353

Re: Choosing Op Amps for DC Servo

« Reply #14 on: June 12, 2014, 07:16:49
AM »

Quote
from: saint gillis on June 12, 2014, 06:41:40 AM

Quote
from: JohnRoberts on June 10, 2014, 10:21:32 AM

When looking at DC servos there can
be a temptation to underestimate the demands on the active circuitry.
With the remedy of passive LPF coming and going, the opamp never sees HF so
can be selected based on optimal DC performance alone.

So if we take the care of
adding an output LPF it doesn't need to be as low as the servo's input LPF (RF
precaution)  ?

Two different constraints.
The
input LPF governs the LF response of the whole circuit, so needs to be <20Hz
for audio applications.
The output LPF is used to get rid of any HF noise in
the servo opamp to reach the main path. Could be chosen as low as 20Hz, but
generally higher because of the size of the components - and stability issues
(having two poles in the feedback loop may introduce unstability.

Quote

Now let's take this exemple (but could
have taken any other)
http://www.johnhardyco.com/Schematics/MPC-1_Schematic.jpg

I have a naive novice question: when we are in high gain, -in of the 990C at
20r from the ground, isn't the servo's output going through a 200k/20r voltage
divider and being divided by a factor of 10000?

Not naïve; you are absolutely right.
There may be cases where the servo opamp hits one of the rails.

Quote

I also have a last
question, why R13 and R14 have to be the same value?

In order to minimize the effects of
bias current; DC voltage developped in the - and + inputs must be
balanced.

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Who's right or wrong is
irrelevant. What matters is what's right or wrong.
Star
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Samuel
Groner

Global Moderator

Member
Zürich, Switzerland

Posts:
2841

Re: Choosing Op Amps for DC Servo

« Reply #15 on: June 12, 2014, 07:40:10
AM »

Quote
from: saint gillis on June 12, 2014, 06:41:40 AM

I have a naive novice question: when we
are in high gain, -in of the 990C at 20r from the ground, isn't the servo's
output going through a 200k/20r voltage divider and being divided by a factor
of 10000?

Correct (as noted already), but the
correction signal of the DC servo is very small. If the offset of the 990 opamp
is 100 uV, the required DC voltage at the servo opamp is still just 1
V.

If the offset and/or bias current of the main opamp is not trimmed,
the required correction signal will be 10x to 100x higher, and we're in
trouble.

Quote
from: abbey road d enfer on June 12, 2014, 07:16:49 AM

Quote
from: saint gillis on June 12, 2014, 06:41:40 AM

I also have a last question, why R13
and R14 have to be the same value?

In order to minimize the effects of
bias current; DC voltage developped in the - and + inputs must be
balanced.

The primary requirement is that the
time constants R13*C5 and R14*C6 match (or the DC servo response will deviate
from the ideal integrator). There's no obvious reason to not use the same
resistor/capacitor values, and there are certain advantages for this (besides
what's been noted e.g. minimized cost).

Samuel

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JohnRoberts

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Hickory, MS

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8911

Re: Choosing Op Amps for DC Servo

« Reply #16 on: June 12, 2014, 01:24:18
PM »

Quote
from: saint gillis on June 12, 2014, 06:41:40 AM

Quote
from: JohnRoberts on June 10, 2014, 10:21:32 AM

When looking at DC servos there can
be a temptation to underestimate the demands on the active circuitry.
With the remedy of passive LPF coming and going, the opamp never sees HF so
can be selected based on optimal DC performance alone.

So if we take the care of
adding an output LPF it doesn't need to be as low as the servo's input LPF (RF
precaution)  ?

Now let's take this exemple (but could have
taken any other)
http://www.johnhardyco.com/Schematics/MPC-1_Schematic.jpg

I have a naive novice question: when we are in high gain, -in of the 990C at
20r from the ground, isn't the servo's output going through a 200k/20r voltage
divider and being divided by a factor of 10000? I'm sure it is here my lack of
understanding Op Amps...

I also have a last question, why
R13 and R14 have to be the same value?

OK, looking at this in the context of my
comments, it is using the NI topology that I prefer because it passively LPF the
input to the servo. The LM11 is now probably obsolete, but that was a DC
precision part, optimized for DC characteristics and I couldn't find a gain
bandwidth figure. They do spec a plot for slew rate but in mV/uSec so not very
fast. Luckily a servo does not need to move very fast.

Regarding an
output filter, the LM11 has a 200k resistor in series which will limit the max
current there to tens of uA. The good part is that the signal at that output has
been band-pass filtered by the input transformer. Since there is no compensation
cap shown in the schematic for the LM11 it will be as fast as can be. Since I am
not aware of any complaints I guess it is fast enough.

Worst case for the
servo output would be low preamp gain and high frequency audio signal input, but
as I noted the transformer will LPF that input.
=====
As others have
noted the simplest analysis of pull-in range for that topology is 10k/200k or
1/20th the +/-15V range. Of course at max preamp gain the DC offset voltage of
the  990 into 20 ohm needs to be matched, but that is why there is a DC
trim to null for that worst case before the servo is connected.

From one
data sheet I found the closed loop output impedance of the LM11c @ 20kHz and for
unity gain as used is 10 ohms. So at 20kHz 10s of uA into 10 ohms will make 100s
of uV at that output node and that is cap coupled to the servo - input, while
that is likely part of the closed loop output impedance measurement.

It is kind of academic to second guess this design at this point since
the LM11c is obsolete, but if stable**, I might have added a passive RC in that
200k path. The good news is that any modern part you drop into that socket will
probably be lower output impedance and faster than the LM11c (perhaps why it is
obsolete).

re: the same value of R13 and R14, to insure best
DC performance you generally want similar DCR at both opamp inputs. The 990 does
not enjoy the same DCR at both inputs with different gain settings thus the
benefit of using a servo.

FWIW I am not criticizing this design that is
highly regarded, just doing a little mental exercise. Not only might I have done
the servo slightly differently, I wouldn't use a transformer or a DOA in
2014.

JR

** I am not suggesting to just add a pole there without
testing. Using two 100k resistors with a small cap in the middle to ground could
rise time limit the small current feeding back into the LM11c output while could
be set high enough frequency to not disrupt stability, while I am reluctant to
just say do this, without testing on the bench.

Note: this is
probably trading one very tiny gain/response error as the effective resistance
rises from 200,000 at LF to 200,010 at 20kHz. Of course any audio frequency pole
in that 200k path creates a gain increase above that pole frequency. so two
different direction tiny gain/response errors. Probably hard to measure, and
harder to hear, and hard to justify doing.

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saint gillis

Member
Brussels - Belgium

Posts:
170

Re: Choosing Op Amps for DC Servo

« Reply #17 on: June 13, 2014, 12:10:38
PM »

Quote
from: JohnRoberts on June 12, 2014, 01:24:18 PM

Worst case for the servo output would
be low preamp gain and high frequency audio signal input, but as I noted the
transformer will LPF that input.
=====
As others have noted the
simplest analysis of pull-in range for that topology is 10k/200k or 1/20th the
+/-15V range. Of course at max preamp gain the DC offset voltage of the
990 into 20 ohm needs to be matched, but that is why there is a DC trim to
null for that worst case before the servo is connected.

Here is the M1 input BIAS
calibration procedure  :

"Repeat steps 4 and 5 (990 output DC offset
at min & max gain) until the DC offset measurements are within 1 millivolt
of each other at minimum and maximum gain. Note that both readings may be
several millivolts, or even tens of millivolts. They might both be positive, or
negative. The important thing is that they are within 1 millivolt of each other,
and the same polarity"
http://www.johnhardyco.com/pdf/990.pdf
Does it make
sense? (guess it does)

Quote

re: the same value of R13 and R14, to
insure best DC performance you generally want similar DCR at both opamp
inputs. The 990 does not enjoy the same DCR at both inputs with different gain
settings thus the benefit of using a servo.

Ok, thank you for making it
clear!

Quote

** I am not suggesting to just add a
pole there without testing. Using two 100k resistors with a small cap in the
middle to ground could rise time limit the small current feeding back into the
LM11c output while could be set high enough frequency to not disrupt
stability, while I am reluctant to just say do this, without testing on the
bench.

Nice idea

Quote

FWIW I am not criticizing this design
that is highly regarded, just doing a little mental exercise. Not only might I
have done the servo slightly differently, I wouldn't use a transformer or a
DOA in 2014.

JR

You already avdised me the use of
THAT1646, which are great. Do you use THAT1240 family for input? what would you
use instead of a DOA?
I know thoose last questions are not really the
subject of this thread...

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JohnRoberts

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Member
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Posts:
8911

Re: Choosing Op Amps for DC Servo

« Reply #18 on: June 13, 2014, 01:44:48
PM »

Quote
from: saint gillis on June 13, 2014, 12:10:38 PM

Quote
from: JohnRoberts on June 12, 2014, 01:24:18 PM

Worst case for the servo output
would be low preamp gain and high frequency audio signal input, but as I
noted the transformer will LPF that input.
=====
As others have noted
the simplest analysis of pull-in range for that topology is 10k/200k or
1/20th the +/-15V range. Of course at max preamp gain the DC offset voltage
of the  990 into 20 ohm needs to be matched, but that is why there is a
DC trim to null for that worst case before the servo is connected.

Here is the M1 input BIAS
calibration procedure  :

"Repeat steps 4 and 5 (990 output DC
offset at min & max gain) until the DC offset measurements are within 1
millivolt of each other at minimum and maximum gain. Note that both readings
may be several millivolts, or even tens of millivolts. They might both be
positive, or negative. The important thing is that they are within 1 millivolt
of each other, and the same polarity"
http://www.johnhardyco.com/pdf/990.pdf
Does it make
sense? (guess it does)

Yes, the concept is to make the dc error
the same at the different gains. This way the servo only has to correct it once
and not correct again every time you change the gain. The servo output is slow
moving so gain switch would click and maybe thump, without the two trims.

Quote

Quote

re: the same value of R13 and R14,
to insure best DC performance you generally want similar DCR at both opamp
inputs. The 990 does not enjoy the same DCR at both inputs with different
gain settings thus the benefit of using a servo.

Ok, thank you for making it
clear!

Quote

** I am not suggesting to just add a
pole there without testing. Using two 100k resistors with a small cap in the
middle to ground could rise time limit the small current feeding back into
the LM11c output while could be set high enough frequency to not disrupt
stability, while I am reluctant to just say do this, without testing on the
bench.

Nice idea

Quote

FWIW I am not criticizing this
design that is highly regarded, just doing a little mental exercise. Not
only might I have done the servo slightly differently, I wouldn't use a
transformer or a DOA in 2014.

JR

You already avdised me the use of
THAT1646, which are great. Do you use THAT1240 family for input? what would
you use instead of a DOA?
I know thoose last questions are not
really the subject of this thread...

I haven't designed a professional
audio path for over 15 years. Back when I did I roiled my own from decent
opamps, low noise transistors, and precision resistors. These days I suspect I
would use one of the several decent off the shelf chip sets that have become
more cost effective. I haven't needed to roll my own DOA since the late '70s.
Modern opamps are quite good.

But don't look to me for personal guidance
DOA and transformers are very popular with DIY crowd. Me not so much.

JR

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Choosing Op Amps for DC Servo

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DC servo or not to DC servo,this is the question NEW INFO

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June 10, 2008, 04:54:36 AM
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August 18, 2011, 05:33:15 PM
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